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What is a battery?

A battery, in concept, can be any device that stores energy for later use. A rock, pushed to the top of a hill, can be considered a kind of battery, since the energy used to push it up the hill (chemical energy, from muscles or combustion engines) is converted and stored as potential kinetic energy at the top of the hill. Later, that energy is released as kinetic and thermal energy when the rock rolls down the hill. Common use of the word, "battery," however, is limited to an electrochemical device that converts chemical energy into electricity, by use of a galvanic cell. A galvanic cell is a fairly simple device consisting of two electrodes (an anode and a cathode) and an electrolyte solution. A battery is an electrical storage device. Batteries do not make electricity, they store it, just as water tank stores water for future use. As chemicals in the battery change, electrical energy is stored or released. In rechargeable batteries this process can be repeated many times. Batteries are not 100% efficient - some energy is lost as heat and chemical reactions when charging and discharging.

Why Cells Fail?

For the applications engineer designing reliable products dependent on batteries for power an understanding of the potential failure modes of the cells employed is essential. This is to enable him to ensure that potential faults have been designed out of the cells themselves and that unsuitable or uncontrolled operating conditions during manufacture or use of the cells can be prevented or avoided. Batteries with different cell chemistries or constructions may fail in different ways. This report outlines some of the most common cell failures and suggests preventative measures which need to be considered when specifying cells for a new battery application. 

Why Cells Fail?

Cell design faults
The logical place to start the analysis is in the design of the cell itself. Unfortunately it is the area where the applications design engineer has least knowledge and which he is least able to influence. Cell design faults such as weak mechanical design, inadequate pressure seals and vents, the specification of poor quality materials and improperly specified tolerances can be responsible for many potential failures. Unless he is qualified in physical chemistry, and has experience in components design and access to detailed cell design data and specialist equipment such as mass spectrometers and electron microscopes, there's not much the applications engineer can do to assure himself of the quality of the cell design just from the specifications. What can be done however is accelerated life testing on sample cells to verify that they meet the desired reliability requirements for the proposed application. Before any cells are adopted for your battery application they should undergo thorough qualification testing to identify any potential weaknesses. For more information see Battery Testing. If you don't have the necessary equipment to carry out these tests your friendly pack designer should be able to do it for you.
Manufacturing processes out of control
This is an area where the applications engineer can begin to have some influence. A cell may be well designed, but once it gets out of the design lab and into the factory its fate is determined by the factory manager. In well managed companies this should not be a problem, but a badly run production facility can introduce numerous potential failure sources into the cell. This is less likely to be a problem in a large automated plant with a well known brand name to protect, but if you are looking for the lowest cost cell manufacturer you need to be conscious that corners may be cut to achieve the cost targets. Some symptoms to watch out for:-
• Manual production methods. - It is very difficult to achieve precision and repeatability using manual assembly methods and lack of precision means potential short circuits, leaks, unreliable connections and contamination. This doesn't just apply to back street operations using low cost labour. Even with well managed companies, when new technologies are introduced, the initial customer requirements are usually supplied by hand made products or products made on semi automated machinery until the demand is established and the investment in automated production machinery is justified.
• Handling damage such as scratches or crushing of the electrode sub-assembly.
• Out of tolerance components create similar problems as with imprecise assembly noted above.
• Burrs on the electrode current collector foils give rise to short circuits.
• Voids reduce cell capacity, increase impedance and impede heat dissipation.
• Contamination of the active chemicals gives rise to unwanted chemical effects which could result in various forms of cell failure such as overheating, pressure build up, reduced capacity, increased impedance and self discharge and short circuits.
• Lubricants or debris left in the casing materials.
• Poor control of electrode particle morphology. Particle size needs to be very small and uniform to achieve the cell's specified power handling capability.
• Process out of control. - A typical example is variable coating thicknesses of the active chemicals on the electrodes. Once again the results could affect cell capacity, impedance and self discharge. Process control also applies to the temperature and humidity of the air in the production plant as well as to the dimensional accuracy of the components.
• Use of unapproved alternative materials. This is not necessarily obvious but it certainly happens. Tests on samples may be needed to verify this.
• Weld/sealing quality - This can result in poor, unreliable connections and localised heat build up.
• Mechanical weaknesses. In smaller cells the most likely problem will be leakage of the electrolyte. Larger cells will be more prone to cracking or splitting, which also cause leakage, or distortion which means the cells may not fit into the enclosure designed for them.
• Poor sealing results in leakage and loss of active chemicals or water ingress, corrosion and potential safety problems.
• Quality systems and quality management. - After the design of the cell itself, these are perhaps the most important factors affecting cell failures. The manufacturing facility needs to have in place, at key points in the production process, controls which set limits to, and monitor continuously , all the parameters which can ultimately affect the quality and reliability of the product. Corrective actions should come into play automatically whenever the specified limits are approached to ensure they are never breached. Not only should the system be in place but it should be seen to be fully operational. Records should be kept as evidence that the system is at all times operating correctly.

All of these points can be verified by the battery applications engineer to give confidence in the proposed cell supplier provided the cell suppliers allow access to their manufacturing plants.


Battery performance gradually deteriorates with time due to unwanted chemical reactions and physical changes to the active chemicals. This process is generally not reversible and eventually results in battery failure. The following are some examples:-

• Corrosion consumes some of the active chemicals in the cell leading to increased impedance and capacity loss
• Chemical loss through evaporation. Gaseous products resulting from over charging are lost to the atmosphere causing capacity loss.
• Change in physical characteristics (morphology) of the working chemicals.
• Crystal formation. Over time the crystal structure at the electrode surface changes as larger crystals are formed. This reduces the effective area of the electrodes and hence their current carrying and energy storage capacity.
• Dendritic growth. This is the formation of small crystals or treelike structures around the electrodes in what should be an aqueous solution. Initially these dendrites may cause an increase in self discharge. Ultimately dendrites can pierce the separator causing a short circuit.
• Passivation. This is a resistive layer which builds up on the electrodes impeding the chemical action of the cell.
• Shorted cells. Cells which were marginally acceptable when new may have contained latent defects which only become apparent as the ageing process takes its toll. This would include poor cell construction, contamination, burrs on metal parts which can all cause the electrodes to come into contact with each other causing a short circuit.
• Electrode or electrolyte cracking. Some solid electrolyte cells such as Lithium polymer can fail because of cracking of the electrolyte.
The ageing process outlined above is accelerated by elevated temperatures.
Uncontrolled operating conditions
Good batteries are not immune to failure which can be provoked by the way they are used or abused. High cell temperature is the main killer and this can be brought about in the following situations.
• Bad applications design
• Unsuitable cell for the application
• Unsuitable charging profile
• Overcharging
• Environmental conditions. High ambient temperatures. Lack of cooling.
• High storage temperature
• Physical damage is also a contributing factor
Most of these conditions result in overheating of the cell which is what ultimately kills it.
Abuse does not just mean deliberate physical abuse by the end user. It also covers accidental abuse which may be unavoidable. This may include dropping, crushing, penetrating, impacts, immersion in fluids, freezing or contact with fire, any of which could occur to an automotive battery for instance. It is generally accepted that the battery may not survive all these trials, however the battery should not itself cause an increased hazard or safety problem in these circumstances.
External Factors
Battery failures may not necessarily be due to natural wearout or abuse by the user. They could well be caused by malfunctions in the systems in which they are installed. Batteries used in automotive applications could be subject to a variety of problems, any of which could wreck the battery, such as:
• Sensor failures
• Circuit interruptor failure
• Fan or pump failures
• Loss of cooling fluid
• Incorrect or missing BMS messages
• BMS failure
• Loss of communications or interference
• Charger failure (overcharging)
To identify the route cause of the failure the vehicle On Board Diagnostics (OBD) should be correlated with the data logging in the BMS.

How Cells Fail

The actions or processes outlined above cause the cells to fail in the following ways:
Active chemicals exhausted
In primary cells this is not classed as a failure since this is to be expected but with secondary cell we expect the active chemicals to be restored through recharging. As noted above however ageing will cause the gradual depletion of the active mass.
Change in molecular or physical structure of the electrodes
Even though the chemical composition of the active chemicals may remain unchanged, changes in their morphology which take place as the cell ages can impede the chemical actions from taking place, ultimately rendering the cell unusable.
Breakdown of the electrolyte
Overheating or over-voltage can cause chemical breakdown of the electrolyte.
Electrode plating
In Lithium cells, low temperature operation or over-current during charging can cause deposition of Lithium metal on the anode resulting in irreversible capacity loss and eventually a short circuit.
Increased internal impedance
The cell internal impedance tends to increase with age as the larger crystals form, reducing the effective surface area of the electrodes.
Reduced capacity
This is another consequence of cell ageing and crystal growth. Is is sometimes recoverable through reconditioning the cell by subjecting the cell to one or more deep discharges.
Increased self discharge
The changing crystal structure of the active chemicals as noted above can cause the electrodes to swell increasing the pressure on the separator and, as a consequence, increasing the self discharge of the cell. As with all chemical reactions this increases with temperature.
Unfortunately these changes are not usually reversible.
Gassing is generally due to over charging. This leads to loss of the active chemicals but In many cases this can also be dangerous. In some cells the released gases may be explosive. Lead acid cells for instance give off oxygen and hydrogen when overcharged.
Pressure build up
Gassing and expansion of the chemicals due to high temperatures lead to the build up of pressure in the cell and this can be dangerous as noted above. In sealed cells it could lead to the rupture or explosion of the cell due to the pressure build up unless the cell has a release vent to allow the escape of the gasses. Pressure build up can cause short circuits due to penetration of the separator (see next) and this is more of a problem in cylindrical cells which tend to resist deformation under pressure compared with prismatic cells whose cases have more "give" thus mitigating the pressure effect somewhat.
Penetration of the separator
Short circuits can be caused by pentration of the separator due dendrite growth, contamination, burrs on the electrodes or softening of the separator due to overheating.
Before the pressure in the cell builds up to dangerous limits, some cells are prone to swelling due to overheating. This is particularly true of Lithium polymer pouch cells. This can lead to capacity loss due to deteriorating contact between the conductive particles within the cell as well as external problems in fitting the cell into the battery enclosure.
Overheating is always a problem and is a contributing factor in nearly all cell failures. It has many causes and it can lead to irreversible changes to the chemicals used in the cells, gassing, expansion of the materials, swelling and distortion of the cell casing. Preventing a cell from overheating is the best way of extending its life.
Thermal runaway
The rate at which a chemical action proceeds doubles for every 10°C increase in temperature. The current flow through a cell causes its temperature to rise. As the temperature rises the electro-chemical action speeds up and at the same time the impedance of the cell is reduced leading to even higher higher currents and higher temperatures which could eventually lead to destruction of the cell unless precautions are taken.

Consequences of Cell Faults

The failure mechanisms noted above to not always lead to immediate and complete failure of the cell. The failure will often be preceded by a deterioration in performance. This may be manifest in reduced capacity, increased internal impedance and self discharge or overheating. If a degraded cell continues in use, higher cell heat dissipation may result in premature voltage cut off by the protection circuits before the cell is fully charged or discharged reducing the effective capacity still further. Measurement of the State of Health of the cells can provide an advance warning of impending failure of the cell. There are several possible failure modes associated with the complete breakdown of the cell, but it is not always possible to predict which one will occur. It depends very much on the circumstances.
• Open circuit - This is a fail safe mode for the cell but may be not for the application. Once the current path is cut and the battery is isolated, the possibility of further damage to the battery is limited. This may not suit the user however. If one cell of a multicell battery goes open circuit then the whole battery will be out of commission.
• Short circuit - If one cell of a battery chain fails because of a short circuit, the rest of the cells may be slightly overloaded but the battery will continue to provide power to its load. This may be important in emergency situations.
Short circuits may be external to the cell or internal within the cell. The battery management system (BMS) should be able to protect the cell from external shorts but there's not much the BMS can do to protect the cell from an internal short circuit.
Within the cell there are different degrees of failure.
Hard Short: solid connection between electrodes causes extremely high current flow and complete discharge resulting in permanent damage to the cell.
Soft Short: small localised contact between electrodes. Possibly self correcting due to melting of the small regions in contact caused by the high current flow which in turn interrupts the current path as in a fuse.
The existence of a soft short could possibly be indicated by an increase in the self discharge of the cell or by a cell with a higher self discharge than the rest of the population. This indicator is unfortunately less ponounced in larger cells where it is most needed.
• Explosion - This is to be avoided at all costs and the battery must incorporate protection circuits or devices to prevent this situation from occurring.
• Fire - This is also possible and as above the battery should be protected from this possibility.
False Alarms
Occasionally you may find that an apparent fault in the battery is actually a fault external to the cells. It could be in the charger or in the protection circuitry. This may occur when a "perfectly good" charger is unable to charge the battery. It is possible that this could be caused by a mismatch in the protection limit settings between the battery and the charger. The charger voltage regulation may not have the range to cope with a fully discharged battery or the current limits may be set too low to allow the initial current inrush into the battery when the charger is switched on.
It is also possible that a fault in the protection circuit could cause the battery to discharge. The possibility of external faults should therefore be verified before the cells are blamed.
Maximizing Battery Life
Applications design: the first step is to ensure that the most appropriate battery is chosen for the application.
Supplier qualification: the second step is to select a cell supplier who can be relied upon to provide a safe reliable product.
Cell qualification: the next step is to verify that the chosen cells meet the desired specification under every expected condition of use and that inbuilt safety devices such as pressure vents function correctly.
Protection circuits: once the cell has been chosen the ancillary electronic circuits can be specified. The most important of these are the safety circuits which ensure that the cells are maintained within their specified operating temperature, current, and voltage limits. This should also include the specification of the charger.
Failure Prevention Design Reviews
The design process for a new cell technology could take up to 10 years or more. Failure prevention sould be an important agenda item during regular design reviews which sould take place during this period. See Failure Modes and Effects Analysis.
Product qualification
When finished battery packs become available, they should be subject to qualification tests as stand alone units and as part of the qualification testing of the product in which they are used and also with the associated charger. These tests should identify whether there are any undesired interactions between these units.
Don't use up the battery's life unnecessarily by storing it at too high temperatures.
After taking such care during the design process, don't let the pack manufacturing introduce potential faults into the battery. Make sure that the factory is implementing the necessary quality systems.
Planned maintenance
Provide the user with recommended operating and maintenance procedures for the battery (including reconditioning if this is possible) and ways of monitoring the battery State of Health.

Battery Types

1.      Cylindrical Cell

The cylindrical cell continues to be one of the most widely used packaging styles for primary and secondary batteries. The advantages are ease of manufacture and good mechanical stability. The tubular cylinder has the ability to withstand internal pressures without deforming. Figure 1 shows a cross section of a cell.






Figure 1: Cross section of a
lithium-ion cylindrical cell

The cylindrical cell design has good cycling ability, offers a long calendar life, is economical but is heavy and has low packaging density due to space cavities.

Typical applications for the cylindrical cell are power tools, medical instruments and laptops. Nickel-cadmium offers the largest variety of cell choices, and some popular formats have spilled over to nickel-metal-hydride. To allow variations within a given size, manufacturers use fractural cell length, such as half and three-quarter formats.

The established standards for nickel-based batteries did not catch on with lithium-ion and the chemistry has established its own formats. One of the most popular cell packages is the 18650, as illustrated in Figure 2. Eighteen denotes the diameter and 65 is the length of the cell in millimeters. The Li-manganese version 18650 has a capacity of 1,200–1,500mAh; the Li-cobalt version is 2,400–3,000mAh. The larger 26650 cells have a diameter of 26mm with a length of 65mm and deliver about 3,200mAh in the manganese version. This cell format is used in power tools and some hybrid vehicles.


Figure 2: Popular 18650 lithium-ion cell

The metallic cylinder measure 18mm in diameter and 65mm the length. The larger 26650 cell measures 26mm in diameter. 

Lead acid batteries come in flooded and dry formats; portable versions are packaged in a prismatic design resembling a rectangular box made of plastic. Some lead acid systems also use the cylindrical design by adapting the winding technique, and the Hawker Cyclone is in this format. It offers improved cell stability, higher discharge currents and better temperature stability than the conventional prismatic design.

Cylindrical cells include a venting mechanism that releases excess gases when pressure builds up. The more simplistic design utilizes a membrane seal that ruptures under high pressure. Leakage and subsequent dry-out may occur when the membrane breaks. The re-sealable vents with a spring-loaded valve are the preferred design. Cylindrical cells make inefficient use of space, but the air cavities that result with side-by-side placement can be used for air-cooling.

2.      Pouch Cell

In 1995, the pouch cell surprised the battery world with a radical new design. Rather than using a metallic cylinder and glass-to-metal electrical feed-through for insulation, conductive foil tabs welded to the electrode and sealed to the pouch carry the positive and negative terminals to the outside. Figure 5 illustrates such a pouch cell.

Figure 3: The pouch cell

The pouch cell offers a simple, flexible and lightweight solution to battery design. Exposure to high humidity and hot temperature can shorten service life.

The pouch cell makes the most efficient use of space and achieves a 90 to 95 percent packaging efficiency, the highest among battery packs. Eliminating the metal enclosure reduces weight but the cell needs some alternative support in the battery compartment. The pouch pack finds applications in consumer, military, as well as automotive applications. No standardized pouch cells exist; each manufacturer builds the cells for a specific application.

Pouch packs are commonly Li-polymer. Its specific energy is often lower and the cell is less durable than Li-ion in the cylindrical package. Swelling or bulging as a result of gas generation during charge and discharge is a concern. Battery manufacturers insist that these batteries do not generate excess gases that can lead to swelling. Nevertheless, excess swelling can occur and most is due to faulty manufacturing, and not misuse. Some dealers have failures due to swelling of as much as three percent on certain batches. The pressure from swelling can crack a battery cover, and in some cases break the display and electronic circuit board. Manufacturers say that an inflated cell is safe. While this may be true, do not puncture a swollen cell in close proximity to heat or fire; the escaping gases can ignite. Figure 6 shows a swelled pouch cell.


Figure 4: Swelling pouch cell

Swelling can occur as part of gas generation. Battery manufacturers are at odds why this happens. A 5mm (0.2”) battery in a hard shell can grow to 8mm (0.3”), more in a foil package.

To prevent swelling, the manufacturer adds excess film to create a “gas bag” outside the cell. During the first charge, gases escape into the gasbag, which is then cut off and the pack resealed as part of the finishing process. Expect some swelling on subsequent charges; 8 to 10 percent over 500 cycles is normal. Provision must be made in the battery compartment to allow for expansion. It is best not to stack pouch cells but to lay them flat side by side. Prevent sharp edges that could stress the pouch cell as they expand.

Summary of Packaging Advantages and Disadvantages

  • A cell in a cylindrical metallic case has good cycling ability, offers a long calendar life, is economical to manufacture, but is heavy and has low packaging density. 
  • The prismatic pouch pack is light and cost-effective to manufacture. Exposure to high humidity and hot temperature can shorten the service life. A swelling factor of 8 - 10 percent over 500 cycles is normal.


Primary Cells

1. Leclanché Cells

2. Alkaline Cells

3. Silver Oxide Cells

4. Zinc Air Cells

5. Lithium Primary Cells

6. Water Activated Batteries

7. Thermal Batteries


Secondary Cells

1.   Lead Acid

2.   Nickel Iron

3.   Nickel Cadmium

4.   Nickel Metal Hydride

5.   Nickel Zinc

6.   Nickel Hydrogen

7.   Lithium Secondary Cells

8.   Sodium Sulphur

9.    Flow Cells (Redox)

10.  Zebra Cells

11.  Other Galvanic Cells

Unusual Batteries

1.    Urine Battery- No its not a joke

2.    Ampoule Batteries

3.    Thin Film Batteries

4.    Homebrew Battery

Common Household-Battery Sizes

Size Shape and Dimensions

D: Cylindrical, 61.5 mm tall, 34.2 mm diameter.

C: Cylindrical, 50.0 mm tall, 26.2 mm diameter.

AA: Cylindrical, 50.5 mm tall, 14.5 mm diameter.

AAA: Cylindrical, 44.5 mm tall, 10.5 mm diameter.

PP3: Rectangular, 48.5 mm tall, 26.5 mm wide, 17.5 mm deep.


Toxicity of Lithium

In case you wondered whether there were any toxic effects associated with Lithium, it is claimed that Lithium on the contrary has therapeutic benefits. The soft drink "7Up" started life in 1929, two months before the Wall Street Crash, with the catchy name "Bib Label Lithiated Lemon-Lime Soda". "7Up" contained Lithium Citrate until 1950 when it was reformulated, some say because of Lithium's association with mental illness. Since the 1940s, Lithium in the form of Lithium Carbonate has been used successfully in the treatment of mental disorder particularly manic depression. As with most chemicals however, small doses may be safe or therapeutic, but too much can be fatal. See more about toxicity on the New Battery Designs and Chemistries page.

Battery Temperature Characteristics

Cell performance can change dramatically with temperature. At the lower extreme, in batteries with aqueous electrolytes, the electrolyte itself may freeze setting a lower limit on the operating temperature. At low temperatures Lithium batteries suffer from Lithium plating of the anode causing a permanent reduction in capacity. At the upper extreme the active chemicals may break down destroying the battery. In between these limits the cell performance generally improves with temperature. See also Thermal Management and Battery Life for more details.

Battery Storage

Battery Storage
The optimum storage conditions for batteries depend on the active chemicals used in the cells. During storage the cells are subject to both self discharge and possible decomposition of the chemical contents. Over time solvents in the electrolyte may permeate through the seals causing the electrolyte to dry out and lose its effectiveness. In all cases these processes are accelerated by heat and it is wise to store the cells in a cool, benign environment to maximise their shelf life. The glove compartment of a car does not qualify as a suitable storage location since temperatures may exceed 60°C shortening dramatically the life of the battery.
For cells with the same nominal cell chemistry, individual manufacturers may add different additives to optimise their cell performance for a particular parameter and this may affect the behaviour of the cells during storage. It is possible to make some general recommendation about storage but the best guidance for storage is to consult the manufacturers' specifications and recommendations for their products. 

The possible storage temperature range for Lithium-Ion batteries is is -20°C to 60°C but for prolonged storage period -20°C to 25°C is recommended and 15°C is ideal. Cells should be stored with a partial charge of between 30% and 50%. Although the cells can be stored fully discharged the cell voltage should not drop below 2.0 Volts per cell and cells should be topped up to prevent over-discharge. The maximum voltage should not exceed 4.25 Volts
If secondary cells must be for a prolonged period the state of charge should be checked regularly and provision should be made for recharging the cells before the cell voltage drops below the recommended minimum after which the cells suffer irreparable deterioration. ( This is particularly true for battery packs which may have associated electronics which add to the self discharge drain on the cells)

Spiral Wound Electrodes

Spiral Wound Electrodes, also called Jelly-roll or Swiss-roll construction. In the quest for higher current carrying capacity, it is necessary to increase the active surface area of the electrodes, however the cell case size sets limits on the size of electrodes which can be accommodated. One way of increasing the electrode surface area is to make the electrodes and the separator from long strips of foil and roll them into a spiral or cylindrical jelly-roll shape. This provides very low internal resistance cells. The downside is that since the electrodes take up more space within the can there is less room for the electrolyte and so the potential energy storage capacity of the cell is reduced. This construction is used extensively for secondary cells. The example above shows a Lithium-Ion cell but this technology is also used for NiCads, NiMH and even some Lead acid secondary cells designed for high rate applications. Spiral wound construction not limited to cylindrical shapes. The electrodes can be wound onto a flat mandrel to provide a flattened shape which can fit inside a prismatic case. The cases may be made from aluminium or steel. This construction is ideally suited for production automation.


Internal impedance higher than equivalent NiCads

For high power applications which require large high cost batteries the price premium of Lithium batteries over the older Lead Acid batteries becomes a significant factor, impeding widespread acceptance of the technology. This in turn has discouraged investment in high volume production facilities keeping prices high and has for some time discouraged take up of the new technology. This is gradually changing and Lithium is also becoming cost competitive for high power applications.

Stability of the chemicals has been a concern in the past. Because Lithium is more chemically reactive special safety precautions are needed to prevent physical or electrical abuse and to maintain the cell within its design operating limits. Lithium polymer cells with their solid electrolyte overcome some of these problems.

Stricter regulations on shipping methods than for other cell chemistries.

Degrades at high temperatures.

Capacity loss or thermal runaway when overcharged.

Degradation when discharged below 2 Volts.

Venting and possible thermal runaway when crushed.

Need for protective circuitry.

Measurement of the state of charge of the cell is more complex than for most common cell chemistries. The state of charge is normally extrapolated from a simple measurement of the cell voltage, but the flat discharge characteristic of lithium cells, so desirable for applications, renders it unsuitable as a measure of the state of charge and other more costly techniques such as coulomb counting have to be employed.

Although Lithium cell technology has been used in low power applications for some time now, there is still not a lot of field data available about long term performance in high power applications. Reliability predictions based on accelerated life testing however shows that the cycle life matches or exceeds that of the most common technologies currently in use.

These drawbacks are far out weighed by the advantages of Lithium cells and are now being used in an ever widening range of applications.


Self-discharge Characteristics

The self discharge rate is a measure of how quickly a cell will lose its energy while sitting on the shelf due to unwanted chemical actions within the cell. The rate depends on the cell chemistry and the temperature.

Cell Chemistry

The following shows the typical shelf life for some primary cells:

1.         Zinc Carbon (Leclanché) 2 to 3 years

2.         Alkaline 5 years

3.         Lithium 10 years or more

Typical self discharge rates for common rechargeable cells are as follows:

1.         Lead Acid 4% to 6% per month

2.         Nickel Cadmium 15% to 20% per month

3.         Nickel Metal Hydride 30% per month

4.         Lithium 2% to 3% per month

Temperature Effects

The rate of unwanted chemical reactions which cause internal current leakage between the positive and negative electrodes of the cell, like all chemical reactions, increases with temperature thus increasing the battery self discharge rate. See also Battery Life . The graph below shows typical self discharge rates for a Lithium Ion battery.

Internal Impedance

The internal impedance of a cell determines its current carrying capability. A low internal resistance allows high currents.

Battery Equivalent Circuit

The diagram on the right shows the equivalent circuit for an energy cell.

Rm is the resistance of the metallic path through the cell including the terminals, electrodes and inter-connections.

Ra is the resistance of the electrochemical path including the electrolyte and the separator.

Cb is the capacitance of the parallel plates which form the electrodes of the cell.

Ri is the non-linear contact resistance between the plate or electrode and the electrolyte.

Typical internal resistance is in the order of milliohms.

Battery Load

Battery discharge performance depends on the load the battery has to supply.

If the discharge takes place over a long period of several hours as with some high rate applications such as electric vehicles, the effective capacity of the battery can be as much as double the specified capacity at the C rate. This can be most important when dimensioning an expensive battery for high power use. The capacity of low power, consumer electronics batteries is normally specified for discharge at the C rate whereas the SAE uses the discharge over a period of 20 hours (0.05C) as the standard condition for measuring the Amphour capacity of automotive batteries. The graph below shows that the effective capacity of a deep discharge lead acid battery is almost doubled as the discharge rate is reduced from 1.0C to 0.05C. For discharge times less than one hour (High C rates) the effective capacity falls off dramatically. The effectiveness of charging is similarly influenced by the rate of charge. An explanation of the reasons for this is given in the section on Charging Times .

Cycle Life

This is one of the key cell performance parameters and gives an indication of the expected working lifetime of the cell. The cycle life is defined as the number of cycles a cell can perform before its capacity drops to 80% of its initial specified capacity.

Each charge - discharge cycle, and the associated transformation cycle of the active chemicals it brings about, is accompanied by a slow deterioration of the chemicals in the cell which will be almost imperceptible to the user. This deterioration may be the result of unavoidable, unwanted chemical actions in the cell or crystal or dendrite growth changing the morphology of the particles making up the electrodes. Both of these events may have the effect of reducing the volume of the active chemicals in the cell, and hence its capacity, or of increasing the cell's internal impedance. Note that the cell does not die suddenly at the end of the specified cycle life but continues its slow deterioration so that it continues to function normally except that its capacity will be significantly less than it was when it was new. The cycle life as defined is a useful way of comparing batteries under controlled conditions, however it may not give the best indication of battery life under actual operating conditions. Cells are seldom operated under successive, complete charge - discharge cycles, they are much more likely to be subject to partial discharges of varying depth before complete recharging. Since smaller amounts of energy are involved in partial discharges, the battery can sustain a much greater number of shallow cycles. Such usage cycles are typical for Hybrid Electric Vehicle applications with regenerative braking. See how cycle life varies with depth of discharge in Battery Life. Cycle life also depends on temperature, both operating and storage temperature. See more details in the section on Lithium Battery Failures.

 A more representative measure of battery life is the Lifetime Energy Throughput. This is the total amount of energy in Watthours which can be put into and taken out of a battery over all the cycles in its lifetime before its capacity reduces to 80% of its initial capacity when new. It depends on the cell chemistry and the operating conditions. Unfortunately this measure is not yet in common use by cell manufacturers and has not yet been adopted as a battery industry standard. Until it comes into general use it will not be possible to use it to compare the performance of cells from different manufacturers in this way but, when available, at least it provides a more useful guide to applications engineers for estimating the useful life of batteries used in their designs. 

Deep Discharge

Cycle life decreases with increased Depth of Discharge (DOD) and many cell chemistries will not tolerate deep discharge and cells may be permanently damaged if fully discharged. Special cell constructions and chemical mixes are required to maximise the potential DOD of deep cycle batteries.


The Restriction of Hazardous Substances-RoHS

The Restriction of Hazardous Substances Directive 2002/95/EC, RoHS, short for Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment, was adopted in February 2003 by the European Union.
The RoHS directive took effect on 1 July 2006, and is required to be enforced and become law in each member state. This directive restricts (withexceptions) the use of six hazardous materials in the manufacture of various types of electronic and electrical equipment. It is closely linked with theWaste Electrical and Electronic Equipment Directive (WEEE) 2002/96/EC which sets collection, recycling and recovery targets for electrical goods and is part of a legislative initiative to solve the problem of huge amounts of toxic e-waste. In speech, RoHS is often spelled out, or pronounced /ˈrɒs/, /ˈrɒʃ/,/ˈroʊz/, or /ˈroʊhɒz/, and refers to the EU standard, unless otherwise qualified.

Under the consideration of environmental protection, in response to the legislated environmental directives being instituted throughout the world, HONCELL has established a comprehensive program that is intended to promote our products Li-ion polymer batteries and the related products to be compliant with the new laws. At this time, the key environmental directives are from the European Community, with many countries/states either following these directives explicitly or instituting their own variations. The most notable directive is the RoHS (Restriction of the Use of Certain Hazardous Substances. The RoHS legislation restricts the use of certain hazardous substances to levels of less than 100-1000 ppm used in electronic & electrical products. The RoHS and equivalent directives prohibit use of the following substances in products placed into the market effective July 1, 2006:

1. Lead (Pb)
2. Hexavalent Chromium (Cr “VI”)
3. Mercury (Hg)
4. Cadmium (Cd)
5. Polybrominated Biphenyls (PBB)
6. Polybrominated Diphenylethers (PBDE)

HONCELL has already strictly restricted the non-RoHS compliance raw materials from our suppliers and request each of our raw material supplier to provide SGS reports before their materials enter into our warehouse. In addition, we have already set up a regular audit plan to make spot-check over all the raw material suppliers to ensure their promise to quality confirms to the published data. In house, we pay more attention to production processing and strictly restrict each step with regard to manufacturing control to ensure the RoHS compliant environment. Currently 100% of HONCELL products are RoHS compliant and allow us an easy access to clients all over the world. Please find out the SGS Reports on our website www.honcell.com.


REACH is the Regulation on Registration, Evaluation, Authorisation and Restriction of Chemicals. It entered into force on 1st June 2007. It streamlines and improves the former legislative framework on chemicals of the European Union (EU).

The main aims of REACH are to ensure a high level of protection of human health and the environment from the risks that can be posed by chemicals, the promotion of alternative test methods, the free circulation of substances on the internal market and enhancing competitiveness and innovation.

REACH makes industry responsible for assessing and managing the risks posed by chemicals and providing appropriate safety information to their users. In parallel, the European Union can take additional measures on highly dangerous substances, where there is a need for complementing action at EU level.

Protection Methods

Cell Protection
The purpose of cell protection is to provide the necessary monitoring and control to protect the cells from out of tolerance ambient or operating conditions and to protect the user from the consequences of battery failures. Cell protection can be external to the battery and this is one of the of the prime functions of the Battery Management System.
Safety measures can also be built into the cells themselves and examples are outlined in the section on Battery Safety.
High power cells can be particularly dangerous. They contain large amounts of energy which, if released in an uncontrolled way through a short circuit or physical damage, can have catastrophic consequences. In the case of short circuits, currents of hundreds of amps can build up in microseconds and protection circuits must be very fast acting to prevent this.
Different applications and different cell chemistries require different degrees of protection. Lithium batteries in particular need special protection and control circuits to keep them within their predefined voltage, current and temperature operating limits. Furthermore, the consequences of failure of a Lithium cell could be quite serious, possibly resulting in an explosion or fire. Cell protection is therefore indispensable in Lithium batteries. The following discussion illustrates some of the principles involved.
In general cell protection should address the following undesirable events or conditions:
Excessive current during charging or discharging.
Short circuit
Over voltage - Overcharging
Under voltage - Exceeding preset depth of discharge (DOD) limits
High ambient temperature
Overheating - Exceeding the cell temperature limit
Pressure build up inside the cell
System isolation in case of an accident
 The diagrams also illustrate how the multiple levels of protection function to ensure safe operating conditions at all times even if one of the devices fails.
Thermal Fuse: excessive temperatures will cause all cells to fail eventually. Most protection circuits therefore incorporate a thermal fuse which will permanently shut down the battery if its temperature exceeds a predetermined limit.

Thermistor: thermistors are circuit devices whose resistance varies with temperature. PTC thermistors have a Positive Temperature Coefficient in that their resistance increases gradually with temperature and over a limited range the resistance can be considered linearly proportional to temperature. Similarly NTC thermistors have a Negative Temperature Coefficient and their resistance decreases as temperature increases. These components are used extensively in monitoring and protection circuits to provide a voltage analogue of temperature or in control circuits designed to provide temperature compensation. They may be used to terminate the charge (dt/dT) or to disconnect the battery from the charger in an over-temperature condition when the temperature cut off point is reached, or they could be used to turn on cooling fans. In some applications the thermistor may be the only means of communication between the battery and the external world. Thermistors can also be used by the charger to determine starting environmental conditions and prevent charging if the battery temperature is too low or too high.
Resettable Fuse: aresettable fuse indicated in the diagram above provides on-battery over-current protection. It has a similar function to a thermal fuse but after opening it will reset once the fault conditions have been removed and after it has cooled down again to its normal state. It requires no manual resetting or replacement and so is very convenient for the user who may not even be aware of its operation.
The fuse is triggered when a particular temperature is reached. The temperature rise can be caused by the resistive self heating of the thermistor due to the current passing through it, or by conduction or convection from the ambient environment. Thus it can be used to protect against both over- current and over-temperature. Also called a PPTC (Polymeric Positive Temperature Coefficient) device, the resettable fuse is a non-linear PTC thermistor based on a thin composite of semi crystalline polymer and conductive particles. Under normal operating conditions, the conductive particles provide a low resistance path allowing current to flow. Under fault conditions that cause excessive temperature, such as excessive current flow or an excessively high ambient temperature, the crystallites in the polymer undergo an abrupt phase change within a very narrow temperature range melting and becoming amorphous causing separation of the particles resulting in a large, non-linear increase in resistance.
The sharp increase in resistance is typically three orders of magnitude or more, reducing the current to a relatively low and safe level. It will hold in this high resistance state until the fault conditions are removed. On cooling the phase change is reversed and the PPTC resets to low resistance state (within certain post trip limits). Devices have a de-rating at elevated temperatures which means that they will trip at a lower current if the temperature is higher. Environmental and electrical details of application must be full understood when designing in resettable fuse protection. These devices are easily integrated into battery design by welding across cell terminals or placing on circuit board.
Fuses: conventional fuses may be used to protect the battery from an overload, but in many situations they may not act quickly enough. This is particularly true if the battery is short circuited. Since the battery has a very low internal impedance, very high instantaneous currents can flow which can seriously damage the battery. Fuses however are very slow to operate in fault conditions and may not act quickly enough if the battery is short circuited.
Fast acting over current and overvoltage protection which can isolate the battery are usually provided by electronic means.
Electronic Protection: over-current protection is normally provided by a current sensing device which detects when the upper current limit of the battery has been reached and interrupts the circuit. Since current is difficult to measure the usual method of current sensing is by measuring the voltage across a low ohmic value, high precision, series, sense resistor in the current path. When the specified current limit has been reached the sensing circuit will trigger a switch which will break the current path. The switch may be a semiconductor device or even a relay. Relays are inexpensive, they can switch very high currents and provide very good isolation in case of a fault but they are very slow to operate. FET switches are normally used to provide fast acting protection but they are limited in their current carrying capability and very costly for high power applications.Once the fault conditions have been removed, the circuit would normally be reconnected automatically, however there are particular circumstances when the circuit would be latched open. This could be to protect an unsuspecting service engineer investigating why a high voltage battery had tripped out.

Cell Voltage
 The above diagram shows a scheme for over and under-voltage, as well as temperature protection. In this case it also shows interaction with the charger. Batteries can be damaged both by over-voltage which can occur during charging and by under-voltage due to excessive discharging. This scheme allows voltage limits to be set for both charging and discharging. Batteries can be particularly vulnerable to overcharging. (See the section on Charging ). By providing the charger with inputs from voltage and temperature sensors in the battery, the charger can be cut off when the battery reaches predetermined control limits. The diagram above only shows a single voltage cut off from the charger, however multiple protection circuits can be implemented to provide a comprehensive protection scheme involving the charger as well as the protection built into the battery. It should be noted that each protection device added into the main current path will increase the effective internal impedance of the battery, as much as doubling it in the case of single cell batteries. This adversely affects the battery's capability of delivering peak power.
Intelligent Batteries
When the charging system involves communications between the battery and the charger it is called an Intelligent Charging System. An example of an Intelligent Battery is provided in the section on Battery Management Systems. An industry standard for specifying the communications link has been defined. This is the SMBus and this is supported by chip sets which have been developed to facilitate this protocol. Although the SMBus is convenient, many manufacturers still prefer to use proprietary solutions.
Monitoring: as well as sending signals to the charger the intelligent battery can turn on warning lights or send signals about the battery condition to the user. Monitoring is an essential component of Battery Management Systems.
Venting: With many cell chemistries the electrochemical process can give rise to the generation of gases, particularly during conditions of over charge. This is called gassing. If the gases are allowed to escape the active mass of chemicals in the cell will be diminished, permanently reducing its capacity and its cycle life. Furthermore the release of chemicals into the atmosphere could be dangerous. Manufacturers have therefore developed sealed cells to prevent this happening. Sealing the cells however gives rise to a different problem. If gassing does occur, pressure within the cell will build up, this will usually be accompanied by a rise in temperature which will make matters worse, until the cell ruptures or explodes. To overcome this second problem sealed cells will normally incorporate some form of vent to release the pressure in a controlled way if it becomes excessive. This is the last line of defence for an abused cell if all the other protection measures fail. Cells are not meant to vent under normal operating conditions.
Circuit Interrupt Device (CID): For smaller cells an alternative method of dealing with excess pressure is available. This is a small mechanical switch which interrupts the current path through the cell if the internal pressure exceeds a predetermined level. This method is not siutable for high power cells because of the difficulty of incorporating switches which can break the high currents typically causing over-pressure in the cell. Unfortunately there is no easy way of monitoring the internal pressure of standard cells to facilitate the implementation of simple pressure control mechanisms particularly for high current applications and the product designer is dependent on the efficacy of the safety vent and the use of systems based on temperature monitoring to provide protection from excessive pressure build up within the cells. There is the possibility of explosion if a sealed cell is encased in such a way that it cannot vent. The vents are often tiny and usually go unnoticed. Standard battery holders won't block the vents, but encapsulating the battery in epoxy resin to make a solid power module certainly will.
Multi-cell applications
In multi-cell applications each cell should have its own over-voltage detection device. Several temperature sensors will also be required since the pack may not have a uniform temperature across all the cells. Series connected cell chains would normally require only a single current monitoring and protection device unless provision is made for charging or bypassing individual cells. In such cases each cell will also require its own current monitor. Such complication is unfortunately necessary in high voltage packs containing long series cell strings. This is because individual cells may become overstressed and cause the premature failure of the whole battery. Why this arises, and how to avoid it, is discussed in the section on Cell Balancing.
System Isolation
While the battery can detect and initiate protective actions for events within the battery system, there are some applications which require the battery to respond to external events. This could be an out of tolerance condition such as a high temperature in some other part of the application which requires the power to be shut off. In the case of an automobile accident for instance, an inertia switch should isolate the battery. In these situations the battery needs to incorporate a switch in the main current path which can be triggered by an external signal. This does not necessarily need to be a separate switch since it could be possible to design the battery's over current protection circuit to accept a trigger from an external source.
Capacitive and Inductive Loads
Capacitive and inductive loads may be subject to large current surges as the load charges up. These surges can be sufficient to trip the current protection circuits but may not be of long enough duration to damage the battery. If the application does not allow the current surge to be designed out, then the protection circuit should incorporate a timer or some other device to delay or disable the current cut-off during expected short duration current pulses.
Current Drain
The object of protection is to maximise the life of the battery. Electronic protection circuits themselves draw current from the battery, reducing the effective capacity of the battery to supply the desired load. Low quiescent current is therefore an essential requirement for protection circuits.
Procedures and Discipline
No amount of electronics will protect a cell from bad management practices.
We know that elevated temperatures are bad for batteries. We should therefore ensure that cells are stored in a cool environment.
We know that shorting the terminals can be dangerous. We should ensure that handling and packing methods prevent this from happening.
We know batteries have a finite life. We should make sure the stores works on a FIFO basis.
Cell manufacturers set operating limits and conditions for their cells. We should ensure that these recommendations are respected during all stages of the procurement, manufacturing and shipping processes.
Protection During Manufacturing
Safe handling procedures for batteries in general are given in the section on User Safety Instructions.In addition, any electronic circuitry included within the battery pack may be susceptible to damage from electrostatic discharges (ESD) caused by mishandling during the production process. Static electricity may build up on the human body due to contact or friction with insulators and other synthetic materials such as plastics and styrofoam cups, plastic bags and clothing. Its effect is particularly strong in a dry atmosphere. If the charged person then touches an object at a lower potential or ground/earth potential such as circuit boards or components, the charge will be dissipated through that path. This charge is sufficient to damage transistors and integrated circuits. Even if the static sensitive devices are not handled directly they can be damaged by touching the pins or connectors on the printed circuit board. Standard precautions to avoid electrostatic damage include, the prohibition of casual handling of items on the production line (by visitors or managers), the wearing of grounding straps by anyone touching components or printed circuit boards, conductive flooring, conductive packaging, the labeling of static sensitive components and the avoidance of static prone materials near the production line.

Protection Circuits

Protection Circuits

Batteries can release high power, and most packs include protection to safeguard against malfunction. The most basic safety device in a battery is a fuse that opens on high current. Some devices open permanently and render the battery useless; others are more forgiving and reset. The Polyswitch™ is such a re-settable device. It creates a high resistance on excess current and reverts back to the low ON position when the condition normalizes. A third method is a solid-state switch that measures the current and disconnects on excessive load conditions. All switching devices have a residual resistance during normal operation, which causes a slight increase in overall battery resistance and a subsequent voltage drop.

Intrinsically Safe Batteries

Intrinsically safe (IS) batteries contain protection circuits that prevent the formation of high currents, which could lead to excess heat, sparks and explosion. Authorities mandate IS batteries for two-way radios, gas detectors and other electronic instruments operating in hazardous areas such as oil refineries, chemical plants and grain elevators. There are several levels of intrinsic safety, each serving a specific hazard level, and the requirements vary from country to country. The provisions are in addition to the protection circuit for lithium-ion, and the approval standards are rigorous. This results in a high price for the battery.

Making Lithium-ion Safe

Battery packs for laptops and other portable devices contain many levels of protection to assure safety under (almost) all circumstances when in the hands of the public. The safety begins with the battery cell, which includes: [1] a built-in temperature switch called PTC that protects against high current surges, [2] a circuit interrupt device (CID) that opens the electrical path if an over-charge raises the internal cell pressure to 1000 kPa (145psi), and [3]a safety vent that releases gas in the event of a rapid increase in cell pressure.

In addition to these internal safeguards, an external electronic protection circuit prevents the charge voltage of any cell from exceeding 4.30V. Furthermore, a fuse cuts the current if the skin temperature of any cell approaches 90°C (194°F). To prevent the battery from over-discharging, a control circuit cuts off the current path at about 2.20V/cell.

Each cell in a string needs independent voltage monitoring. The higher the cell count, the more complex the protection circuit becomes. Four cells in series had been the practical limit for consumer applications. Today, new chips accommodate 5–7, 7–10 and 13 cells in series. For specialty applications, such as the hybrid or electric vehicle delivering several hundred volts, specialty protection circuits are made, which sharply increases the overall cost of the battery. Monitoring two or more cells in parallel to get higher current is less critical than controlling voltages in a string configuration.

Protection circuits can only shield abuse from the outside, such as an electrical short or faulty charger. If, however, a defect occurs within the cell, such as contamination caused by microscopic metal particles, the external protection circuit has little effect and cannot arrest the reaction. Reinforced and self-healing separators are being developed for cells used in electric powertrains, but this makes the batteries large and expensive. While a Li-ion for a laptop provides a capacity of 170–200Wh/kg, the EV Li-ion has only 100–110Wh/kg.

The gas released by venting of a Li-ion cell as part of pressure buildup is mainly carbon dioxide (CO2). Other gases that form through abusive heating are vaporized electrolyte consisting of ethylene and/or propylene. Burning gases include combustion products of the organic solvents.

Li-ion commonly discharges to 3.0V/cell. This is the threshold at which most portable equipment stops working. The lowest “low-voltage” power cut-off is 2.5V/cell, and during prolonged storage, the self-discharge causes the voltage to drop further. This causes the protection circuit to turn off and the battery goes to sleep as if dead. Most chargers ignore Li-ion packs that have gone to sleep and a charge is no longer possible.

While in the ON position, the internal protection circuit has a resistance of 50 to 100mOhm. The circuit typically consists of two switches connected in series; one is responsible for the high cut-off, and the other for the low cut-off. The protection circuit of some smaller cellular batteries can be relaxed, and some get away with only the cell’s intrinsic protection and/or an external fuse. The absence of a full protection circuit saves money, but a new problem arises. Here is what can happen.

Some low-cost chargers rely solely on the battery’s protection circuit to terminate charge current. Without a functioning voltage termination switch in the battery, the cell voltage can rise too high and overcharge the battery. Heat buildup and bulging are early indications of pending failures before potential disintegration occurs. Figure 1 shows a battery that has fragmented while charging in a car.


Figure 1: 
Exploded cellular phone

Generic cell phone disintegrated while charging in the back of a car.Combination of unsafe battery and charger can have a lethal effect. Manufacturers advise only to use approved batteries and chargers.

By owner’s permission


A concern also arises if static electricity or a faulty charger has destroyed the battery’s protection circuit. This can fuse the solid-state switches into a permanent ON position without the user’s knowledge. A battery with a faulty protection circuit may function normally but fail to provide the required safety.

Low price makes generic replacement batteries from Asia popular with cell phone users. While the quality and performance of these batteries is improving, some do not provide the same high safety as the original branded version. A wise shopper spends a little more and replaces the battery with an approved model.

I receive many questions on www.BatteryUniversity.com from visitors wanting to know why the aftermarket does not provide low-cost laptop batteries as readily as cellular batteries. This is mainly due to safety. While a 1,400mAh cellular battery stores only 4Wh of energy, a laptop battery holds about 60Wh, 15 times more. Many manufacturers of consumer batteries protect the batteries with a security inscription that very few can break. Although aftermarket batteries are available, many do not offer all the functions of the branded version. Typical problems are fuel-gauge errors and not being able to charge correctly.

Manufacturers of lithium-ion batteries do not mention the word “explosion” and refer to “venting with flame” or “rapid disassembly.” Although seen as a slower and more controlled process than explosion, venting with flame, or rapid disassembly, can nevertheless be violent and inflict injury to those in close proximity. The court hears many legal cases involving laptops and other batteries that are said to have caused property damage, fire and personal injury. This is also a large concern in the aviation industry. Most of the batteries for consumer products are shipped by air just in time for improved inventory control.

Battery Pack Design

Battery Pack Design
The purpose of a battery pack is to provide a convenient integrated power source for portable applications. The advantages of using custom designs are outlined in the section on Benefits of Custom Packs. The pack may fulfil several functions:-
It enables higher voltage or higher capacity batteries to be built up from low voltage, low capacity cells.
It houses a cell or a bank of cells together with the associated interconnections in a single convenient pack.
It accommodates any necessary monitoring and electronic protection devices or circuits within the pack.
It can accommodate additional circuitry such as indicator lights, heaters, cooling ducts and solar panels.
It matches and meshes with the cavity in the product which the battery is intended to power providing both electrical and mechanical interfaces.
It can provide unique electrical and mechanical interfaces to ensure compatibility both of the battery with the intended product and the charger with the battery.
Two examples of battery packs from Axeon Power are shown below.
The pack on the left is a 12 Volt 30 Ah Lithium Ion battery used for seismic instrumentation. It uses 32 pouch cells in a 4 series, 8 parallel configuration. The pack incorporates heaters which enable it to work down to -30°C and a solar panel which keeps the battery charged.
The pack on the right is a 3.6 Volt 800 mAh battery employing three Nickel Metal Hydride cells used in mobile phones. The gold plated connector is moulded into the plastic frame.

See also Cell Construction
Capacity and Voltage
With a simple series chain of cells, the battery capacity in AmpHours is the same as the capacity of the individual cells since the current flows equally through all the cells in the chain.
High battery voltages are achieved by adding more cells in a series chain. The voltage of the battery is the voltage of a single cell multiplied by the number of cells in the chain. This does not increase the AmpHour capacity of the battery , but it increases the WattHour capacity, or the total stored energy, in proportion to the number of cells in the chain.
Battery capacity can increased through adding more parallel cells. This increases the AmpHour capacity as well as the WattHour capacity without increasing the battery voltage. For batteries with parallel chains the capacity of the battery is the capacity of the individual chain multiplied by the number of parallel chains.
Whereas cell voltage is fixed by the cell chemistry, cell capacity depends on the surface area of the electrodes and the volume of the electrolyte, - that is, the physical size of the cell. If at all possible the number of cells in a pack should be minimised to simplify the design and to minimise potential reliability problems. Fewer cells require fewer support electronics. Thus parallel chains should be avoided by specifying the highest capacity cells available. Design issues for multi-cell batteries are considered further in the section on Cell Balancing.
NOTE Cells with different capacities or cell chemistries should not be mixed in a single battery pack.
Pack Design Options
The design of the outer package or housing of the battery depends to a great extent on the components it has to accommodate and the physical protection it has to provide for them. These components are not just the cells, but also protection devices, electronic circuits, interconnections and connectors which must all be specified before the final battery case can be designed. For high power, high energy batteries robust packaging is required for safety reasons.
Cell Configuration
The ultimate shape and dimensions of the battery pack are mostly governed by the cavity which is planned to house it within the intended application. This in turn dictates the possible cell sizes and layouts which can be used. Prismatic cells provide the best space utilization, however cylindrical cells provide simpler cooling options for high power batteries. The use of pouch cells provides the product designer more freedom in specifying the shape of the battery cavity permitting very compact designs.
The orientation of the cells is designed to minimise the interconnections between t he cells.
Battery Electronics
Besides the cells many battery packs now incorporate associated electronic circuits. These may be protection devices and circuits, monitoring circuits, charge controllers, fuel gauges, and indicator lights. Electronics for high power multi-cell packs also include cell balancing and communications functions.
The packs may also be designed to deliver more than one voltage from the basic cell combination, although applications requiring multiple voltage sources are more likely to make provision for this within the application. See Multiple Voltages
In addition to the basic battery support electronics the battery pack may include other functions such as heaters to extend the lower working temperature or solar cells to keep the battery fully charged. These circuits in turn have their own control circuits.
Space, fixing points and methods and interconnections need to be allocated for all these electronic circuits.
Sofware is a major component of Lithium batteries, particularly for automotive applications. See the section on Battery Management Systems (BMS) . Control systems are required to keep the cells within their specified operating range and to protect them from abuse. Fuel gauging needs complex algorithms to estimate the state of charge (SOC). Communications with other vehicle systems are needed for monitoring the battery status and controlling energy flows.
Internal Interconnections
Low power cells are usually connected together using nickel strips which are welded to the cell terminals or the case. Soldering is not recommended since the soldering process is apt to apply large, uncontrolled amounts of heat to the battery components which may damage the separators or the vents which are normally made of plastic. Modern computer controlled resistance welders allow much more precise control of the welding process, both limiting the amount of heat applied to the battery and localising the heat to a small desired area. Welding also provides a stronger, low resistance joint. The interconnecting strips often have complex shapes and profiles which may be stamped out of flat strip in a progressive die.
High power cells may use solid copper bus bars or braided straps.
The electronic components are usually mounted on a conventional printed circuit board (PCB).
Flexible PCBs may cost more than rigid PCBs but they can be used to reduce the overall product costs. Not only do they save weight and space but they also provide more packaging options and they simplify physical interconnections and assembly operations as well as eliminating the need for connectors. Connectors may in fact be specified to facilitate assembly and disassembly if the design requires that individual battery components need to be changed or serviced but there is usually a cost and reliability penalty associated with such designs.
External Connections
The type of terminals or connections to the external circuits depend on, the current to be carried, the frequency with which the battery may be connected and disconnected and the design of the design of the circuit to which the battery will be connected.
For low power circuits, gold plated contacts are the terminals of choice for connectors which are subject to frequent insertions. Gold is hard wearing, it has low contact resistance and doesn't oxidise. Flying leads with spade terminals or snap on studs are also used for low power applications. Metal tabs are also used on pouch cells.
Terminals for high power applications are usually threaded metal studs to ensure a reliable connection. Safety requirements on high voltage batteries may also dictate shrouded terminals to prevent accidental exposure of the operator to dangerous voltages or of the battery to short circuits. Keyed or terminals or connections are also advisable to prevent connection to incorrect chargers or loads.
Thermal Design
Thermal management is a major issue in high power designs, particularly for automotive applications. See details in the Thermal Management section. As part of the battery system, it may be necessary to provide air or water cooling ducts, pumps or fans and heat exchangers for high temperature working or heaters for operating in low temperature environments. The layout of the cells should be conducive to managing heat flows within the pack.
Battery Packaging
The battery casing has to provide the mechanical and electrical interfaces to the product it is designed to power as well as to contain all the components outlined above.
The simplest and least expensive packaging for small batteries is shrink wrap or vacuum formed plastic. These solutions are only possible if the battery is intended to be completely enclosed by the finished product.
Injection moulded plastics are used to provide more precision packs. For enclosed packs designs using a minimum of materials are based around which a plastic frame holds the components in place thus minimising the cost, the weight and the size of the pack. The overall product cost can be further reduced by using insert mouldings in which the interconnection strips and the terminals are moulded into the plastic parts to eliminate both materials and assembly costs. Overmoulding may also be used to encapsulate and protect small components or sub-assemblies.

Case for 3 AAA Cells

Case for a Single Prismatic Cell
Insert Mouldings Showing Cell Interconnecting Strips
In some designs the battery pack forms part of the outer case of the end product. The colours and textures of the plastic must match the plastics of the rest of the product even though they may come from a completely different supplier. These designs are usually required to incorporate a mechanical latch to hold the battery in place. Again this latch as well as the terminals must interface with plastic parts from a different supplier so high precision and tight tolerances are essential. ABS polymers are the materials typically used for this purpose.
Batteries for traction applications are usually very large and heavy and subject to large physical forces as well as vibrations so substantial fixings are required to hold the cells in place. This is particularly necessary for batteries made up from pouch cells which are vulnerable to physical damage. Automotive battery packs must also withstand abuse and possible accidental damage so metal casings will normally be specified. The metal pack casing also serves to confine any incendiary event resulting from the failure of a cell or cells within the battery and to provide a measure of protection for the user. At the same time the case must also protect the cells and the electronics from the harsh operating environments of temperature extremes, water ingress, humidity and vibration in which these batteries work.
Usually the complete pack is replaced when the battery has reached the end of its useful life. In certain circumstances however, for instance when the pack incorporates a lot of electronic circuits, it may be desirable to design the pack such that the cells within the pack can be replaced.

14.4V 12Ah Lithium battery pack in an off-the-shelf case.
If the design requires provision for replacement of the cells the casing of the battery pack must be designed to clip or screw together. Normally the parts of the plastic housing will be ultrasonically welded together both for security and for low cost as well as to prevent unauthorised tampering with the cells and the electronics.
Other Considerations
Thermal effects need to be taken into account and, tolerances must allow for potential swelling of the cells. Some Lithium pouch cells may swell as much as 10% or more over the lifetime of the cell. For this reason potting is not recommended. In low power designs groups of pouch cells may be shrink wrapped but for higher power applications plastic or metal frames may be used both to provide physical protection of the cells as well as to allow for swelling.
The battery pack should not normally be airtight or sealed since many batteries release hydrogen or oxygen during operation which could cause bursting of the pack or an explosion if the gases are allowed to accumulate. Lithium cells do not emit gases under normal circumstances, but in the case of failure and thermal breakdown, inflammable gases may be vented by the clells. Some form of ventilation or purging should be provided to avoid these problems.
Tolerances should also allow for the use of alternative cells from other manufacturers. While the cells may be "standard" sizes, there could still be differences between cells from different vendors.
High power batteries may need special ventilation or channels between the cells to permit forced air or liquid cooling.
The pack design must incorporate some form of identification to indicate the manufacturer's name, the cell chemistry, the voltage and the capacity as well as the country of manufacture. Most manufacturers will also include a date stamp and or serial number to assist traceability in case of problems. This information is usually provided on a label but it may also be printed directly on to the battery casing.
Pack Costs
The costs involved in designing custom battery packs are often underestimated.
As an indication of the order of magnitude, some very rough cost estimates are given below. They assume that the manufacturer possesses all the necessary standard production resources and they include the pack maker's profit margin and warranty provision. Costs could be lower if the packs are designed and made in house, but then some investment in capital equipment may be required and possibly some recruitment and training costs.
Design Engineering Costs
Low power batteries are usually designed for very high volume production and costs may be calculated to one thousandth of a cent. Most battery packs include some form of battery management electronics, even the smallest designs used in applications such as mobile phones incorporate integrated circuits mounted on a printed circuit board . The mechanical engineering effort however is the activity most often underestimated. It involves the design of precision thin section plastic parts and their associated complex moulding tools as well as metal stampings requiring precision stamping dies. Component sourcing as well as cell selection and qualification also add to the costs.
For low power packs these engineering costs could amount to $20,000 to $50,000.
High power batteries bring an additional set of challenges. Systems integration is much more complex due to the wider range of system functions and demands to be accommodated. For automotive applications the accuracy of the SOC estimation must be much higher and this may also require a major cell characterisation programme. The components are much larger and more expensive and the enormous energy content of the cells demands special safety considerations to prevent physical and electrical abuse and accidental damage. This requires robust steel frames and enclosures and fail safe electronics. Thermal management is also very important and designs may include both heating and forced cooling circuits. Expensive cable forms are needed to connect the cell voltage and temperature sensing signals to the BMS processing unit. All of these requirements add to the complexity, costs and timescales of the associated systems software.
Enginering costs for EV and HEV applications could be upwards of $200,000 and probably much more.
Tooling Costs
High volume products may require multi-cavity moulding tools and progessive stamping dies. In addition automated transfer mechanisms and assembly jigs and fixtures will be required for the manufacturing operations.All this could cost a minimum of $1,500. This assumes the manufacturing plant is already equipped with standard engineering, production and test facilities such as CAD and CAM, PCB assembly machines, conveyer belts, welders, presses, power supplies and electrical test equipment.
For manufacturing high power batteries, material handling and operator safety become major factors because of the heavy weight of the packs and the high voltages involved. Tooling costs may be double those needed for low power packs starting at $200,000 or more.
Prototypes could cost double the cost of volume production. Low volume purchases are more expensive and a considerable amount of manual work is involved. This is only significant for high power batteries.
Production Costs
The manufacturing costs for low power batteries used in mobile phones could be as low as $2.50 whereas a high capacity EV battery could cost upwards of $10,000. In both cases the major cost is the cells. In small batteries this may be 80% to 85% of the total costs. Large batteries use more electronics and higher power components. They are also more labour intensive. For large batteries the cost of the cells could be between 60% and 80% of the total costs depending on the battery specification. Since most cells are sourced from Asia, shipping costs also contribute significantly to the costs.
Two conclusions can be made from this .
Large production volumes are required to justify the development of custom battery packs.
For safety reasons, batteries for electric vehicles involve very high unavoidable engineering development costs, even for a single vehicle.

Material Safety Data Sheet-MSDS

A material safety data sheet (MSDS), safety data sheet (SDS), or product safety data sheet (PSDS) is an important component of product stewardshipand occupational safety and health. It is intended to provide workers and emergency personnel with procedures for handling or working with that substance in a safe manner, and includes information such as physical data (melting point, boiling point, flash point, etc.), toxicity, health effects, first aid, reactivity, storage, disposal, protective equipment, and spill-handling procedures. MSDS formats can vary from source to source within a country depending on national requirements. SDSs are a widely done system for cataloging information on chemicals, chemical compounds, and chemical mixtures. SDS information may include instructions for the safe use and potential hazards associated with a particular material or product. These data sheets can be found anywhere where chemicals are being used.

There is also a duty to properly label substances on the basis of physico-chemical, health and/or environmental risk. Labels can include hazard symbols such as the European Union standard black diagonal cross on an orange background, used to denote a harmful substance.
An SDS for a substance is not primarily intended for use by the general consumer, focusing instead on the hazards of working with the material in an occupational setting. In some jurisdictions, the SDS is required to state the chemical's risks, safety, and effect on the environment. It is important to use an SDS specific to both country and supplier, as the same product (e.g. paints sold under identical brand names by the same company) can have different formulations in different countries. The formulation and hazard of a product using a generic name (e.g. sugar soap) may vary between manufacturers in the same country.

Battery Manufacturing

Electrode Coating

The anodes and cathodes in Lithium cells are of similar form and are made by similar processes on similar or identical equipments. The active electrode materials are coated on both sides of metallic foils which act as the current collectors conducting the current in and out of the cell. The anode material is a form of Carbon and the cathode is a Lithium metal oxide. Both of these materials are delivered to the factory in the form of black powder and to the untrained eye they are almost indistinguishable from eachother. Since contamination between the anode and cathode materials will ruin the battery, great care must be taken to prevent these materials from coming into contact with eachother. For this reason the anodes and cathodes are usually processed in different rooms.

Particle size must be kept to a minimum in order to achieve the maximum effective surface area of the electrodes needed for high current cells. Particle shape is also important. Smooth spherical shapes with rounded edges are desirable since sharp edges or flaky surfaces are susceptible to higher electrical stress and decomposition of the anode passivating SEI layer, which can lead to very large heat generation and possible thermal runaway when the cells are in use.

The metal electrode foils are delivered on large reels, typically about 500 mm wide, with copper for the anode and aluminium for the cathode, and these reels are mounted directly on the coating machines where the foil is unreeled as it is fed into the machine through precision rollers.

The first stage is to mix the electrode materials with a conductive binder to form a slurry which is spread on the surface of the foil as it passes into the machine. A knife edge is located just above the foil and the thickness of the electrode coating is controlled by adjusting the gap between the knife edge and the foil. Since it is not unusual for the gravimetric or volumetric energy storage capacity of the anode material to be different from that of the cathode material, the thickess of the coating layers must be set to allow the energy storage per unit area of the anode and cathode electrodes to be matched.

From the coater, the coated foil is fed directly into a long drying oven to bake the electrode material onto the foil. As the coated foil exits the oven it is re-reeled. The coated foils are subsequently fed into slitting machines to cut the foil into narrower strips suitable for different sizes of electrodes. Later they are cut to length. Any burrs on the edges of the foil strips could give rise to internal short circuits in the cells so the slitting machine must be very precisely manufactured and maintained.

Cell Assembly

In the best factories cell assembly is usually carried out on highly automated equipment, however there are still many smaller manufacturers who use manual assembly methods.

The first stage in the assembly process is to build the electrode sub-assembly in which the separator is sandwiched between the anode and the cathode. Two basic electrode structures are used depending on the type of cell casing to be used, a stacked structure for use in prismatic cells and a spiral wound structure for use in cylindrical cells.

Prismatic Cells

Prismatic cells are often used for high capacity battery applications to optimise the use of space. These designs use a stacked electrode structure in which the anode and cathode foils are cut into individual electrode plates which are stacked alternately and kept apart by the separator. The separator may be cut to the same size as the electrodes but more likely it is applied in a long strip wound in a zig zag fashion between alternate electrodes in the stack. While this case design makes optimum use of space when used in a battery pack, it has the disadvantage that it uses multiple electrode plates which need a clamping mechanism to connect all the anodes together and to the main terminal post and a similar mechanism for the cathodes. This all adds to the complexity and labour content of the cell and consequently to the costs.Some prismatic cells are also made by the simpler method of winding the electrodes on a flat mandrel. Stacked electrodes are also used for the production of pouch cells.

Cylindrical Cells

For cylindrical cells the anode and cathode foils are cut into two long strips which are wound on a cylindrical mandrel, together with the separator which keeps them apart, to form a jelly roll (Swiss roll in the UK). Cylindrical cells thus have only two electrode strips which simplifies the construction considerably. A single tab connects each electrode to its corresponding terminal, although high power cells may have multiple tabs welded along the edges of the electrode strip to carry the higher currents.

The next stage is to connect the electrode structure to the terminals together with any safety devices and to insert this sub-assembly into the can. The can is then sealed in a laser welding or heating process, depending on the case material, leaving an opening for injecting the electrolyte into the can. The following stage is to fill the cell with the electrolyte and seal it. This must be carried out in a "dry room" since the electrolyte reacts with water. Moisture will cause the electrolyte to decompose with the emission of toxic gases. Lithium Hexafluoride (LiPF6) for instance, one of the most commonly used electrolyte materials, reacts with water forming toxic hydrofluoric acid (HF).

Afterwards the cell is given an identification with a label or by printing a batch or serial number on the case.


Once the cell assembly is complete the cell must be put through at least one precisely controlled charge / discharge cycle to activate the working materials, transforming them into their useable form. Instead of the normal constant current - constant voltage charging curve, the charging process begins with a low voltage which builds up gradually. This is called the Formation Process. For most Lithium chemistries this involves creating the SEI (solid electrolyte interface) on the anode. This is a passivating layer which is essential for moderating the charging process under normal use.

During formation, data on the cell performance such as capacity and impedance, are gathered and recorded for quality analysis and traceability. The spread of the performance measurements also gives an indication of whether the process is under control. (Beware of manufacturers who use this process for sorting their cells into different performance groups for sale with alternative specifications).

Although not the prime purpose of formation, the process allows a significant percentage of early life cell failures due to manufacturing defects, the so called "infant mortalities", to occur in the manufacturer's plant rather than at the customers' premises.

Process Control

Tight tolerances and strict process controls are essential throughout the manufacturing process. Contamination, physical damage and burrs on the electrodes are particularly dangerous since they can cause penetration of the separator giving rise to internal short circuits in the cell and there are no protection methods which can prevent or control this.

Support Services

Cleanliness is essential to prevent contamination and cells are normally manufactured in clean room conditions with controlled access to the assembly facilities often via air showers.

Apart from the production test equipment, a battery manufacturer should be expected to have a materials laboratory equipped to carry out a full analysis of the materials used in the production of the cells as well as to carry out failure analysis. The following list shows some of the major equipment used.

-Scanning electron microscope (SEM) for investigating the physical structure of the materials

-Mass spectrometer for analysing the chemical content of the materials

-Calorimeters for checking the thermal properties of the materials and the cells

-Programmable charge/discharge cycle test equipment to exercise the cells and verify their lifetime

-Environmental chambers and vibration tables for investigating the performance of the cells under their expected operating conditions

-Mechanical stress tesing equipment

Lithium Based Batteries

Other Lithium Cathode Chemistry Variants

Numerous variants of the basic Lithium-ion cell chemistry have been developed. Lithium Cobalt and Lithium Manganese were the first to be produced in commercial quantities but Lithium Iron Phoshate is taking over for high power applications because of its improved safety performance. The rest are either at various stages of development or they are awaiting investment decisions to launch volume production.


Doping with transition metals changes the nature of the active materials and enables the internal impedance of the cell to be reduced.

The operating performance of the cell can also be be "tuned" by changing the identity of the transition metal. This allows the voltage as well as the specific capacity of these active materials to be regulated. Cell voltages in the range 2.1 to 5 Volts are possible.


While the basic technology is well known, there is a lack of operating experience and hence system design data with some of the newer developments which also hampers their adoption. At the same time patents for these different chemistries tend to be held by rival companies undertaking competitive developments with no signs of industry standardisation or adoption of a common product. (The original patent on Lithium Cobalt technology has now expired which is perhaps one explanation for its popularity).


Lithium Cobalt LiCoO2

Lithium Cobalt is a mature, proven, industry-standard battery technology that provides long cycle life and very high energy density. The polymer design makes the cells inherently safer than "canned" construction cells that can leak acidic electrolyte fluid under abusive conditions. The cell voltage is typically 3.7 Volts. Cells using this chemistry are available from a wide range of manufacturers.

The use of Cobalt is unfortunately associated with environmental and toxic hazards.


Lithium Manganese LiMn2O4

Lithium Manganese provides a higher cell voltage than Cobalt based chemistries at 3.8 to 4 Volts but the energy density is about 20% less. It also provides additional benefits to Lithium-ion chemistry, including lower cost and higher temperature performance. This chemistry is more stable than Lithium Cobalt technology and thus inherently safer but the trade off is lower potential energy densities. Lithium Manganese cells are also widely available but they are not yet as common as Lithium Cobalt cells.

Manganese, unlike Cobalt, is a safe and more environmentally benign cathode material.

Manganese is also much cheaper than Cobalt, and is more abundant.


Lithium Nickel LiNiO2

Lithium Nickel based cells provide up to 30% higher energy density than Cobalt but the cell voltage is lower at 3.6 Volts. They also have the highest exothermic reaction which could give rise to cooling problems in high power applications. Cells using this chemistry are therefore not generally available.


Lithium (NCM) Nickel Cobalt Manganese - Li(NiCoMn)O2

Tri-element cells which combine slighlty improved safety (better than Cobalt oxide) with lower cost without compromising the energy density but with slightly lower voltage. Different manufacturers may use different proportions of the three constituent elements, in this case Ni, Co and Mn.


Lithium (NCA) Nickel Cobalt Aluminium - Li(NiCoAl)O2

As above, another tri-element chemistry which combines slighlty improved safety (better than Cobalt oxide) with lower cost without compromising the energy density but with slightly lower voltage.


Lithium Iron Phosphate LiFePO4

Phosphate based technology possesses superior thermal and chemical stability which provides better safety characteristics than those of Lithium-ion technology made with other cathode materials. Lithium phosphate cells are incombustible in the event of mishandling during charge or discharge, they are more stable under overcharge or short circuit conditions and they can withstand high temperatures without decomposing. When abuse does occur, the phosphate based cathode material will not burn and is not prone to thermal runaway. Phosphate chemistry also offers a longer cycle life.

Recent developments have produced a range of new environmentally friendly cathode active materials based on Lithiated transition metal phosphates for Lithium-ion applications.


Phosphates significantly reduce the drawbacks of the Cobalt chemistry, particularly the cost, safety and environmental characteristics. Once more the trade off is a reduction of 14% in energy density, but higher energy variants are being explored.

Due to the superior safety characteristics of phosphates over current Lithium-ion Cobalt cells, batteries may be designed using larger cells and potentially with a reduced reliance upon additional safety devices.

The use of Lithium Iron Phosphate chemistry is the subject of patent disputes and some manufacturers are investigating other chemistry variants mainly to circumvent the patent on the LiFePO4 chemistry.


Lithium Metal Polymer

Developed specifically for automotive applications employing 3M polymer technology and independently in Europe with technology from the Fraunhofer Institute, they have been trialled successfully in PNGV project demonstrators in the USA. They use metallic Lithium anodes rather than the more common Lithium Carbon based anodes and metal oxide (Cobalt) cathodes.


Some versions need to work at temperatures between 80 and 120ºC for optimum results although it is possible to operate at reduced power at ambient temperature.

The Fraunhofer technology uses an organic electrolyte and the cell voltage is 4 Volts. It is claimed that their the cell chemistry is more tolerant to abuse.

These products are not yet in volume production.


Lithium Sulphur Li2S8

Lithium Sulphur is a high energy density chemistry, significantly higher than Lithium-ion metal oxide chemistries. This chemistry is under joint development by several companies but it is not yet commercially available. A major issue is finding suitable electrolytes which are not subject to the numerous unwanted side reactions which plague the current designs.

Lithium Sulphur cells are tolerant of over-voltages but current versions have limited cycle life. The cell voltage is 2.1 Volts

See also Dissolution of the Electrodes on the New Cell Designs and Chemistries page.


Alternative Anode Chemistry (LTO)

The anodes of most Lithium based secondary cells are based on some form of carbon (graphite or coke). Recently Lithium Titanate Spinel (Li4Ti5O12) has been introduced for use as an anode material providing high power thermally stable cells with improved cycle life.

This has the following advantages

Does not depend on SEI Layer for stability

No restriction on ion flow hence significantly higher charge and discharge rates possible as well as better low temperature performance.

Lower internal impedance of the cell

Higher temperatures can be tolerated.

No SEI build up over time means very long cycle life possible (10,000 deep cycles)

Public domain technology (No patent disputes)

Disadvantages are

Lower anode reactivity means cell voltage reduced to 2.25 Volts when used with Spinel cathode. (Other cathode chemistries possible)

25% to 30% Lower energy density hence bulkier cells


Lithium Air Cells

Originally conceived as primary cells (see Lithium Pimary Cells), Lithium air cells offer a very high energy density. Rechargeable versions are now under development which promise energy densities of 10 times more than the current generation of Lithium cells, approaching that of Gasoline/Petrol.

The anode is Lithium and the cathode is not air but in fact gaseous Oxygen from the air. Because the cell does not have a solid cathode in the conventional sense it eliminates the weight and volume of the cathode as well as its mechanical supporting structure.

This would enable very small batteries to be made with the same range as current technology, or alternatively, electric drive ranges of several hundred miles could be obtained from batteries the same physical size as those available today.


The Lithium is oxidised by the Oxygen during discharging and charging drives the Oxygen off again, a relatively simple chemistry. There are however problems in preventing the other constituents of the air from poisoning the Lithium electrode. There are also potential safety concerns with the metallic Lithium anodes. The cells demonstrate very high hysteresis with the charging voltage considerably higher than the discharge voltage This corresponds to a low Coulombic efficiency, currently only about 60% to 70%.

The cell voltage is 2.5 Volts.

.See also Energy Density on the New Cell Designs and Chemistries page.


See note on the Toxicity of Lithium

Characteristics of some common Lithium chemistries used in high power batteries


International Battery Standards

National and international standards organisations were set up to facilitate trade by encouraging greater product interoperability and compatibility as well as setting standards for acceptable product safety, quality and reliability.

Below are listed some of the most common standards applicable to battery applications and some of the organisations who issue them and or carry out quality assurance and conformance testing. InEurope, European standards are gradually being adopted in replacement of the previous national standards.


Copies of the relevant standards can be obtained directly from the issuing organisations or from public libraries.

Standards Setting and Safety Testing Organisations




Asociación Española de Normalización y Certificación (Spain)


American National Standards Institute sponsored by NEMA


Australian Standard


Association Suisse des Electriciens (Swiss)


American Society for Quality Control


American Society for Testing and Materials


Explosive Atmospheres (Safety directive)


Battery Council International (Publishes AutomotiveBatteryStandards)


British Standards


California Air Resources Board (Automotive Emission Standards)


Conformance with EU directives


European Committee for Normalisation (Standards Committee)


European Committee for Electrotechnical Standardisation


International Special Committee on Radio Interference


Committee on Data for Science and Technology (Committee of ICSU)


Canadian Standards Association


Defence Standards (UK)


Danmarks Electriske Materielkontrol (Denmark)


Deutsches Institut für Normung (German Institute for Standardisation)


Economic Commission forEuroperegulations.


Electronics Industry Association (USA)


European Norms (Standards)


Federal Communications Commission (USA)


Finnish Electrical Inspectorate


Foundation for Intelligent Physical Agents (Interoperability standards)


Guo Biao = National Standard (People's Republic ofChina)


Health & Safety Executive (UK)


International Council for Science


International Electrotechnical Commission


Institution of Electrical Engineers (UK)


Institute of Electrical and Electronics Engineers (USA)


Instituto ItalianodelMarchio de Qualitá


Ingress Protection


International Standards Organisation


International TelecommunicationsUnion


Japanese Industrial Standard


Keuring van Elektrotechnishe Materialen (Netherlands)


KoreanInstituteofStandardsand Technology


Military Standards (USA)


Motor Industry Software Reliability Association (UK)


Motor Vehicle Emission Group (EU Emission standards)


National Measurement Accreditation Service (UKCalibration)


National Electric Manufacturers Association (USA)


Norges Electriske Materiellkontroll (Norway)


Norme Française (France)


National Fire Protection Association (USA)


National Institute of Justice (USA)


US Department of Labor - Occupational Safety & Health Administration


Osterreichischer Verband für Elektrotechnik (Austria)


Automotive 42 VoltBatteryStandard


Rehabilitation Engineering & Assistive Technology Society ofNorth America


Society of Automotive Engineers (USA)


Svenska Elektriska Materielcontrollanstalten (Sweden)


Schweitzerischer Elektrotechnische Verein (Swiss)


NATO Standards Agreements


DTI Standards and Technical Regulations Directorate (UK)


Telecommunications Industry Association (USA)


Technical Report (Used by IEC)


TÜV Rheinland Group (TUV - Technical Inspection Asssociation)


UK Accreditation Service (Assessment of test services)/(Calibration)


Underwriters Laboratories Requirements (USA)


United States AdvancedBatteryConsortium


United States National Electrical Code


Union Technique de l'Electriciteé (France)


Verband Deutscher Elektrotechniker (Germany)




Standard Number


IEC 60050

International electrotechnical vocabulary. Chapter 486: Secondary cells and batteries.

IEC 60086-1, BS 387 

Primary Batteries - General

IEC 60086-2, BS

Batteries - General


Portable Primary Cells and Batteries with Aqueous Electrolyte - General and Specifications


Portable Rechargeable Cells and Batteries - General and Specifications


Portable Lithium Primary Cells and Batteries - General and Specifications

UL 2054

Safety of Commercial and HouseholdBatteryPacks - Testing

IEEE 1625

Standard for Rechargeable Batteries forMobileComputers

USNEC Article 480

Storage Batteries

ISO 9000

A series of quality management systems standards created by the ISO. They are not specific to products or services, but apply to the processes that create them.

ISO 9001: 2000

Model for quality assurance in design, development, production, installation and servicing.

ISO 14000

A series of environmental management systems standards created by the ISO.

ISO/IEC/EN 17025

General Requirements for the Competence of Calibration and Testing Laboratories


Standard Number


BS 2G 239:1992

Specification for primary active lithium batteries for use in aircraft

BS EN 60086-4:2000, IEC 60086-4:2000

Primary batteries. Safety standard for lithium batteries

BS EN 61960-1:2001, IEC 61960-1:2000

Secondary lithium cells and batteries for portable applications. Secondary lithium cells

BS EN 61960-2:2002, IEC 61960-2:2001

Secondary lithium cells and batteries for portable applications. Secondary lithium batteries

02/208497 DC

IEC 61960. Ed.1. Secondary cells and batteries containing alkaline or other non-acid electrolytes. Secondary lithium cells and batteries for portable applications

02/209100 DC

IEC 62281. Ed.1. Safety of primary and secondary lithium cells and batteries during transport

BS G 239:1987

Specification for primary active lithium batteries for use in aircraft

BS EN 60086-4:1996, IEC 60086-4:1996

Primary batteries. Safety standard for lithium batteries

UL 1642

Safety of Lithium-Ion Batteries - Testing

GB /T18287-2000

Chinese National Standard for Lithium Ion batteries for mobile phones



United Nations recommendations on the transport of dangerous goods



Battery Related Glossary

AC Inverter - An electrical circuit which generates a sine-wave output (regulated and without breaks) using the DC current supplied by the rectifier-charger or the battery. The primary elements of the inverter are the DC/AC converter, a regulation system and an output filter.
A/D Converter (ADC) Analogue/Digital Converter. A device which converts continuously varying analogue signals into a binary coded digital form.
Acid - A proton donor. A compound containing hydrogen which dissociates in aqueous solution producing positively charged hydrogen ions (H+). An acidic solution has a pH less than 7.0
Active material - The chemically reactive materials in an energy cell which react with each other converting from one chemical composition to another while generating electrical energy or accepting electric current from an external circuit.
Ageing - Permanent loss of capacity with frequent use or the passage of time due to unwanted irreversible chemical reactions in the cell.
AGM (Absorbtive Glass Mat) battery - A lead acid battery using a glass mat to promote recombination of the gases produced by the charging process.
Alkali - A compound which dissolves in water producing negatively charged hydroxide ions. Alkaline solutions are strongly basic and neutralise acids forming a salt and water.
Alkaline battery - A battery which uses an aqueous alkaline solution for its electrolyte.
Allotrope - Two or more forms of the same element in the same physical state (solid, liquid, or gas) that differ from each other in their physical, and sometimes chemical properties. The term allotropy applies to elements only, not compounds. The more general term, used for any crystalline material, is polymorphism. See also isotope.
Ambient temperature - The average temperature surrounding the battery, typically air.
Amorphous - Without definite shape or structure, without crystalline structure.
Ampere (Amp) - The unit of current flow equal to one coulomb per second.
Ampere hours (Ah) or Amphours - The unit of measure used for comparing the capacity or energy content of a batteries with the same output voltage. For most batteries it defines the battery's C rate. For automotive (Lead Acid) batteries the SAE defines the Amphour capacity as 20 times the current delivered for a period of 20 hours when the battery is discharged at 1 twentieth of the C rate until the cell voltage drops to 1.75 Volts.
Strictly - One Ampere hour is the charge transferred by one amp flowing for one hour. 1Ah = 3600 Coulombs.
The true capacity of any battery is its energy content and this is measured in WattHours (Wh). It is the battery's Amphour capacity multiplied by the battery voltage.
Ampoule battery - A battery in which the electrolyte is stored in a separate chamber from the cell electrodes until the battery is needed.
Analogue (Analog) circuit - An electronic circuit in which an electrical value (usually voltage or current, but sometimes frequency, phase) represents something in the physical world.The magnitude of the electrical value varies with with the intensity of an external physical quantity.
Also - An electrical circuit which provides a continuous quantitative output ( as opposed to a digital output which may be a series of pulses or numbers) in response to its input.
Anechoic chamber - A room whose walls do not reflect either electromagnetic or acoustic waves.
Anion - Particles in the electrolyte of a galvanic cell carrying a negative charge and moving toward the anode during operation of the cell. See also cation
Anisotropic - Showing differences of property or of effect in different directions.
Anode - The electrode in an electrochemical cell where oxidation takes place, releasing electrons. During discharge the negative electrode of the cell is the anode. During charge the situation reverses and the positive electrode of the cell is the anode.
ANSI - The American National Standards Institute publish standards for batteries jointly with NEMA. (See below)
Aqueous solution - Chemical components in liquid or gel form.
Arrhenius Equation - The relationship between the rate at which a chemical reaction proceeds and its temperature. In general terms, heat speeds up the chemical action.
Assembled battery - A battery composed of two or more cells.
Atomic Number - Specific to individual elements - represents the number of protons in the atomic nucleus. The same as the number of electrons.
Atomic Mass - The number of nucleons (protons and neutrons) in the atomic nucleus.
Auger analysis - Similar to ESCA but does not provide information on the chemical state (oxidation etc.) of the elements.
Authentication - Verification that an item is from an approved source and/or that it is able to meet its declared specification.
Avogadro's Number (NA) - The number of atoms in 12grams of Carbon-12 (definition) = 6.022 x 1023. By extension, the number of particles in 1 mole of a substance.
Base - A proton acceptor. A compound containing hydrogen which dissociates in aqueous solution producing negatively charged hydroxide (OH-) or other ions. Alkalis are bases and a basic solution has a pH greater than 7.0
Battery - Two or more electrochemical energy cells connected together to provide electrical energy.
Battery Management System (BMS) - Electronic circuits designed to monitor the battery and keep it within its specified operating conditions and to protect it from abuse during both charging and discharging.
Battery Monitoring - Sometimes confused with BMS (above) of which it is an essential part, these circuits monitor the key operating parameters (current, voltage, temperature, SOC, etc.) of the battery and provide information to the user.
Bobbin - A cylindrical cell design utilizing an internal cylindrical electrode and an external electrode arranged as a sleeve inside the cell container.
Bootstrap - To do something seemingly impossible using only the available resources. In the context of DC battery power circuits it means generating a DC voltage higher than the battery voltage.
British Thermal Units (BTU) - A unit of heat energy defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. One Btu is equal to about 252 calories, or 778 foot pounds, or 1.055 kilojoules or 0.293 watt hours.
Buck regulator - A switching regulator which incorporates a step down DC-DC converter. A transformerless design in which the lower output voltage is achieved by chopping the input voltage with a series connected switch (transistor) which applies pulses to an averaging inductor and capacitor.
Butler Volmer equation - Used by cell designers to predict the current which will flow in a battery. It is the sum of the anodic and cathodic contributions and is directly proportional to the surface area of the electrodes, increasing exponentially with temperature.
Button cell - Miniature cylindrical cell with a characteristic disc shape.

C Programming Language - The preferred programming language for embedded software used in many battery management applications. Robust, fast and powerful, it allows low level access to information and commands while still retaining the portability and syntax of a high level language.
C Rate - C is a value which expresses the rated current capacity of a cell or battery. A cell discharging at the C rate will deliver its nominal rated capacity for 1 hour. Charging and discharging currents are generally expressed as multiples of C. The time to discharge a battery is inversely proportional to the discharge rate.
• NC is a charge or discharge rate which is N times the rated current capacity of the battery where N is a number (fraction or multiple)
• CN is the battery capacity in AmpHours which corresponds to complete discharge of the battery in N hours (N is usually a subscript). Also written as the N-Hour rate.
Calendar life - The expected life time duration of a cell whether it is active use or in storage
CAN Bus - Controller Area Network The automotive industry standard for on-board vehicle communications. It is a two wire, serial communications bus which is used for networking intelligent sensors and actuators
Calorimeter - A device or chamber for measuring the heat generated by objects placed inside it.
Capacitance (C) - A measure of the ability of a device to store charge per unit of voltage applied across the device. C=Q/V Farads.
The capacitance of a parallel plate capacitor is given by C = ε A/d where ε is the permittivity of the dielectric, A is the area of the plates (electrodes) and d the distance between them.
1 Farad = 1 Coulomb per Volt. (Q / V)
The current through the capacitor is given by the relationship i =C d/dtV(t)
Capacitor - A passive electrical device that stores energy in an electric field.
Capacity - The electric energy content of a battery expressed in "Watt hours". Batteries with the same output voltage also use "Ampere hours" for comparing capacities.
Capacity offset - A correction factor applied to the rating of a battery if discharged under different C-rates from the one rated.
Catalyst - A chemical agent which promotes or influences a chemical reaction without itself being permanently changed by the reaction. Used in recombinant cells and fuel cells
Cathode - The electrode in an electrochemical cell where reduction takes place, gaining electrons. During discharge the positive electrode of the cell is the cathode. During charge the situation reverses and the negative electrode of the cell is the cathode.
Cation - Particles in the electrolyte of a galvanic cell carrying a positive charge and moving towards the cathode during operation of the cell. See also anion
CCA - Cold Cranking Amperes - A measure used to specify the cold cranking capability of automotive SLI batteries. For Lead Acid batteries it is the constant current a battery can deliver during a continuous discharge over a period of 30 seconds at -18°C without the terminal voltage dropping below a minimum of 1.2 Volts/cell.
CE - The CE marking indicates that the product has been designed and manufactured in conformity with the essential requirements of all relevant EU directives, and submitted to the relevant conformity assessment procedure.
Cell - A closed electrochemical power source. The minimum unit of a battery.
Cell balancing - The process used during charging to ensure that every cell is charged to the same state of charge. Also called "Equalisation".
Cell chemistry - The active materials used in the energy cell.
Cell reversal - A condition which may occur multi cell series chains in which an over discharge of the battery can cause one or more cells to become completely discharged. The subsequent volt drop across the discharged cell effectively reverses its normal polarity.
Charge - The process of replenishing or replacing the electrical charge in a rechargeable cell or battery. see also Electric charge
Charge acceptance - The ability of a secondary cell to convert the active material to a dischargeable form. A charge acceptance of 90% means that only 90% of the energy can become available for useful output. Also called Coulombic Efficiency or Charge Efficiency. See alternative definition below.
Charge carriers - The particle carrying the electrical charge during the flow of electrical current . In metallic conductors the charge carriers are electrons , while ions carry the charges in electrolyte solutions .
Charge efficiency - The ratio (expressed as a percentage) between the energy removed from a battery during discharge compared with the energy used during charging to restore the original capacity. Also called the Coulombic Efficiency or Charge Acceptance. See alternative definition above.
Charge pump - A power supply which uses capacitors instead of inductors to store and transfer energy to the output. A voltage doubler or tripler.
Charge rate - The current at which a cell or battery is charged. Generally expressed as a function of rated capacity C.
Charge retention - The ability of a battery to retain its charge in zero current conditions. Charge retention is much poorer at high temperatures. See also Self Discharge
Charge, state of - The available or remaining capacity of a battery expressed as a percentage of the rated capacity.
Charge transport - The movement of electrical charge from one part of the system to another, occurring through the drift of ions under the influence of electrical potential difference. Also called Electromigration.
Chemical species - Atoms, molecules, molecular fragments, ions, etc., as entities being subjected to a chemical process or to a measurement.
CID Circuit Interrupt Device - A small mechanical switch which interrupts the current through an energy cell if the internal pressure exceeds a predetermined limit. Usually applied in small cells only.
Coercivity - The resistance of a ferromagnetic material to becoming demagnetised. Measured in Oersteds.
Coin cell - Small cylindrical cell with a disc shape.
Conditioning - Cycle charging and discharging to ensure that formation (see below) is complete when a cell enters service or returns to service after a period of inactivity
Conductance - Strictly speaking the Conductance applies to resistive circuits and is the reciprocal of the Resistance. Battery manufacturers have their own definition which applies to the frequency dependent elements of the circuit, that is - C= I/E where C is the conductance, I is the test current applied to a component (the cell) and E is the in phase component of the ac voltage E producing it.(Compare with Ohm's Law R=E/I) Measuring the conductance of a battery gives a good indication of its state of health.
Conducting polymer - Plastic materials which have some of the properties of metals. Used as solid electrolytes in batteries. Also used in the construction of fuel cell membranes, capacitor electrodes and in applications requiring anti-static plastics. (See also Polymerbelow)
Constant current charge CC - A charging scheme which maintains the current through the cell at a constant value.
Constant voltage charge CV - A charging scheme which maintains the voltage across the battery terminals at a constant value.
Contacts - The battery output terminals.
Conversion Efficiency - The percentage of the input energy of a process that is converted to energy of the desired type.
Coulomb - A unit of electric charge. One coulomb (1C) is equal to the charge transferred by a current of one ampere in one second.
Coulomb Counting - A method of determining the state of charge of a battery by integrating the ingoing and outgoing discharge currents of a battery over time.
Coulombic Efficiency - The ratio (expressed as a percentage) between the energy removed from a battery during discharge compared with the energy used during charging to restore the original capacity. Also called Charge Efficiency or Charge Acceptance.
Coup de fouet (Whiplash) - A dramatic initial voltage drop when a battery is suddenly called upon to supply a heavy load. The voltage recovers after a short time once the electro-chemical discharge process stabilises.
Critical Temperature (Superconductor) - The temperature below which a superconducting material must be cooled in order to exhibit the property of superconductivity.(See below)
CSA - The Canadian Standards Association is a not-for-profit membership-based association serving business, industry, government and consumers in Canada and the global marketplace.
Curie point or Curie temperature - The temperature above which a ferromagnets and some other materials undergo a sharp change in their magnetic properties losing their ability to possess a net spontaneous or remanent magnetization in the absence of an external magnetic field.
Current limit - The maximum current drain under which the particular battery will perform adequately under a continuous drain.
Current shunt - A current shunt is an low value resistance, whose value is accurately known, placed in series between the battery and the load. The voltage drop across the shunt is used to determine the value of the current using Ohm's Law. Used in series, it is not ashunt in the literal sense of the word. Its name derives from the fact that early ammeters could not handle high currents and the shunt was used to bypass most of the current around the meter.
Cut-off voltage - The specified voltage at which the discharge of a cell is considered complete. See also End voltage and Termination voltage
CVT - Constant Voltage Transformer
Cycle - A single charge and discharge of a battery.
Cycle life - The number of cycles a battery can perform before its nominal capacity falls below 80% of its initial rated capacity. See also Float life below.
Cylindrical cell - A cell in which the electrodes are rolled up in a spiral and placed into a cylindrical container.

D/A Converter (DAC) Digital/Analogue Converter - A device which converts a digitally coded signal into an equivalent analogue signal.
DC-DC Converter - An electronic circuit which takes a DC input voltage and converts it to a different, desired DC output voltage.
Deep cycle battery - A battery designed to be discharged to below 80% Depth of Discharge. Used in marine, traction and EV applications.
Deep discharge - Discharge of at least 80% of the rated capacity of a battery.
Delta V - The voltage drop which occurs in some cells, notably NiCads, which indicates that the cell is fully charged.
Dendritic growth - The formation from small crystals in the electrolyte of tree like structures which degrade the performance of the cell.
Depth of discharge DOD - The ratio of the quantity of electricity or charge removed from a cell on discharge to its rated capacity.
Diamagnetism - The property of a substance which is repelled instead of attracted by a magnet. A diamagnetic material will be repelled from a magnet no matter what pole it is near. It is exhibited by all common materials, but is very weak and often swamped by stronger paramagnetic or ferromagnetic effects. Metals such as bismuth, copper, gold, silver and lead, as well as many nonmetals such as graphite, water and most organic compounds are diamagnetic. See also Ferromagnetism and Paramagnetism.
Dielectric - A nonconductor of electricity, such as an insulator, or a substance in which an electric field can be maintained with a minimum loss of power. The material used between two conducting plates to form a capacitor. When a dielectric or insulator is placed in an electric field, electric charges do not flow through the material but shift only slightly from their average equilibrium positions causing the dielectric to become polarised with a positive charge on one side and a negative charge on the other.
Dielectric Constant - Used to determine the ability of an insulator to store electrical energy. The dielectric constant is the ratio of the capacitance induced by two metallic plates with an insulator between them to the capacitance of the same plates with air or a vacuum between them.
Discharge - The change from chemical energy within the cell into electrical energy to operate a external circuit.
Discharge capacity - The amount of energy taken from the battery when discharged at the rated current and ambient temperature until the discharge end voltage is reached. Generally expressed in units of Watt hours (or Ampere hours for batteries with the same voltage).
Discharge rate - The current delivered by the cell during discharging. Expressed in Amperes or multiples of the C rate.
Discharge voltage - The voltage between the terminals of a cell or battery under load, during discharge.
DOD - Depth of Discharge (see above)
Dropout - In a voltage regulator, the lower limit of the AC input voltage where the power supply just begins to experience insufficient input to maintain regulation. The dropout voltage for linears is quite load dependent. For most switchers it is largely design dependent, and to a smaller degree, load dependent.
Dry Cell - A Leclanché cell with a gel electrolyte.
DST - Dynamic Stress Test. Accelerated battery life tests specified by the USABC. Cycling down to 80% DOD twice per day at different temperatures.
Duty Cycle - The load current or power a battery is expected to supply for specified time periods.
dT/dt - The rate of change of temperature with time. The rapid rate of temperature rise is used to detect the end of the charging cycle in NiMH batteries.

Earth Leakage Trip - See Ground Fault Interruptor
ECE-15 - The United Nations Economic Commission for Europe specification for urban driving cycle simulation.
E Rate - Discharge or charge power, in watts, expressed as a multiple of the rated capacity of a cell or battery which is expressed in watt-hours. For example, the E/10 rate for a cell or battery rated at 23.4 watt-hours is 2.34 watts. (This is similar to the method for calculating C-Rate.)
Elastomer - elastic or plastic materials that resemble rubber which resume their original shape when a deforming force is removed.
Electret - The electrostatic equivalent of the permanent magnet. Dielectric materials that have been permanently electrically charged or polarised.
Electric charge is a physical property of matter which causes it to experience a force when near other electrically charged matter. The charge may be positive or negative. Similar charges repel each other while opposite charges attract each other. The unit of electric charge is the Coulomb (C).
Electrochemical equivalent - The weight of a substance which is deposited by the passage of one coulomb of current.
Electrode - Conducting element within a cell in which an electrochemical reaction occurs.
Electrode potential - The voltage developed by a single electrode, determined by its propensity to gain or lose electrons.
Electrolysis - Chemical modifications, oxidation and reduction produced by passing an electric current through an electrolyte. See also Faraday's Law of Electrolysis
Electrolyte - A substance which dissociates into ions (charged particles) when in aqueous solution or molten form and is thus able to conduct electricity. It is the medium which transports the ions carrying the charge between the electrodes during the electrochemical reaction in a battery.
Electromotive Force EMF - The ability of an electrical source to deliver energy. It is the difference of potentials which exists between the two electrodes of opposite polarity in an electrochemical cell. Also known as the Cell voltage. The unit of EMF is the Volt.
Embedded System - A special-purpose computer system, which is completely encapsulated within the device it controls, usually performing a limited range of specific pre-determined tasks. This allows the use of simpler or cheaper dedicated microprocessors providing only the minimum functionality required by the application, or alternatively the entire processing power of the microprocessor can be focused on a single task. Battery Management Systems will normally be implemented with an embedded system.
EMC - Electromagnetic compatibility (EMC) is the ability of electronic and electrical equipment and systems to operate without adversely affecting other electrical or electronic equipment or being affected by other sources of electromagnetic interference. (RFI)
End voltage - The prescribed voltage that indicates that the discharge is complete. (see also Cut-off voltage)
Endothermic - Describes a chemical action in which heat is absorbed.
Energy Content - The absolute amount of energy stored in a battery expressed in Wh or Joules
Energy density - The amount of energy stored in a battery. It is expressed as the amount of energy stored per unit volume or per unit weight (Wh/L or Wh/kg).
Enthalpy - The amount of energy released or absorbed by a chemical reaction. The "Free Enthalpy" (also called the " Change in Gibbs Free Energy") in a reaction is the maximum amount of chemical energy available from a system that can be converted into electrical or mechanical energy and vice versa. (discharge and charge respectively)
Entropy - A measure of the disorder of a system. Used as a measure of heat content.
EPROM - Electronically Erasable Programmable Read-Only Memory. Re-writable memory that does not lose data if power is lost to the system (non-volatile). Available in three types:
• OTP One Time Programmable non-erasable.
• Windowed (ultraviolet light erasable) used for prototyping and development work.
• EEPROM Electronically Erasable Programmable Read-Only memory.
Equalisation - The process of bringing every cell in a battery chain to the same state of charge (SOC)
ESCA - Electron Spectroscopy for Chemical Analysis. Equipment using x-ray irradiation to identify the presence of individual chemical elements particularly for surface coatings and thin films where it can be used for selected element depth profiling. A machine typically costs about $750,000
ESD - Electrostatic Discharge
EUDC - Extra Urban Driving Cycle. European additional specification for urban driving cycle simulation.
EUROBAT - The Association of European Storage Battery Manufacturers. (Mainly Lead acid)
Eutectic - A mixture in such proportions that the melting-point is as low as possible, the constituents melting simultaneously.
EV - Electric Vehicle
Exercise - Commonly describes the discharging to one volt per cell and subsequent charging. Used to maintain or condition NiCad and NiMH cells.
Exothermic - Describes a chemical action in which heat is produced.

Farad - The charge in Coulombs necessary to change the potential between the plates of a capacitor by 1 volt.
1 Farad = 1 Coulomb per Volt. (Q / V)
Faraday cage - An enclosure with no apertures (holes, slits, windows or doors) made of a perfectly conducting material. No electric fields are produced within the Faraday cage by the incidence of external fields upon it or by currents flowing on the perfect conductor such that the perfectly conducting enclosure is a perfect electromagnetic shield.
Faraday constant- The magnitude of electric charge per mole of electrons or protons. It is equal to Avogadro's Number times the charge on the electron. F= NA.e
Faraday's Law of Electrolysis - The mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electrical charge (measured in Coulombs) transferred at that electrode.
Faraday's Law of Induction - The induced EMF in a closed circuit is proportional to the rate of change of the magnetic flux through the circuit. see also Inductance
Fast charge - Charging in less than one hour at about 1.0C rate. Needs special charger.
FCC - The Federal Communications Commission is an independent United States government agency charged with regulating interstate and international communications by radio, television, wire, satellite and cable.
Ferromagnetism - The property of a substance which is attracted to a magnet. Iron, cobalt, nickel, gadolinium, dysprosium and alloys containing these elements are ferromagnetic. See also Diamagnetism and Paramagnetism.
FET - Field Effect Transistor - A semiconductor device designed for fast, current switching applications.
Firmware - Instructions programmed into a micro-controller that controls its operation. A combination of hardware and software.
FlexRay Bus - A fault tolerant, high speed data communications bus designed for complex automotive control applications.
Float charge - An arrangement in which the battery and the load are permanently connected in parallel across the DC charging source, so that the battery will supply power to the load if the charger fails. Compensates for the self-discharge of the battery.
Float life - The expected lifetime in hours of a battery when used in a float charge application. See also Cycle life above.
Flooded Lead Acid cell -In "flooded" batteries, the oxygen created at the positive electrode is released from the cell and vented into the atmosphere. Similarly, the hydrogen created at the negative electrode is also vented into the atmosphere. This can cause an explosive atmosphere in an unventilated battery room. Furthermore the venting of the gasses causes a net loss of water from the cell. This lost water needs to be periodically replaced. Flooded batteries must be vented to prevent excess pressure from the build up of these gasses. See also Sealed Lead Acid (SLA) Cells which overcome these problems.
Flow battery - A battery in which the electrolyte flows or is pumped through the electrodes
Flywheel battery - A flywheel stores kinetic energy in a high speed (up to 100,000 rpm) rotating cylinder and is "charged" and "discharged" via an integral motor/generator. High power availability but low capacity.
Formation - Electrochemical processing of a cell electrode(or plate) between manufacturing and first discharge, which transforms the active material into its useable form.
FPGA - Field Programmable Gate Array. A microchip which can be made with thousands of programmable logic gates. Often used for prototype or custom designs, they permit short development times and low production costs.
FUDS - Federal Urban Driving Schedule specification for urban driving cycle simulation.
Fuel Cell - An electrochemical generator in which the reactants are stored externally and may be supplied continuously to a cell.
Fuel Gauge - An indication of the State of Charge (SOC) or how much charge is remaining in a battery. Also called a Gas Gauge.
Fuzzy Logic - A method of deriving precise answers from vague data.

Galvanic cell - An electrolytic cell in which chemical energy is converted to electrical energy on demand
Gas chromatography - The separation and identification of individual chemical components from a sample. A typical machine costs over $250,000..
Gas gauge - An electrical circuit which indicates the amount of charge remaining in a battery.
Gassing - The generation of a gaseous product at one or both electrodes as a result of the electrochemical action. In Lead Acid batteries gassing produces hydrogen and oxygen.
Gel cell - A battery which uses gelled electrolyte, an aqueous electrolyte that has been fixed by the addition of a gelling agent.
Gibbs Free Energy - See Enthalpy
GMR (Giant MagnetoResistance) A spintronic effect that produces a large change in resistance of the conducting layers that occurs when thin stacked layers of ferromagnetic and nonmagnetic materials are exposed to a magnetic field. "Giant" refers to its very large electrical signal. The technology is used to manufacture current and magnetic sensors.
Gravimetric Energy Density (WhKg) - The energy output per unit weight of a battery.
Gravimetric Power Density (W/Kg) - The power output per unit weight of a battery.
Ground Fault Interruptor - Also called an Earth Leakage Trip - A safety device which disconnects the mains power if an earth leakage (unsafe) condition is detected. A sensing coil detects fault currents from the live wire to the earth (ground) wire and switches off the power when a predetermined threshold is reached. The device is designed to protect the electrical installation from faults and does not sense fault currents from the live wire to any other earthed body. See also Residual Current Device (RCD) which also protects the user.
Ground Loop - An unintentionally induced feedback loop or crosstalk caused by two or more circuits sharing a common electrical ground.

Half Cell Reaction - The electrochemical reaction between the electrode and the electrolyte.
Hall effect - When a fixed conductor carrying an electric current is placed in an external magnetic field perpendicular to the current there is voltage drop across the conductor at right angles to the current which is proportional to the magnetic field. Used to measure magnetic field strength.
Heavy Duty battery - An ill defined battery characteristic. See Battery Performance.
Henry (H) - The unit of inductance. The inductance L in a circuit =1 Henry if the rate of change of the current of 1 Ampre per second in the circuit produces an EMF of 1 Volt.
1 Henry = 1 Weber per Amp (Wb / A)
Hertz (Hz) - The standard unit of frequency of one cycle per second.
HEV - Hybrid Electric Vehicle (See below)
Hibernation state - A state in which the the status of the various functions of a circuit has been saved in memory and the circuit has been switched off save energy. When power is reapplied, data taken from the memory is used to restore the circuit to the status it had before switch off. (See also "Standby state" below)
High Energy battery - An ill defined battery characteristic. See Battery Performance.
High rate discharge - Discharge at a current of 2C or more.
Horse Power (Hp) - The rate of doing work. 1 Hp = 746 Watts or 550 foot pounds per second.
Hybrid Electric Vehicle (HEV) - A vehicle which has two forms of motive power one of which is electric.
Hydrometer - A device used for measuring the specific gravity of a fluid. In the case of lead acid batteries the specific gravity provides a measure of the state of charge of the cell.
Hygrometer - An instrument for measuring humidity. Often confused with a hydrometer.
Hysteresis - A property of physical and chemical systems that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state. In the case of magnetic systems, when an external magnetic field is applied to a magnetic material, the material becomes magnetised absorbing some of the external field. When the external field is removed the material remains magnetised to some extent, retaining some magnetic field. See also hysteresis in batteries.

IEC - The International Electrotechnical Commission (IEC), founded in London in 1906, is the leading global organization that prepares and publishes international standards for all electrical, electronic and related technologies. See also Standards
IGBT - Insulated Gate Bipolar Transistor. It has the output switching and conduction characteristics of a bipolar power transistor but is voltage controlled like the MOSFET giving the high current carrying capability of the bipolar transistor but the ease of control of the MOSFET.
Immobilised electrolyte - A construction technique used in lead-acid batteries. The electrolyte (the acid) is held in place against the plates instead of being a free-flowing liquid. The two most common techniques are Gel Cell and Absorbed Glass Mat.
Impedance - A measure of the response of an electric circuit to an electric current. The actual value is frequency dependent. The current is opposed by the capacitance, inductance and resistance of the circuit.
Impedance testing - Determination of the battery's internal impedance by measuring the voltage drop across a cell when it carries a sample alternating current.
Inductance (L) - A measure of the ability of a device to store magnetic flux per unit of rate of change of current passing through the device. Measured in Henries. 1 Henry = 1 Weber per Amp (Wb / A)
See also Faraday's Law of Induction
The voltage across the inductor is given by the relationship v = - L d/dtI(t)
Inductive charging - A charger in which the charging current is induced by an external induction coil into a secondary transformer winding housed within the battery together with rectifying and charge control circuits.
Inductor - A passive electrical device that stores energy in a magnetic field
Infra red radiation - The spectrum of the heat radiated by a warm body.
Inhibitor - A substance added to the electrolyte to prevent or slow down an unwanted electrochemical process. Used to prevent corrosion of the electrodes or the formation of dendrites.
Insert mouldings - Plastic parts containing metal inserts used to simplify product assembly and reduce costs. Inserts made from metal or other materials are placed in the mould prior to the injection of plastic. The plastic flows around the inserts and fixes their position.
Intelligent battery - Battery containing circuitry enabling some communication between the battery and the application or with the charger.
Intelligent charger - Charger which is able to react to inputs from an intelligent battery to control or optimise the charging process.
Intelligent Energy Manager IEM - A system for reducing the demands that power hungry applications place on the battery.
Intercalation - This insertion of ions into the crystalline lattice of a host electrode without changing its crystal structure.
Internal impedance - Resistance to the flow of AC current within a cell. It takes into account the capacitive effect of the plates forming the electrodes.
Internal resistance - Resistance to the flow of DC electric current within a cell, causing a voltage drop across the cell in closed circuit proportional to the current drain from the cell. A very low internal impedance is usually required for a high rate (high power) cell.
Inverter - An electrical circuit which generates a sine-wave output (regulated and without breaks) using the DC current supplied by the rectifier-charger or the battery. The primary elements of the inverter are the DC/AC converter, a regulation system and an output filter.
Ion - An atom or group of atoms which is electrically charged. Depending on how they were created - through release or absorption of electrons - ions can be either positively charged (Cations) or negatively charged (Anions). See also Ionisation
IP Code - Ingress Protection Rating. It consists of the letters IP followed by two digits. The first digit represents the degree of protection against dust and solids. The second digit represents the degree of protection against moisture and water.
IR drop - The voltage drop across a battery due to its internal impedance. See also Ohmic loss below.
I2R loss - The energy generated or lost as heat due to the internal resistance of the battery. Also known as the Joule heating effect.
ISO - A network of national standards institutes from 148 countries, founded in 1946, working in partnership with international organizations, governments, industry, business and consumer representatives.The name, "ISO" was not intended as an acronym for anInternational Standards Organisation but was derived from the Greek word "isos" meaning "equal". See also Standards
Isotope - Atoms of the same element with the same atomic number ( the same number of protons) but with different numbers of neutrons an hence different weights. See also allotrope.

Josephson effect - The flow of electric current through nonconductive material when placed between two superconductors. Used to detect very weak magnetic fields.
Joule - "J" A measure of work, energy or cell capacity. For electrical energy, one Joule is one Amp at one Volt for one Second, or one WattSecond. 1 Wh = 3.6kJ. For mechanical energy one Joule is a force of one Newton acting over one metre i.e. One newton metre.
Joule heating - The I2R loss or heating effect of a current I flowing through a resistance R.

Kalman Filter - A mathematical technique for deriving accurate information from inaccurate data.
Kelvin Bridge - An electrical circuit for measuring very low impedances such as battery internal impedance, contact resistance and resistance of circuit elements such as wires and cables. Also known as the Kelvin Connection for voltage sensing.
Keyed connectors - Plug and socket pairs with a unique mechanical profile which can only be mated with eachother in a particular orientation and which do not allow mating with connectors of a different design.

LDO (Low Drop Out) Regulator - An LDO is a type of linear regulator. Dropout voltage is the minimum input to output voltage differential required for the regulator to sustain an output voltage within 100mV of its nominal value.
Leakage - The escape of electrolyte to the outer surface of the battery or cell.
Leclanché Cell - A zinc carbon or zinc chloride cell.
Lifetime Energy Throughput - The total amount of energy in Watthours which can be taken out of a rechargeable battery over all the cycles in its lifetime before its capacity reduces to 80% of its initial capacity when new.
LIN Bus - Local Interconnect Network An automotive industry standard for on-board vehicle communications. It is a single wire, serial communications bus which is used for networking intelligent sensors and actuators
Linear charger - Charger which uses a series regulator. The simplest and cheapest type but less efficient than a Switch mode charger.
Linear Regulator - A linear, or Series, regulator is a voltage regulator which uses a transistor or FET in series with the load, operating in its linear region, to subtract excess voltage from the applied input voltage, producing a regulated output voltage.
Lithium Ion Cell - A secondary lithium cell in which both the negative and positive electrodes are lithium insertion (intercalation) compounds. Also known as rocking chair, shuttlecock or swing cell.
Lithium Polymer Cell - A lithium ion cell with a solid polymer electrolyte.
Load current - The discharge current provided by a battery, or drawn by a battery powered device.
Long Life battery - An ill defined battery characteristic. See Battery Performance.

Magnetic flux ( Φ ) - is a measure of the magnetic field strength. Measured in Webers
Magnetic flux density (B) - is the magnetic flux per unit area. B = (Φ / A) Teslas. The flux density resulting from a magnetic field is given by B = μH where μ is the permeability of the medium.
Magnetic field strength (H) - is a measure of the magnetic field surrounding a wire (or moving charge). H = I / (2 π r) Amps per metre, where I is the current in the wire and r the distance from the wire.
Magnetic Resonance Imaging (MRI) - A method of looking inside the human body without using surgery, harmful dyes or x-rays based on Nuclear Magnetic Resonance (NMR).
Magnetohydrodynamic Generator MHD - The production of electricity by passing a conducting fluid or plasma through a magnetic field.
Magnetomotive Force (MMF) - is the strength of a magnetic field, or magnetic potential, in a current carrying coil of wire. It is the work that would be required to carry a hypothetical isolated magnetic pole of unit strength completely around a magnetic circuit and is equivalent to the current I multipled by the number of turns N in the coil producing the field. It is expressed in units called ampere-turns (At). The MMF = ampere-turns = NI = Number of turns (N) X Current in the wire (I)
Magnetostriction - A property which causes the shape or dimensions of ferromagnetic materials to change during the process of magnetisation.
Mass spectrometer - A device which produces a mass spectrum of a sample to find out its composition by ionising the sample and separating ions of differing masses and recording their relative abundance by measuring intensities of ion flux. Mass spectroscopy allows detection of compounds by separating ions by their unique mass. A typical machine costs around $250,000
Memory effect - Reversible, progressive capacity loss in nickel based batteries found in NiCad and to a lesser extent in NiMH batteries. It is caused by a change in crystalline formation from the desirable small size to a large size which occurs when the cell is recharged before it is fully discharged.
Mechanical charging - Charging by replacing one or more of the active chemicals in the cell.
Meissner effect - When a superconducting material is cooled below its critical temperature it will exclude or repel a magnetic field. A magnet moving by a conductor induces currents in the conductor. This is the principle upon which the electric generator operates. But, in a superconductor the induced currents exactly mirror the field that would have otherwise penetrated the superconducting material causing the magnetic field to be excluded and magnet to be repulsed. This phenomenon is known as diamagnetism (see above) and is so strong that a magnet can actually be levitated over a superconductive material.
Metal hydride - A metallic compound which is able to absorb hydrogen. Used as the negative electrode (anode) of a Nickel Metal Hydride battery.
Microcycles - Rapid, shallow charge and discharge cycles which occur in automotive battery applications, particularly those which involve regenerative braking.
MISRA - UK Motor Industry Software Reliability Association.
Mole (n) - The amount of substance of a system that contains as many "elemental entities" (e.g., atoms, ions, electrons, molecules) as there are atoms in 12 grams of carbon-12 (Avogadro's number of particles). It is an amount, not a physical quality. 1 mole of a pure substance has a mass in grams equal to its molecular mass (M).
Molar mass - The mass in grams of one mole (or 6.02 x 1023 molecules) of any chemical compound.
Monomer - A small molecule that may become chemically bonded to other monomers to form a polymer. From Greek mono "one" and meros "part".
Morphology - The microstructure of the solid phases of materials. The grain shapes and structure of crystals of the chemical components of a battery.
MOSFET - A Field Effect Transistor made using Metal Oxide Semiconductor technology. Controlled by voltage rather than current like a bipolar transistor. MOSFET's have a significantly higher switching speed than bipolar power transistors. Suitable for high power circuits, they generate almost no loss (little heat generation), enabling fast response, excellent linearity, and high efficiency. The positive temperature coefficient inhibits thermal runaway. (Degrades to an SFET - Smoke and Fire Emitting Transistor if subject to excessive voltages). See also IGBT.
MSDS - Material Safety Data Sheet. Information provided by battery or cell manufacturers about any hazardous materials used in their products.
Multiplexer - A multiplexer is a device which enables several communications links or signals to share a single communications channel. At the receiving end of the link a demultiplexer separates the signals again. Various coding schemes are possible which enable the signals to be transmitted simultaneously or sequentially.

Nano - From the latin word meaning "dwarf". One billionth or 10 -9. One micron = 1000 nanometers. One nanometer is about the diameter of 3 to 6 atoms (depending on the element).
Nanotechnology - Nanomaterials (nanocrystalline materials) are materials possessing grain sizes on the order of a billionth of a meter. Used for electrodes and separator plates in NiMH and Lithium ion batteries and also in supercapacitors. Their foam-like (aerogel) structure provides a very large effective surface area which can hold considerably more energy than their conventional counterparts.
Nanobattery - Very small battery built using nano technology. Of microscopic size 1 micron diameter they deliver 3.5 volts. The electrodes are ceramic or carbon particles and the electrolyte is a solid polymer impregnated in an aluminium oxide membrane.
Negative Delta V (NDV) - The NDV is the drop in the battery voltage which occurs when NiCad or NiMH cells reaches their fully charge state. Used to detect the end of the charging cycle in Nicads.
Negative electrode - The electrode which has a negative potential. The anode.
NEMA - The National Electric Manufacturers Association in the USA publish standards for batteries jointly with ANSI. (See above)
Nernst equation - Used by cell designers to calculate the voltage of a chemical cell from the standard electrode potentials, the temperature and to the concentrations of the reactants and products.
Neural Network - A powerful data modeling tool that is able to capture and represent complex input/output relationships. It is used as a basis for self learning systems.
NIH - Not Invented Here. Used to describe engineers and managers who are reluctant to accept ideas from another organisation.
Nominal capacity - Used to indicate the average capacity of a battery. It is the average capacity when batteries are discharged at 0.2C within one hour of being charged for 16 hours at 0.1C and 20± 5°C. (or discharge at 0.05C for automotive batteries - SAE) Definition depends on the conditions. See Ampere Hours Ah above
Nominal voltage - Used to indicate the voltage of a battery. Since most discharge curves are neither linear nor flat, a typical value is generally taken which is close to the voltage during actual use.
NRE - Non-Recurring Engineering costs. A one time charge for design and implementation of custom battery packs or other products.
NTC - A thermistor with a negative temperature coefficient, whose resistance decreases with increasing temperature.
Nuclear fission - Occurs when the atomic nucleus splits into two or more smaller nuclei plus some by-products. These by-products include free neutrons and photons (usually gamma rays). Fission releases substantial amounts of energy (the nuclear binding energy ). The neutrons released by the fission process may collide with other nuclei causing them in turn to undergo fission initiating to a chain reaction.
Nuclear fusion - A process in which two nuclei join together to form a larger nucleus and releasing energy. It takes considerable energy to overcome the repulsion between the two positively charged nuclei to force them to fuse. The fusion of lighter nuclei, which creates a heavier nucleus and a free neutron, will generally release even more energy than it took to force them together. It is an exothermic process which could produce self-sustaining reactions.
Nuclear Magnetic Resonance (NMR) - The interaction of atomic nuclei placed in an external magnetic field with an applied electromagnetic field oscillating at a particular frequency . Magnetic conditions within the material are measured by monitoring the radiation absorbed and emitted by the atomic nuclei. Used in MRI scanners and as a spectroscopy technique to obtain physical, chemical, and electronic properties of molecules.

OEM Original Equipment Manufacturer - A company with the prime responsibility for conceiving, designing, manufacturing and distributing a particular product line.
Ohmic loss - The voltage drop across the cell during passage of current due to the internal resistance of the cell. Also known as IR loss or IR drop.
Open circuit voltage OCV - The voltage of a cell or battery with no load attached measured with a voltmeter at room temperature.
Operating voltage - Voltage between the two terminals of the battery with a load connected.
Operational amplifier (Op amp) - A high gain DC amplifier with a voltage gain of 100 to 100,000 or more and a very high (ideally infinite) input impedance and very low (ideally zero) output impedance. Op-amps are the basic building block of linear integrated circuits used for analogue circuit applications. They have positive and negative inputs which allow circuits which use feedback to achieve a wide range of functions.
Opportunity charging - Intermittent charging from sources whenever or wherever power is available.
Opto-isolator - Also called opto-coupler. An isolation device using optical techniques (an LED transmitting across a small gap to a photocell) to isolate the electrical connections between a transmitter and a receiver. Used to pass signals between high voltage and low voltage circuits and to replace switches and relays. Having no electrical connection they also help to cut down on ground loops.
Osmosis - The diffusion of a solvent through a semi permeable membrane from a region of low solute concentration to a region of high solute concentration. The semi permeable membrane is permeable to the solvent, but not to the solute, resulting in a chemical potential difference across the membrane which drives the diffusion. The solvent flows from the side of the membrane where the solution is weakest to the side where it is strongest to equalise the concentration on both sides.
Over-charge - Continuous charging of the battery after it reaches full charge. Generally overcharging will have a harmful influence on the performance of the battery which could lead to unsafe conditions. It should therefore be avoided.
Over-current - Exceeding the manufacturer's recommended maximum discharge current for a cell or battery.
Over-discharge - Discharging a battery below the end voltage or cut-off voltage specified for the battery.
Overmoulding - An injection moulding technique used to encapsulate and protect components or small sub-assemblies, usually by moulding a soft, flexible, cosmetically attractive plastic over the components which must be able to withstand the temperatures and pressures of the moulding process. Used for cable connectors, gaskets, and for incorporating small components into cables. Two shot moulds may be used to provide soft plastic grips over a hard plastic shell. It provides rugged, almost unbreakable protection with built in strain relief.
Over-voltage - The difference between the actual potential at which an electrochemical reaction occurs, and its theoretical equilibrium potential.
Oxidation - The loss of electrons by a chemical species

Packaging - In a battery, the mechanical structure used to contain and protect its components (cells, electronic circuits, contacts etc.).
Parallel connection - The connection together of, two or more, similar cells to form a battery of higher capacity by connecting together all the cell terminals of the same polarity.
Paramagnetism - The property of a substance which is attracted to a magnet. It is similar to ferromagnetism except that the attraction is weaker. When a paramagnetic material is placed in a strong magnetic field, it becomes a magnet as long as the strong magnetic field is present. But when the strong magnetic field is removed the magnetic effect is lost. Below the substance's Curie temperature a paramagnetic material becomes ferromagnetic. Paramagnetism is exhibited by materials containing transition elements, rare earth elements and actinide elements. Liquid oxygen and aluminium are also examples of paramagnetic materials. See also Diamagnetism and Ferromagnetism.
Passivation layer - A resistive layer that forms on the electrodes in some cells after prolonged storage impeding the chemical reaction. This barrier must be removed to enable proper operation of the cell. Applying charge/discharge cycles often helps in preparing the battery for use. In other applications, passivation is used as a method of shielding a metal surface from attack.
Periodic Table of the Elements - A tabular display of the known chemical elements. The elements are arranged by electron structure so that many chemical properties vary regularly appearing in groups with common properties across the table. Each element is listed by its atomic number and chemical symbol .
Permanent charge - The charging current which can safely be continuously supported by the battery, regardless of the state of the charge.
Permeability (μ) - The measure of the characteristic of a medium to support the formation of a magnetic field. It indicates degree of magnetisation that a material obtains in response to an applied magnetic field. It is measured in units of Henries per metre (H / m)
Permittivity (ε) - The measure of the characteristic of a medium to resist the formation of an electric field. It gives an indication of how much electrical charge a material can store in a given volume. It is measured in units of Farads per metre (F/ m)
Peukert's equation An empirical formula that approximates how the available capacity of a battery changes according to the rate of discharge. The equation shows that at higher currents, there is less available energy in the battery.
Peukert number A value that indicates how well a battery performs under heavy currents. A value close to 1 indicates that the battery performs well; the higher the number, the more capacity is lost when the battery is discharged at high currents. The Peukert number of a battery is determined empirically.
pH - (potential (of) hydrogen) is a logarithmic measure of the concentration of hydrogen ions (H + ) in a solution and, therefore, its acidity or alkalinity (basicity). pH = -log[H + ]
The "pH" scale extends from 0 to 14 (in aqueous solutions at room temperature). A pH value of 7 indicates a neutral (neither acidic nor basic) solution. A pH value of less than 7 indicates an acidic solution, the acidity increases with decreasing pH value. A pH value of more than 7 indicates an alkaline or basic solution, the alkalinity or basicity increases with increasing pH value.
Photovoltaic cell - A device that directly converts the energy in light into electrical energy. Also called a photocell, a solar cell or a PV cell.
Photovoltaic effect - The generation of an electromotive force as a consequence of the absorption of radiation. In practice a current which flows across the junction of two dissimilar materials when light falls upon it.
Pilot Cell - A selected cell whose condition is assumed to indicate the condition of the entire battery.
Plates - The electrodes used in a flat plate cell.
PNGV - Partnership for a New Generation of Vehicles. A partnership between government, industry and academia in the USA to improve all aspects of automotive design in which batteries figure highly.
Polarisation - The change in the potential of a cell or electrode from its equilibrium value caused by the passage of an electric current through it. There are two irreversible electrochemical components, the "electrode polarisation" at the electrodes and the "concentration polarisation" in the electrolytic phase plus an ohmic loss component due to the electrical resistance of the cell. Also due to the build up of gas bubbles on the electrodes.
Polarity reversal - Reversal of the polarity of a battery or cell due to over discharge.
Polymer - Strictly it is a substance made of long repeating chains of molecules called monomers which may be identical or different. The term polymer is often used in place of plastic, rubber or elastomer. In battery technology "polymer" usually refers to a solid (plastic) ionic conductor that is an electrical insulator but passes ions. (See also Conducting Polymer above)
Polymorphism - The ability of solid materials or compounds with the same chemical composition to exist in more than one form or crystal structure giving rise to materials with different physical or chemical properties. When the material consists of a single element, the property is known as allotropy.
Polyswitch - A resettable fuse. (See below)
Positive electrode - The electrode which has a positive potential. The cathode. Electric current from this electrode flows into the external circuit.
Pouch cell - A battery or cell contained in a flexible metal foil pouch.
Power density - The amount of power available from a battery. It is expressed as the power available per unit volume or per unit weight (W/L or W/kg).
PowerNet - The standard proposed for next generation of automotive batteries. Nominally 42 Volt systems.
Power transistor - A high current, bipolar transistor controlled by the current through the gate. Used in linear (series) regulators as the voltage dropper between the unregulated voltage input and the regulated output. Also used as a high current switching device in control and protection circuits. Needs a high current to turn it on and is slow to turn off and its negative temperature coefficient makes it prone to thermal runaway. For these reasons it was mostly superceded by MOSFETs in high power battery switching applications. See also Thyristor and IGBT.
ppm - Parts Per Million
Precursor - A chemical compound that participates in a chemical reaction which produces another compound.
Primary battery - A battery that is non-rechargeable.
Prismatic cell - A slim rectangular sealed cell in a metal case. The positive and negative plates are stacked usually in a rectangular shape rather than rolled in a spiral as done in a cylindrical cell.
Progressive dies - Multi-stage stamping tools for producing complex metal components from flat metal strip in a hydraulic or eccentric press. The die consists of two or more stages each of which carries out punching, drawing or folding operations with each down stroke of the press. Between each stroke, the strip moves from stage to stage through the die. Complex profiles and three dimensional shapes can be built up from a series of simpler operations which take place progressively at each stage as the strip passes through the die.
Protection - A facility incorporated into battery packs to protect the cells from out of tolerance working conditions or misuse.
PTC - A thermistor with a positive temperature coefficient, whose resistance increases with temperature.
PPTC - A Polymeric Positive Temperature Coefficient device. It is a non-linear thermistor, more commonly called a resettable fuse.
Pulse charger - Versatile, hybrid charger having some of the advantages of both switch-mode and linear chargers. More costly than both.
Pulse discharge - A high rate discharge, usually of 1 second or less.

Quick charge - Charging in three to six hours at about 0.3C rate. Needs special charger.
Quiescent current - The current which continues to be drawn from the battery when the application it powers is in standby or hibernation mode.

Ragone Plot - The graphical illustration of the specific energy of a cell as a function of its specific power.
RAM cells - Rechargeable Alkaline Manganese cells.
RAPS - Remote-Area Power Supplies - Power systems deriving their energy from local solar or wind sources using a battery for energy storage and supplying the load through DC-DC converters or AC inverters.
Rare earth elements - The rare earth metals belong to group 3 of the periodic table in two blocks, the Lanthanide series and the Actinide series. Originally found in small quantities they are not particularly rare. They are silver, silvery-white, or grey metals with a high electrical conductivity and a bright lustre which tarnishes readily in air.
Rate - When applied to cells it usually means the cells current carrying capacity.
Rated capacity - The specified capacity of a battery.
Reconditioning - One or more deep discharges below 1.0 V/cell with a very low controlled current, causing a change to the molecular structure of the cell and a rebuilding of its chemical composition. Reconditioning helps break down large crystals to a more desirable small size, often restoring the battery to its full capacity. Applies to nickel-based batteries. See also refurbishment (below)
Recombinant system - Sealed secondary cells in which gaseous products of the electrochemical charging cycle are made to recombine to recover the active chemicals. A closed cycle system preventing loss of active chemicals. Used in Nicads and SLA batteries.
Recovery - The lowering of the polarization of a cell during rest periods.
Recycling - Reclamation of materials without endangering human health and the environment.
Redox - A contraction of the words "reduction" and "oxidation". The two chemical reactions on which cell chemistries depend.
Redox Battery - A battery in which the chemical energy is stored in two dissolved ionic reactants separated by a membrane.
Reduction - The gain of electrons by a chemical species.
Refurbishing - The repair of worn out or damaged batteries. This is not the same as reconditioning (see above).
Regenerative braking - This uses the electrical drive motor in an electric vehicle to act as a generator returning energy to the battery when overdriven mechanically by the vehicle wheels. This provides a powerful braking effect and at the same time captures energy which would otherwise be wasted or dissipated in the brakes.
Regenesys - A high power Sodium Polysulfide Bromine "Flow Battery".
Regulator - See Voltage regulator.
Relay - A mechanical switch operated by a solenoid.
Resealable safety vent - The resealable vent internal to a cell to release excessive internal pressure.
Reserve battery - Batteries which are stored in an inactive state without their electrolyte. They are only activated when needed by the introduction of the electrolyte. See also Water-activated batteries and Ampoule batteries.
Reserve capacity - The number of minutes at which the battery can be discharged at 25 Amps and maintain a terminal voltage higher than 1.75 volts per cell, on a new, fully charged battery at 80degrees Fahrenheit (27 °C). Defines a battery's ability to power a vehicle with an inoperative alternator or fan belt. Used for comparing automotive SLI batteries.
Resettable fuse - A fuse which protects against excessive current and temperature by interrupting the flow of current. After opening it will reset after the fault conditions have been removed but only after it has cooled. It requires no manual resetting or replacement. The "Polyswitch" is an example of this.
Residual Circuit Breaker (RCCB), or Residual Current Device (RCD) - an electrical safety device which interrupts a circuit whenever it detects that the current is not balanced between the live (high voltage) conductor and the return neutral conductor. It can be used as a safety device by cutting off the supply voltage when it detects current leakage through the body of a person who is earthed (grounded) accidentally touching a live part of the circuit. See also Ground Fault Interruptor / Earth Leakage Trip.
Resistance welding - Resistance welding is a process used to join metallic parts with electric current. There are several forms of resistance welding, including spot welding, seam welding, projection welding, and butt welding.
Rest periods - Interruptions to the charging process to allow the chemical reactions in the battery to stabilise.
Reversible reaction - A chemical reaction which can be reversed to reconstitute the original components.
RFI - Radio Frequency Interference. Transmitted/emitted RFI affects other external equipment. Susceptibility measures the immunity of equipment from received RFI. See also EMC and Electromagnetic Radiation
RFID - Radio Frequency Identification. Small tags incorporating a radio transmitter which can be used to identify or track items of value.
Rocking Chair Cell- A lithium ion cell
RS232 connection - A standard for serial transmission of data between two devices.
RS485 connection - A standard for serial transmission of data between multiple devices.

SAE - Society of Automotive Engineers. The SAE Technical Standards Board issues and recommends industry standards. See also Standards
Safety vent - A safety mechanism that is activated when the internal gas pressure rises above a normal level.
Sampling Rate - The repetition frequency at which digital samples are taken of an analogue quantity.
Sealed cells - A cell which remains closed and does not release gas or liquid when operated within the limits of charge and temperature specified by the manufacturer. An essential component in recombinant cells.
Secondary battery - A battery which can be recharged and used repeatedly.
Self-discharge - Capacity loss during storage due to the internal current leakage between the positive and negative plates.
SEM (Scanning Electron Microscope) - Apparatus used to investigate the physical structure of cell components and surfaces. They typically cost about $500,000 or more.
Semiconductor - An insulator whose conductivity can be manipulated by the addition of impurities ( doping ), by introduction of an electric field, by exposure to light , or by other means.
Separator - A non-conductive semi-permeable film or grid to separate 2 electrodes to prevent them from contacting each other and short-circuiting but which allows the passage of ions through it.
Series connection - The connection of, two or more, similar cells in a chain to form a battery of higher voltage by connecting the positive terminal of each cell to the to negative terminal of the next cell.
Series regulator - Another name for a Linear regulator
Service life - The period of useful life of a battery before a predetermined end point is reached.
Shaft encoder - An electro-mechanical or optical device which converts the angular position or motion of a shaft or axle to an analogue or digital electrical signal. Also called a rotary encoder.
Shedding - The loss of material from the plates of Lead Acid batteries.
Shelf life - The duration a cell can be kept in storage and still retain its ability to give a specified performance. See also Battery Storage
Shrouded terminals - Terminals surrounded by an insulating shroud which prevents accidental contact with the terminal.
Shunt - A device which allows electric current to pass around another point in the circuit.
Shunt regulator - A voltage regulator which uses a transistor or FET, in parallel with the load, which shorts out the excess voltage when the applied input voltage exceeds a specified limit producing a regulated output voltage. It is a simple but lossy design.
Shuttlecock cell - A lithium ion cell.
Sintering - Heating a mixture of powdered metals, sometimes under pressure, to the melting-point of the metal in the mixture which has the lowest melting-point, the melted metal binding together the harder particles.
SLA Battery - Sealed Lead Acid battery. In sealed batteries the generated oxygen combines chemically with the lead and then the hydrogen at the negative electrode, and then again with reactive agents in the electrolyte, to recreate water. A recombinant system. The net result is no significant loss of water from the cell. See also Flooded Lead Acid cell.
SLA - Equipment used for rapid prototyping. See StereoLithography Apparatus below.
SLI Battery - Common automotive battery used for Starting Lighting and Ignition
Slow charge - Charging overnight in 14 to 16 hours at about 0.1C rate. Safe and simple.
Smart Battery - An intelligent battery which contains information about its specification, its status and its usage profile which can be read by its charger or the application in which it is used.
SMBus - System Management Bus. A two wire, 100 KHz, serial bus for interconnecting Smart Batteries which have built in intelligence, with their associated chargers or applications.
Solar cell - A photovoltaic cell. Solar cells convert sunlight energy into electric current. They do not store energy.
Solar panel - An array of photocells providing an increased output.
Solenoid - A coil containing an iron plunger which moves when a current is passed through the coil.
Solid State Battery - Cells with solid electrolytes. Lithium polymer cells are examples of this technology
SOC - State of Charge. See below.
SOH - State of Health. See below.
Specific Energy - Same as Gravimetric Energy Density (Wh/Kg)
Specific Gravity SG - The ratio of the weight of a solution compared with the weight of an equal volume of water at a specified temperature. It is used to determine the charge condition in lead acid batteries.
Specific Power - Same as Gravimetric Power Density (W/Kg)
Spintronics - A technology used in solid state devices which exploits the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge. Also known as magnetoelectronics.
Spiral Wound - Battery construction in which the electrodes with the electrolyte and separator between them are rolled into a spiral like a jelly roll (Swiss roll).
Stacked Electrodes -
Standard charge - The normal C/10 charge used to recharge a cell or battery in 10 hours. Other definitions (charging periods) also apply.
Standby power - A fully charged battery ready to take over supplying a load in case of emergency.
Standby state - A state in which the main functions of a circuit have been powered down to save energy, but power remains applied to the circuit ready to make a rapid restart. (See also "Hibernation state" above)
State of Charge- SOC - The available capacity of a battery expressed as a percentage of its rated capacity.
State of Health- SOH - A measurement that reflects the general condition of a battery and its ability to deliver the specified performance compared with a fresh battery. It takes into account such factors as charge acceptance, internal resistance, voltage and self-discharge. It is not as precise as the SOC determination.
Stereolithography (SLA) - A Rapid Prototyping (RP) system for creating plastic parts directly from 3D CAD files. The RP model speeds design validation and is also finds use as a master pattern.
Stoichiometry - The branch of chemistry that deals with the numerical proportions in which substances react.
Storage life - The length of time a cell or battery can be stored on open circuit without permanent deterioration of its performance. See also Battery Storage
Studs - Threaded bolt connectors used on high power cells
Sulphation - Growth of lead sulphate crystals in Lead-Acid batteries which inhibits current flow. Sulphation is caused by storage at low state of charge.
Supercapacitor - A capacitor that can store a large amount of energy. Also called Ultracapacitor or Booster capacitor.
Superconductivity - A phenomenon occurring below a very low, characteristic critical temperature in certain materials (superconductors), characterised by the complete absence of electrical resistance and the damping of the interior magnetic field (the Meissner effect). Superconductors can carry currents that will not decay.
Swelling - Distortion of cells caused by expansion of the active chemicals due to temperature and pressure effects.
Swing cell - A lithium ion cell
Switcher - A switch mode regulator.
Switch mode charger - Charger which uses a switch mode regulator. More efficient but more costly than a Linear charger.
Switch mode regulator - A switching regulator is a voltage regulator which uses an output stage, switched repetitively on and off, together with energy storage components (capacitors and inductors) to generate a DC output voltage. Regulation is achieved through Pulse Width Modulation (PWM). Output voltages can be generated that are greater than or less than the input voltage, and multiple output voltages can be generated with a single regulator.

Tabs - Flat connectors used on pouch cells.
Tafel equation - The relationship between the internal electrode potentials in a battery and the current which flows. This is an exponential relationship based on empirical results which quantifies the elecrochemical reactions. It is analogous to the Arrhenius equationwhich quantifies the thermochemical process relating the temperature to the rate at which a chemical action progresses.
Taper charge - In quick chargers the charging current is is progressively reduced in a controlled way by controlling the supply voltage. In slow chargers the voltage is fixed and the charging current reduces in an uncontrolled way due to increase in the cell voltage as the charge builds up.
Temperature cut-off - A temperature sensing method which detects heat rise in a cell at overcharge and switches the charger off or to a lower rate of charge.
Temperature sensor - An electronic device which provides a voltage analogue of the temperature of the surface on which it is mounted. A thermistor is an example.
Termination voltage - The maximum voltage which can be tolerated by a cell during charging without damaging the cell. The cell voltage at which the charging process should be terminated.
Tesla (T) - The unit of magnetic flux density. 1 Tesla = 1 Weber / metre2
Thermal Capacity - The amount of energy required to raise the temperature of an object by one degree Celsius. Expressed in Joules/Kg.
Thermal fuse - A safety device which interrupts a circuit when it detects excessive temperature.
Thermal imaging - A photographic technique which displays the range of temperatures of a warm body in the form of a colour spectrum. Used as a design verification tool for detecting hot spots in battery and other equipment designs.
Thermal management - The means by which a battery is maintained within its operating temperature limits during charging and discharging.
Thermal runaway - A condition in which an electrochemical cell will overheat and destroy itself through internal heat generation. This may be caused by overcharge or high current discharge and other abusive conditions.
Thermistor - An electrical device whose resistance varies with temperature. They are used as temperature-measuring devices or in electrical circuits to compensate for temperature variations of other components.
Thyristor - Also called a Silicon-Controlled Rectifier or SCR, it is a solid-state high current semiconductor switching device similar to a diode, with an extra terminal which is used to turn it on. Once turned on, the thyristor will remain on (conducting) as long as there is a significant current flowing through it. If the current falls to zero, the device switches off. See also Power transistor.
Traction battery - A high power deep cycle secondary battery designed to power electric vehicles or heavy mobile equipment.
Transient response - The ability of an electrical or other device to respond faithfully to sudden changes to the input conditions.
Trickle charge - A continuous charge at low rate, balancing losses through local action and/or periodic discharge, to maintain a cell or battery in a fully charged condition. Normally at a C/20 to C/30 rate.
TÜV - TÜV Rheinland Group (TUV - Technical Inspection Association) is an international service company which documents the safety and quality of new and existing products, systems and services.

UL - Underwriters Laboratories Inc - (UL) is an independent, not-for-profit product safety testing and certification organization based in the USA. UL marking indicates that the product conforms with the safety standards laid down by Underwriters Laboratories.
Ultracapacitor - See "Supercapacitor" above.
Ultrasonic welding - Ultrasonic welding involves the use of high frequency sound energy to soften or melt the thermoplastic at the joint. Parts to be joined are held together under pressure and are then subjected to ultrasonic vibrations usually at a frequency of 20, 30 or 40kHz.
UPS - Uninterruptible Power Supply
USABC - The United States Advanced Battery Consortium

Valence - The combining capacity of an atom expressed as the number of single bonds the atom can form or the number of electrons an element gives up or accepts when reacting to form a compound.
Venting - The release of excessive internal pressure from a cell in a manner intended by design to preclude explosion.
Voltage cutoff - A voltage sensing device which will end a charge or discharge at a preset voltage value.
Voltage limit - A voltage value a battery is not permitted to rise above on charge and/or fall below on discharge
Voltage regulator - A circuit which provides a fixed or controlled voltage output from a variable voltage input. Used in power supplies and chargers. Switching regulators , Linear regulators and Shunt regulators are the most common types.
Voltaic efficiency - The ratio (expressed as a percentage) between the voltage necessary to charge a secondary cell and the corresponding discharge voltage.
Volumetric Energy Density (Wh/L) - The energy output per unit volume of a battery
Volumetric Power Density (W/L) -The power output per unit volume of a battery
VRLA battery - Valve Regulated Lead Acid Battery

Ward-Leonard controller - A motor-generator system which uses a AC motor driving a variable voltage DC generator which drives a DC motor to provide a variable power transmission. Used for high power load testing.
Watt - A unit of power, the rate of doing work. Watts = Amps X Volts = One Joule per second.
WattHours (Wh) - A measure of the energy capacity of a battery. The amount of work done in one hour.
1 Wh = 3.6 kJ.
Weber (Wb) - The unit of the magniude of the magnetic flux. A flux density of one Wb/m2 (1 Weber per square meter) is 1 Tesla
Well to wheel efficiency - The ratio between the mechanical energy ultimately delivered to the road wheels of a vehicle and the chemical energy content of the oil consumed in providing it. It is used to compare the fuel efficiencies of different methods of powering road vehicles and takes into account the refining process, the energy loss in the distribution process (in the case of hydrogen, the energy used to compress it) and the conversion efficiency of the vehicle's power unit.
Wet Cell - A cell with free flowing liquid electrolyte.

X-ray Crystallography - The use of the property of X-ray diffraction by crystals to determine their physical structure.


Zapping - A desperation measure to revive a shorted cell suffering from dendrites. A very high current, low voltage pulse from a large capacitor used in an attempt to vaporise the dendrites.
Zebra battery - A high temperature Sodium Nickel Chloride battery delivering high power.

Galvanic Cell Component

The basic components of a battery are the electrodes with terminals to connect to the external circuit, a separator to keep the electrodes apart and prevent them from shorting, the electrolyte which carries the charged ions between the electrodes and a case to contain the active chemicals and hold the electrodes in place.


Each galvanic or energy cell consists of at least three and sometimes four components

1.         The anode or negative electrode is the reducing or fuel electrode. It gives up electrons to the external circuit and is oxidised during the elecrochemical (discharge) reaction. It is generally a metal or an alloy but hydrogen is also used. The anodic process is the oxidation of the metal reducing agent to form metal ions.

( LEO Lose Electrons - Oxidation)


(OIL - Oxidation is Loss)

2.         The cathode or positive electrode is the oxidising electrode. It accepts electrons from the external circuit and is reduced during the electrochemical (discharge) reaction. It is usually an metallic oxide or a sulfide but oxygen is also used. The cathodic process is the reduction of the oxidising agent (oxide) to leave the metal.

(GER Gain Electrons - Reduction). Remember the mnemonic of the lion growling.


(RIG - Reduction is Gain) Alternative mnemonic - OIL RIG

3.         The electrolyte (the ionic conductor) which provides the medium for transfer of charge as ions inside the cell between the anode and cathode. The electrolyte is typically a solvent containing dissolved chemicals providing ionic conductivity. It should be a non-conductor of electrons to avoid self discharge of the cell.

Metal ions are metal atoms missing electrons and are thus positively charged. Particles missing electrons are called cations and during discharge they move through the electrolyte towards the cathode.

Anions are atoms or particles with excess electrons and thus negatively charged. During discharge they are attracted towards the anode.

4.         The separator which electrically isolates the positive and negative electrodes.



5.         Electrodes: The electrodes material may be a rigid metallic grids as in Lead acid batteries or the active electrode material may impregnated into or coated onto a spiral rolled metallic foil which simply acts as a current collector as in many Nickel and Lithium based cells. See also Battery Manufacturing

6.         Separator: The separator may be a mechanical spacer, fibreglass cloth or a flexible plastic film made from nylon, polyethylene or polypropylene. It must be porous and very thin to permit the charged ions to pass without impediment and it should take up the minimum of space to allow for the maximum use of the available space for the active chemicals. At the same time it must be resistant to penetration by burrs or dendrite growths on the electrode plates or from contamination of the electrode coating to prevent the possibility of short circuits between the electrodes. These characteristics should be maintained at high operating temperature when softening of the plastic material could clog the pores or reduce its resistance to penetration. The breakdown or penetration of the separator is a potential area of weakness in high power cells and special separator materials have been developed to overcome this problem.

7.         Terminals: There are many ways of connecting to the electrodes ranging from spring contacts, through wires or tags to mechanical studs. The main requirement is that the terminals should be able to handle the maximum current without overheating, either the terminal itself or the electrode connected to it. This needs careful design of the connection to the electrodes to take off the current through the maximum possible area of electrode material so as not to cause any hot spots. See also notes about external connections in the section on Battery Pack Design.

8.         Electrolyte: For many years all electrolytes were in aqueous or gel form. Recently solid polymer electrolytes have been developed which do not suffer from leakage or spillage. As well as being safer in case of an accident and they also bring new degrees of freedom to cell design allowing mechanical designs to be shaped to fit into odd shaped cavities. Polymer electrolytes are typically used in Lithium batteries.

Battery Energy Density


The energy density is a measure of the amount of energy per unit weight or per unit volume which can be stored in a battery. Thus for a given weight or volume a higher energy density cell chemistry will store more energy or alternatively for a given storage capacity a higher energy density cell will be smaller and lighter. The chart below shows some typical examples.

In general higher energy densities are obtained by using more reactive chemicals. The downside is that more reactive chemicals tend to be unstable and may require special safety precautions. The energy density is also dependent on the quality of the active materials used in cell construction with impurities limiting the cell capacities which can be achieved. This is why cells from different manufacturers with similar cell chemistries and similar construction may have a different energy content and discharge performance.


Note that there is often a difference between cylindrical and prismatic cells. This is because the quoted energy density does not usually refer to the chemicals alone but to the whole cell, taking into account the cell casing materials and the connections. Energy density is thus influenced or limited by the practicalities of cell construction.


Electrodes-Energy Power Trade-Offs

For a given cell chemistry and within the space available inside a given cell case, the cell performance can be optimised for capacity or power.

Increasing the surface area of the electrodes increases the cell's current handling capability. Thus the cell can both deliver more power and it can be charged more quickly.

Increasing the volume of electrolyte in the cell increases the cell's energy storage capacity.

The prime trade off is between the area of the electrodes and the volume of the electrolyte which can be contained within the volume available in the cell case.


High power cells require electrodes with a large surface area as well as enlarged current collectors which take up more of the available space within a given cell, displacing the electrolyte and reducing the cell capacity.


The effective surface area of an electrode can be increased without increasing its physical size by making its surface porous and using materials with very fine particle size. This can increase the effective surface area of the electrodes by 1000 to 100,000 times enabling higher current rates to be achieved.


High capacity cells require large volumes of electrolyte which must be accommodated between the electrodes. This has a double effect in reducing the cell power handling capability. First, the electrodes must be smaller and further apart to make space for the extra electrolyte and hence they can carry less current. Secondly, because of the increased volume of the electrolyte, it takes longer for the chemical actions associated with charging and discharging to propagate completely through the electrolyte to complete the chemical conversion process. (More details are given in the section on Charging Times).


As an example - Lithium Ion cells optimised for capacity may typically handle peak currents of 2C or 3C for short periods, whereas Lithium Ion cells optimised for power could possibly deliver pulsed currents of 30C to 40C.


Four of the most common constructions are shown below. Over the years there have been many thousands of variants of these basic types used for many different cell chemistries.


High power cells usually incorporate special safety devices. See "Designed in" safety measures.



Battery Definition

Lithium-ion Polymer

Lithium-ion polymer batteries use liquid Lithium-ion electrochemistry in a matrix of ion conductive polymers that eliminate free electrolyte within the cell. The electrolyte thus plasticizes the polymer, producing a solid electrolyte that is safe and leak resistant. Lithium polymer cells are often called Solid State cells.


Because there's no liquid, the solid polymer cell does not require the heavy protective cases of conventional batteries. The cells can be formed into flat sheets or prismatic (rectangular) packages or they can be made in odd shapes to fit whatever space is available. As a result, manufacturing is simplified and batteries can be packaged in a foil. This provides added cost and weight benefits and design flexibility. Additionally, the absence of free liquid makes Lithium-ion polymer batteries more stable and less vulnerable to problems caused by overcharge, damage or abuse.


Solid electrolyte cells have long storage lives, but low discharge rates.


There are some limitations on the cell construction imposed by the thicker solid electrolyte separator which limits the effective surface area of the electrodes and hence the current carrying capacity of the cell, but at the same time the added volume of electrolyte provides increased energy storage. This makes them ideal for use in high capacity low power applications.


Despite the above comments there are some manufacturers who make cells designated as Lithium polymer which actually contain a liquid or a gel. Such cells are more prone to swelling than genuine solid polymer cells.



Pouch cell - Also known as Lipo cells

Vulnerable, Inexpensive, Design freedom on dimensions, Difficult packaging, High energy density but reduced by support packaging needed, Prone to swell and leak, Less danger of explosion (cell bursts), Good heat dissipation, Made in very high volumes, Economical for small volumes, Sizes up to 240 Ah


Pouch casings are typically used for Lithium Polymer cells with solid electrolytes, providing a low cost "flexible" (sometimes in unintended ways) construction. The electrodes and the solid electrolyte are usually stacked in layers or laminations and enclosed in a foil envelope. The solid electrolyte permits safer, leak-proof cells. The foil construction allows very thin and light weight cell designs suitable for high power applications but because of the lack of rigidity of the casing the cells are prone to swelling as the cell temperature rises. Allowance must be made for the possibility of swelling when choosing cells to fit a particular cavity specified for the battery compartment. The cells are also vulnerable to external mechanical damage and battery pack designs should be designed to prevent such possibilities.

The Honcell example illustrated uses spiral wound electrodes and a solid polymer electrolyte.


This construction, using stacked electrodes is suitable for making odd shaped cells but few applications make use of this opportunity.

Battery Cost

The price of Lithium cells continues to fall as the technology gains more acceptance.

The target price for high power cells is around $300/kWh but cell makers are still quite some way from achieving that.

Although Lithium secondary batteries may cost two or three times more than the cost of equivalent Lead acid batteries and even more when the necessary battery management electronics are thaken into account, this is more than compensated for by their longer cycle life which may be five to ten times the life of Lead acid batteries. Valid cost comparisons should therefore take into account the lifetime costs as well as the initial capital costs.

Battery Connection in Series or Parallel

Serial and Parallel Battery Configurations

Battery packs achieve the desired operating voltage by connecting several cells in series, with each cell adding to the total terminal voltage. Parallel connection attains higher capacity for increased current handling, as each cell adds to the total current handling. Some packs may have a combination of serial and parallel connections. Laptop batteries commonly have four 3.6V Li-ion cells in series to achieve 14.4V and two strings of these 4 cells in parallel (for a pack total of 8 cells) to boost the capacity from 2,400mAh to 4,800mAh. Such a configuration is called 4S2P, meaning 4 cells are in series and 2 strings of these in parallel. Insulating foil between the cells prevents the conductive metallic skin from causing an electrical short. The foil also shields against heat transfer should one cell get hot.

Most battery chemistries allow serial and parallel configuration. It is important to use the same battery type with equal capacity throughout and never mix different makes and sizes. A weaker cell causes an imbalance. This is especially critical in a serial configuration and a battery is only as strong as the weakest link.

Imagine a chain with strong and weak links. This chain can pull a small weight but when the tension rises, the weakest link will break. The same happens when connecting cells with different capacities in a battery. The weak cells may not quit immediately but get exhausted more quickly than the strong ones when in continued use. On charge, the low cells fill up before the strong ones and get hot; on discharge the weak are empty before the strong ones and they are getting stressed.

Single Cell Applications

The single-cell design is the simplest battery pack. A typical example of this configuration is the cellular phone battery with a 3.6V lithium-ion cell. Other uses of a single cell are wall clocks, which typically use a 1.5V alkaline cell, as well as wristwatches and memory backup.

The nominal cell voltage of nickel is 1.2V. There is no difference between the 1.2V and 1.25V cell; the marking is simply preference. Whereas consumer batteries use 1.2V/cell as the nominal rating, industrial, aviation and military batteries adhere to the original 1.25V. The alkaline delivers 1.5V, silver-oxide 1.6V, lead acid 2V, primary lithium 3V, Li-phosphate 3.3V and regular lithium-ion 3.6V. Li-manganese and other lithium-based systems sometimes use 3.7V. This has nothing to do with electrochemistry and these batteries can serve as 3.6V cells. Manufacturers like to use a higher voltage because low internal resistance causes less of a voltage drop with a load. Read more: Confusion with Voltages

Serial Connection

Portable equipment needing higher voltages use battery packs with two or more cells connected in series. Figure 3-8 shows a battery pack with four 1.2V nickel-based cells in series to produce 4.8V. In comparison, a four-cell lead acid string with 2V/cell will generate 8V, and four Li-ion with 3.6V/cell will give 14.40V. If you need an odd voltage of, say, 9.5 volts, you can connect five lead acid, eight NiMH/NiCd), or three Li-ion in series. The end battery voltage does not need to be exact as long as it is higher than what the device specifies. A 12V supply should work; most battery-operated devices can tolerate some over-voltage.

Figure 1: Serial connection of four NiCd or NiMH cells 
Adding cells in a string increases the voltage; the current remains the same.

Courtesy of Cadex

A higher voltage has the advantage of keeping the conductor size small. Medium-priced cordless power tools run on 12V and 18V batteries; high-end power tools use 24V and 36V. The car industry talked about increasing the starter battery from 12V (14V) to 36V, better known as 42V, by placing 18 lead acid cells in series. Logistics of changing the electrical components and arcing problems on mechanical switches derailed the move. Early hybrid cars run on 148V batteries; newer models have batteries with 450–500V. Such a high-voltage battery requires 400 nickel-based cells in series. Li-ion cuts the cell count by three.

High-voltage batteries require careful cell matching, especially when drawing heavy loads or when operating in cold temperatures. With so many cells in series, the possibility of one failing is real. One open cell would break the circuit and a shorted one would lower the overall voltage.

Cell matching has always been a challenge when replacing a faulty cell in an aging pack. A new cell has a higher capacity than the others, causing an imbalance. Welded construction adds to the complexity of repair and for these reasons, battery packs are commonly replaced as a unit when one cell fails. High-voltage hybrid batteries, in which a full replacement would be prohibitive, divide the pack into blocks, each consisting of a specific number of cells. If one cell fails, the affected block is replaced.

Figure 2 illustrates a battery pack in which “cell 3” produces only 0.6V instead of the full 1.2V. With depressed operating voltage, this battery reaches the end-of-discharge point sooner than a normal pack and the runtime will be severely shortened. The remaining three cells are unable to deliver their stored energy when the equipment cuts off due to low voltage. The cause of cell failure can be a partial short cell that consumes its own charge from within through elevated self-discharge, or a dry-out in which the cell has lost electrolyte by a leak or through inappropriate usage.

Figure 2: Serial connection with one faulty cell
Faulty “cell 3” lowers the overall voltage from 4.8V to 4.2V, causing the equipment to cut off prematurely. The remaining good cells can no longer deliver the energy.

Courtesy of Cadex

Parallel Connection

If higher currents are needed and larger cells with increased ampere-hour (Ah) ratings are not available or the design has constraints, one or more cells are connected in parallel. Most chemistries allow parallel configurations with little side effect. Figure 3 illustrates four cells connected in parallel. The voltage of the illustrated pack remains at 1.2V, but the current handling and runtime are increased fourfold.



Figure 3: Parallel connection of four cells

With parallel cells, the current handling and runtime increases while voltage stays the same.

Courtesy of Cadex

A high-resistance cell, or one that is open, is less critical in a parallel circuit than in serial configuration, however, a weak cell reduces the total load capability. It’s like an engine that fires on only three cylinders instead of all four. An electrical short, on the other hand, could be devastating because the faulty cell would drain energy from the other cells, causing a fire hazard. Most so-called shorts are of mild nature and manifest themselves in elevated self-discharge. Figure 4 illustrates a parallel configuration with one faulty cell.


Figure 4: Parallel/connection with one faulty cell

A weak cell will not affect the voltage but will provide a low runtime due to reduced current handling. A shorted cell could cause excessive heat and become a fire hazard.

Courtesy of Cadex

Serial/Parallel Connection

The serial/parallel configuration shown in Figure 5 allows superior design flexibility and achieves the wanted voltage and current ratings with a standard cell size. The total power is the product of voltage times current, and the four 1.2V/1000mAh cells produce 4.8Wh. Serial/parallel connections are common with lithium-ion, especially for laptop batteries, and the built-in protection circuit must monitor each cell individually. Integrated circuits (ICs) designed for various cell combinations simplify the pack design.



Figure 5: Serial/ parallel connection of four cells

This configuration provides maximum design flexibility.

Courtesy of Cadex

Simple Guidelines for Using Household Primary Batteries

  • Keep the battery contacts clean. A four-cell configuration has eight contacts (cell to holder and holder to next cell); each contact adds resistance.
  • Never mix batteries; replace all cells when weak. The overall performance is only as good as the weakest link in the chain.
  • Observe polarity. A reversed cell subtracts rather than adds to the cell voltage.
  • Remove batteries from the equipment when no longer in use to prevent leakage and corrosion. While spent alkaline normally do not leak, spent carbon-zinc discharge corrosive acid that can destroy the device.
  • Don’t store loose cells in a metal box. Place individual cells in small plastic bags to prevent an electrical short. Don’t carry loose cells in your pockets.
  • Keep batteries away from small children. If swallowed, the current flow of the battery can ulcerate the stomach wall.The battery can also rupture and cause poisoning.
  • Do not recharge non-rechargeable batteries; hydrogen buildup can lead to an explosion. Perform experimental charging only under supervision.

Simple Guidelines for Using Household Secondary Batteries

  • Observe polarity when charging a secondary cell. Reversed polarity can cause an electrical short that can lead to heat and fire if left unattended.
  • Remove fully charged batteries from the charger. A consumer charger may not apply the optimal trickle charge and the cell could be stressed with overcharge.


Charging Battery From a USB Port

Charging from a USB Port

The Universal Serial Bus (USB) was introduced in 1996 and has since become one of the most widespread and convenient interfaces for electronic devices. The USB port is a bi-directional data port that provides a supply voltage to power memory sticks, keyboards, mice, wireless interfaces, cameras, MP3 players and chargers.

With 5V and 500mA of available current, the USB bus can charge a small single-cell Li-ion pack, but there is a danger of overloading the USB hub when attaching too many gadgets. Plugging in a charger that draws 500mA along with other devices will exceed the port’s current limit, leading to a voltage drop and a possible system failure. To prevent overload, some hosts include current-limiting circuits that shut down the supply when overdrawn. Another method is limiting the current of all attachments to 400mA to reserve 100mA for housekeeping.

The most common USB chargers are designed for single-cell Li-ion. The charge begins with a constant current charge to 4.20V/cell, at which point the voltage caps and the current begins to decrease. Due to a voltage drop in the cable, which is about 350mV, and losses in the charger circuit, it is possible that the 5V supply cannot supply the battery’s 4.2V charge threshold. This is no problem; the battery does not suffer but will deliver shorter than expected runtimes.

The rectangular Type A USB plug has four connector pins and a shield. The rightmost contact is number 1 and carries 5V; the leftmost contact is number 4 and forms the ground. The two shorter pins in the middle are reserved for data transfer and have no function in the USB charger. Figure 1 illustrates the rectangular Type A USB plug.

Figure 1: Rectangular Type A USB plug

The rightmost contact is number 1 and carries +5VDC; the leftmost pin is number 4 and is the ground. The housing connects to the ground and provides shielding. Pins 2 and 3 carry data.

With the USB supply current limited to 500mAh, doing active work on a laptop or watching a video on a tablet with a bright screen can result in a net discharge. A larger internal load than what the charger can provide will gradually drain the battery; however, the pack will replenish itself when the activity ends. 

Some USB chargers plugging into the AC main or the cigarette lighter of a car deliver higher peak currents than 500mAh. This allows connecting several devices via a USB bar without causing overload.

Note that the USB port is unidirectional and cannot take power from an outside source. In other words, power only flows out.

Battery Chargers and Charging Method

Charging Schemes

The charger has three key functions

·                     Getting the charge into the battery (Charging)

·                     Optimising the charging rate (Stabilising)

·                     Knowing when to stop (Terminating)


The charging scheme is a combination of the charging and termination methods.


Charge Termination

Once a battery is fully charged, the charging current has to be dissipated somehow. The result is the generation of heat and gasses both of which are bad for batteries. The essence of good charging is to be able to detect when the reconstitution of the active chemicals is complete and to stop the charging process before any damage is done while at all times maintaining the cell temperature within its safe limits. Detecting this cut off point and terminating the charge is critical in preserving battery life. In the simplest of chargers this is when a predetermined upper voltage limit, often called the termination voltage has been reached. This is particularly important with fast chargers where the danger of overcharging is greater.


Safe Charging

If for any reason there is a risk of over charging the battery, either from errors in determining the cut off point or from abuse this will normally be accompanied by a rise in temperature. Internal fault conditions within the battery or high ambient temperatures can also take a battery beyond its safe operating temperature limits. Elevated temperatures hasten the death of batteries and monitoring the cell temperature is a good way of detecting signs of trouble from a variety of causes. The temperature signal, or a resettable fuse, can be used to turn off or disconnect the charger when danger signs appear to avoid damaging the battery. This simple additional safety precaution is particularly important for high power batteries where the consequences of failure can be both serious and expensive.


Charging Times

During fast charging it is possible to pump electrical energy into the battery faster than the chemical process can react to it, with damaging results.

The chemical action can not take place instantaneously and there will be a reaction gradient in the bulk of the electrolyte between the electrodes with the electrolyte nearest to the electrodes being converted or "charged" before the electrolyte further away. This is particularly noticeable in high capacity cells which contain a large volume of electrolyte.

There are in fact at least three key processes involved in the cell chemical conversions.

·                     One is the "charge transfer", which is the actual chemical reaction taking place at the interface of the electrode with the electrolyte and this proceeds relatively quickly.

·                     The second is the "mass transport" or "diffusion" process in which the materials transformed in the charge transfer process are moved on from the electrode surface, making way for further materials to reach the electrode to take part in the transformation process. This is a relatively slow process which continues until all the materials have been transformed.

·                     The charging process may also be subject to other significant effects whose reaction time should also be taken into account such as the "intercalation process" by which Lithium cells are charged in which Lithium ions are inserted into the crystal lattice of the host electrode. See also Lithium Plating due to excessive charging rates or charging at low temperatures.

All of these processes are also temperature dependent.


In addition there may be other parasitic or side effects such as passivation of the electrodes, crystal formation and gas build up, which all affect charging times and efficiencies, but these may be relatively minor or infrequent, or may occur only during conditions of abuse. They are therefore not considered here.


The battery charging process thus has at least three characteristic time constants associated with achieving complete conversion of the active chemicals which depend on both the chemicals employed and on the cell construction. The time constant associated with the charge transfer could be one minute or less, whereas the mass transport time constant can be as high as several hours or more in a large high capacity cell. This is one of the the reasons why cells can deliver or accept very high pulse currents, but much lower continuous currents.(Another major factor is the heat dissipation involved). These phenomena are non linear and apply to the discharging process as well as to charging. There is thus a limit to the charge acceptance rate of the cell. Continuing to pump energy into the cell faster than the chemicals can react to the charge can cause local overcharge conditions including polarisation, overheating as well as unwanted chemical reactions, near to the electrodes thus damaging the cell. Fast charging forces up the rate of chemical reaction in the cell (as does fast discharging) and it may be necessary to allow "rest periods" during the charging process for the chemical actions to propagate throughout the bulk of the chemical mass in the cell and to stabilise at progressive levels of charge.


See also the affects of Chemical Changes and Charging Rate in the section on Battery Life.


A memorable though not quite equivalent phenomenon is the pouring of beer into a glass. Pouring very quickly results in a lot of froth and a small amount of beer at the bottom of the glass. Pouring slowly down the side of the glass or alternatively letting the beer settle till the froth disperses and then topping up allows the glass to be filled completely.



The time constants and the phenomena mentioned above thus give rise to hysteresis in the battery. During charging the chemical reaction lags behind the application of the charging voltage and similarly, when a load is applied to the battery to discharge it, there is a delay before the full current can be delivered through the load. As with magnetic hysteresis, energy is lost during the charge discharge cycle due to the chemical hysteresis effect.


The diagram below shows the hystersis effect in a Lithium battery.



Allowing short settling or rest periods during the charge discharge processes to accommodate the chemical reaction times will tend to reduce but not eliminte the voltage difference due to hysteresis.

The true battery voltage at any state of charge (SOC) when the battery is in its "at rest" or quiecent condition will be somewhere between the charge and discharge curves. During charging the measured cell voltage during a rest period will migrate slowly downwards towards the quiescent condition as the chemical transformation in the cell stabilises. Similarlly during discharging, the measured cell voltage during a rest period will migrate upwards towards the quescent condition.


Fast charging also causes increased Joule heating of the cell because of the higher currents involved and the higher temperature in turn causes an increase in the rate of the chemical conversion processes.


The section on Discharge Rates shows how the effective cell capacity is affected by the discharge rates.

The section on Cell Construction describes how the cell designs can be optimised for fast charging.


Charge Efficiency

This refers to the properties of the battery itself and does not depend on the charger. It is the ratio (expressed as a percentage) between the energy removed from a battery during discharge compared with the energy used during charging to restore the original capacity. Also called the Coulombic Efficiency or Charge Acceptance.


Charge acceptance and charge time are considerably influenced by temperature as noted above. Lower temperature increases charge time and reduces charge acceptance.


Note that at low temperatures the battery will not necessarily receive a full charge even though the terminal voltage may indicate full charge. See Factors Influencing State of Charge.


Basic Charging Methods

·                     Constant Voltage A constant voltage charger is basically a DC power supply which in its simplest form may consist of a step down transformer from the mains with a rectifier to provide the DC voltage to charge the battery. Such simple designs are often found in cheap car battery chargers. The lead-acid cells used for cars and backup power systems typically use constant voltage chargers. In addition, lithium-ion cells often use constant voltage systems, although these usually are more complex with added circuitry to protect both the batteries and the user safety.

·                     Constant Current Constant current chargers vary the voltage they apply to the battery to maintain a constant current flow, switching off when the voltage reaches the level of a full charge. This design is usually used for nickel-cadmium and nickel-metal hydride cells or batteries.

·                     Taper Current This is charging from a crude unregulated constant voltage source. It is not a controlled charge as in V Taper above. The current diminishes as the cell voltage (back emf) builds up. There is a serious danger of damaging the cells through overcharging. To avoid this the charging rate and duration should be limited. Suitable forSLAbatteries only.

·                     Pulsed charge Pulsed chargers feed the charge current to the battery in pulses. The charging rate (based on the average current) can be precisely controlled by varying the width of the pulses, typically about one second. During the charging process, short rest periods of 20 to 30 milliseconds, between pulses allow the chemical actions in the battery to stabilise by equalising the reaction throughout the bulk of the electrode before recommencing the charge. This enables the chemical reaction to keep pace with the rate of inputting the electrical energy. It is also claimed that this method can reduce unwanted chemical reactions at the electrode surface such as gas formation, crystal growth and passivation. (See also Pulsed Charger below). If required, it is also possible to sample the open circuit voltage of the battery during the rest period.


The optimum current profile depends on the cell chemistry and construction.


·                     Burp charging Also called Reflex or Negative Pulse Charging Used in conjunction with pulse charging, it applies a very short discharge pulse, typically 2 to 3 times the charging current for 5 milliseconds, during the charging rest period to depolarise the cell. These pulses dislodge any gas bubbles which have built up on the electrodes during fast charging, speeding up the stabilisation process and hence the overall charging process. The release and diffusion of the gas bubbles is known as "burping". Controversial claims have been made for the improvements in both the charge rate and the battery lifetime as well as for the removal of dendrites made possible by this technique. The least that can be said is that "it does not damage the battery".

·                     IUI Charging This is a recently developed charging profile used for fast charging standard flooded lead acid batteries from particular manufacturers. It is not suitable for all lead acid batteries. Initially the battery is charged at a constant (I) rate until the cell voltage reaches a preset value - normally a voltage near to that at which gassing occurs. This first part of the charging cycle is known as the bulk charge phase. When the preset voltage has been reached, the charger switches into the constant voltage (U) phase and the current drawn by the battery will gradually drop until it reaches another preset level. This second part of the cycle completes the normal charging of the battery at a slowly diminishing rate. Finally the charger switches again into the constant current mode (I) and the voltage continues to rise up to a new higher preset limit when the charger is switched off. This last phase is used to equalise the charge on the individual cells in the battery to maximise battery life. See Cell Balancing.

·                     Trickle charge Trickle charging is designed to compensate for the self discharge of the battery. Continuous charge. Long term constant current charging for standby use. The charge rate varies according to the frequency of discharge. Not suitable for some battery chemistries, e.g. NiMH and Lithium, which are susceptible to damage from overcharging. In some applications the charger is designed to switch to trickle charging when the battery is fully charged.

·                     Float charge. The battery and the load are permanently connected in parallel across the DC charging source and held at a constant voltage below the battery's upper voltage limit. Used for emergency power back up systems. Mainly used with lead acid batteries.

·                     Random charging All of the above applications involve controlled charge of the battery, however there are many applications where the energy to charge the battery is only available, or is delivered, in some random, uncontrolled way. This applies to automotive applications where the energy depends on the engine speed which is continuously changing. The problem is more acute in EV and HEV applications which use regenerative braking since this generates large power spikes during braking which the battery must absorb. More benign applications are in solar panel installations which can only be charged when the sun is shining. These all require special techniques to limit the charging current or voltage to levels which the battery can tolerate.


Charging Rates

Batteries can be charged at different rates depending on the requirement. Typical rates are shown below:

·                     Slow Charge = Overnight or 14-16 hours charging at 0.1C rate

·                     Quick Charge = 3 to 6 Hours charging at 0.3C rate

·                     Fast Charge = Less than 1 hour charging at 1.0C rate


Slow charging

Slow charging can be carried out in relatively simple chargers and should not result in the battery overheating. When charging is complete batteries should be removed from the charger.

·                     Nicads are generally the most robust type with respect to overcharging and can be left on trickle charge for very long periods since their recombination process tends to keep the voltage down to a safe level. The constant recombination keeps internal cell pressure high, so the seals gradually leak. It also keeps the cell temperature above ambient, and higher temperatures shorten life. So life is still better if you take it off the charger.

·                     Lead acid batteries are slightly less robust but can tolerate a short duration trickle charge. Flooded batteries tend to use up their water, and SLAs tend to die early from grid corrosion. Lead-acids should either be left sitting, or float-charged (held at a constant voltage well below the gassing point).

·                     NiMH cells on the other hand will be damaged by prolonged trickle charge.

·                     Lithium ion cells however can not tolerate overcharging or overvoltage and the charge should be terminated immediately when the upper voltage limit is reached.


Fast / Quick Charging

As the charging rate increases, so do the dangers of overcharging or overheating the battery. Preventing the battery from overheating and terminating the charge when the battery reaches full charge become much more critical. Each cell chemistry has its own characteristic charging curve and battery chargers must be designed to detect the end of charge conditions for the specific chemistry involved. In addition, some form of Temperature Cut Off (TCO) or Thermal Fuse must be incorporated to prevent the battery from overheating during the charging process.


Fast charging and quick charging require more complex chargers. Since these chargers must be designed for specific cell chemistries, it is not normally possible to charge one cell type in a charger that was designed for another cell chemistry and damage is likely to occur. Universal chargers, able to charge all cell types, must have sensing devices to identify the cell type and apply the appropriate charging profile.


Note that for automotive batteries the charging time may be limited by the available power rather than the battery characteristics. Domestic 13 Amp ring main circuits can only deliver 3KW. Thus, assuming no efficiency loss in the charger, a ten hour charge will at maximum put 30 KWh of energy into the battery. Enough for about 100 miles. Compare this with filling a car with petrol.

It takes about 3 minutes to put enough chemical energy into the tank to provide 90 KWh of mechanical energy, sufficient to take the car 300 miles. To put 90 KWh of electrical energy into a battery in 3 minutes would be equivalent to a charging rate of 1.8 MegaWatts!!


Charge Termination Methods

The following chart summarises the charge termination methods for popular batteries. These are explained in the section below.




Charge Termination Methods





Slow Charge

Trickle OK

Tolerates Trickle


Voltage Limit

Fast Charge 1




Imin at Voltage Limit

Fast Charge 2

Delta TCO




Back up Termination 1





Back up Termination 2






TCO = Temperature Cut Off

Delta TCO = Temperature rise above ambient

I min = Minimum current


Charge Control Methods

Many different charging and termination schemes have been developed for different chemistries and different applications. The most common ones are summarised below.


Controlled charging

Regular (slow) charge

·     Semi constant current Simple and economical. Most popular. Low current therefore does not generate heat but is slow, 5 to 15 hours typical. Charge rate 0.1C. Suitable for Nicads

·     Timer controlled charge system Simple and economical. More reliable than semi-constant current. Uses IC timer. Charges at 0.2C rate for a predetermined period followed by trickle charge of 0.05C. Avoid constantly restarting timer by taking the battery in and out of the charger since this will compromise its effectiveness. The incorporation of an absolute temperature cut-off is recommended. Suitable for Nicad and NiMH batteries.


Fast charge (1 to 2 hours)

·      Negative delta V (NDV) Cut-off charge system

This is the most popular method for rapid charging for Nicads.


Batteries are charged at constant current of between 0.5 and 1.0 C rate. The battery voltage rises as charging progresses to a peak when fully charged then subsequently falls. This voltage drop, -delta V, is due to polarisation or oxygen build up inside the cell which starts to occur once the cell is fully charged. At this point the cell enters the overcharge danger zone and the temperature begins to rise rapidly since the chemical changes are complete and the excess electrical energy is converted into heat. The voltage drop occurs regardless of the discharge level or ambient temperature and it can therefore be detected and used to identify the peak and hence to cut off the charger when the battery has reached its full charge or switch to trickle charge.

This method is not suitable for charging currents less than 0.5 C since delta V becomes difficult to detect. False delta V can occur at the start of the charge with excessively discharged cells. This is overcome by using a timer to delay the detection of delta V sufficiently to avoid the problem. Lead acid batteries do not demonstrate a voltage drop on charge completion hence this charging method is not suitable forSLAbatteries.


·     dT/dt Charge system NiMH batteries do not demonstrate such a pronounced NDV voltage drop when they reach the end of the charging cycle as can be seen in the graph above and so the NDV cut off method is not reliable for ending the NiMH charge. Instead the charger senses the rate of increase of the cell temperature per unit time. When a predetermined rate is reached the rapid charge is stopped and the charge method is switched to trickle charge. This method is more expensive but avoids overcharge and gives longer life. Because extended trickle charging can damage a NiMH battery, the use of a timer to regulate the total charging time is recommended.


·     Constant-current Constant-voltage (CC/CV) controlled charge system. Used for charging Lithium and some other batteries which may be vulnerable to damage if the upper voltage limit is exceeded. The manufacturers' specified constant current charging rate is the maximum charging rate which the battery can tolerate without damaging the battery. Special precautions are needed to maximise the charging rate and to ensure that the battery is fully charged while at the same time avoiding overcharging. For this reason it is recommended that the charging method switches to constant voltage before the cell voltage reaches its upper limit. Note that this implies that chargers for Lithium Ion cells must be capable of controlling both the charging current and the battery voltage.

In order to mainain the specified constant current charging rate, the charging voltage must increase in unison with the cell voltage to overcome the back EMF of the cell as it charges up. This occurs quite rapidly during the constant current mode until the cell upper voltage limit of the cell is reached, after which point the charging voltage is maintained at that level, known as the float level, during the constant voltage mode. During this constant voltage period, the current decreases to a trickle charge as the charge approaches completion. Cut off occurs when a predetermined minimum current point, which indicates a full charge, has been reached. See also Lithium Batteries - Charging and Battery Manufacturing - Formation.


Note 1: When Fast Charging rates are specified, they usually refer to the constant current mode. Depending on the cell chemistry this period could be between 60% and 80% of the time to full charge. These rates should not be extrapolated to estimate the time to fully charge the battery because the charging rate tails off quickly during the constant voltage period.

Note 2: Because it is not possible to charge Lithium batteries at the charging C rate specified by the manufacturers for the full duration of the charge, it is also not possible to estimate the time to charge a battery from empty simply by dividing the AmpHour capacity of the battery by the specified charging C rate, since the rate changes during the charging process. The following equation however gives a reasonable approximation of the time to fully charge an empty battery when the standard CC/CV charging method is used:

Charging time (hrs) = 1.3 * (Batterycapacity in Ah) / (CC mode charging current)


·     Voltage controlled charge system. Fast charging at rates between 0.5 and 1.0 C rate. The charger switched off or switched to trickle charge when predetermined voltage has been reached. Should be combined with temperature sensors in the battery to avoid overcharge or thermal runaway.

·     V- Taper controlled charge system Similar to Voltage controlled system. Once a predetermined voltage has been reached the rapid charge current is progressively reduced by reducing the supply voltage then switched to trickle charge. Suitable forSLAbatteries it allows higher charge level to be reached safely. (See also taper current below)

·     Failsafe timer

Limits the amount of charge current that can flow to double the cell capacity. For example for a 600mAh cell, limit the charge to a maximum of 1,200mAH. Last resort if cut off not achieved by other means.

·     Pre-charging

As a safety precaution with high capacity batteries a pre-charging stage is often used. The charging cycle is initiated with a low current. If there is no corresponding rise in the battery voltage it indicates that there is possibly a short circuit in the battery.

·     Intelligent Charging System
Intelligent charging systems integrate the control systems within the charger with the electronics within the battery to allow much finer control over the charging process. The benefits are faster and safer charging and battery longer cycle life. Such a system is described in the section on Battery Management Systems.



Most chargers provided with consumer electronics devices such as mobile phones and laptop computers simply provide a fixed voltage source. The required voltage and current profile for charging the battery is provided (or should be provided) from electronic circuits, either within the device itself or within the battery pack, rather than by the charger. This allows flexibility in the choice of chargers and also serves to protect the device from potential damage from the use of inappropriate chargers.


Voltage Sensing

During charging, for simplicity, the battery voltage is usually measured across the charger leads. However for high current chargers, there can be a significant voltage drop along the charger leads, resulting in an underestimate of the true battery voltage and consequent undercharging of the battery if the battery voltage is used as the cut-off trigger. The solution is to measure the voltage using a separate pair of wires connected directly across the battery terminals. Since the voltmeter has a high internal impedance there will be minimal voltage drop in the voltmeter leads and the reading will be more accurate. This method is called a Kelvin Connection. See also DC Testing.


Charger Types

Chargers normally incorporate some form of voltage regulation to control the charging voltage applied to the battery. The choice of charger circuit technology is usually a price - performance trade off. Some examples follow:

·     Switch Mode Regulator (Switcher) - Uses pulse width modulation to control the voltage. Low power dissipation over wide variations in input and battery voltage. More efficient than linear regulators but more complex.
Needs a large passive LC (inductor and capacitor) output filter to smooth the pulsed waveform. Component size depends on curent handling capacity but can be reduced by using a higher switching frequency, typically 50 kHz to 500kHz., since the size of the required transformers, inductors and capacitors is inversely proportional to the operating frequency.
Switching heavy currents gives rise to EMI and electrical noise.

·     Series Regulator (Linear) - Less complex but more lossy - requiring a heat sink to dissipate the heat in the series, voltage dropping transistor which takes up the difference between the supply and the output voltage. All the load current passes through the regulating transistor which consequently must be a high power device. Because there is no switching, it delivers pure DC and doesn't need an output filter. For the same reason, the design doesn't suffer from the problem of radiated and conducted emissions and electrical noise. This makes it suitable for low noise wireless and radio applications.
With fewer components they are also smaller.

·     Shunt Regulator - Shunt regulators are common in photovoltaic (PV) systems since they are relatively cheap to build and simple to design. The charging current is controlled by a switch or transistor connected in parallel with the photovoltaic panel and the storage battery. Overcharging of the battery is prevented by shorting (shunting) the PV output through the transistor when the voltage reaches a predetermined limit. If the battery voltage exceeds the PV supply voltage the shunt will also protect the PV panel from damage due to reverse voltage by discharging the battery through the shunt. Series regulators usually have better control and charge characteristics.

·     Buck Regulator A switching regulator which incorporates a step down DC-DC converter. They have high efficiency and low heat losses. They can handle high output currents and generate less RF interference than a conventional switch mode regulator. A simple transformerless design with low switch stress and a small output filter.

·     Pulsed Charger. Uses a series transistor which can also be switched. With low battery voltages the transistor remains on and conducts the source current directly to the battery. As the battery voltage approaches the desired regulation voltage the series transistor pulses the input current to maintain the desired voltage. Because it acts as a switch mode supply for part of the cycle it dissipates less heat and because it acts as a linear supply part of the time the output filters can be smaller. Pulsing allows the battery time to stabilise (recover) with low increments of charge at progressively high charge levels during charging. During rest periods the polarisation of the cell is lowered. This process permits faster charging than possible with one prolonged high level charge which could damage the battery since it does not permit gradual stabilisation of the active chemicals during charging. Pulse chargers usually need current limiting on the input source for safety reasons, adding to the cost.

·     Universal Serial Bus (USB) Charger

The USB specification was developed by a group of computer and peripheral device manufacturers to replace a plethora of proprietary mechanical and electrical interconnection standards for transferring data between computers and external devices. It included a two wire data connection, a ground (earth) line and a 5 Volt power line provided by the host device (the computer) which was available to power the external devices. An unintended use of the USB port has been to provide the 5 Volt source not only to power peripheral devices directly, but also to charge any batteries installed in these external devices. In this case the peripheral device itself must incorporate the necessary charge control circuitry to protect the battery. The original USB standard specified a a data rata of 1.5 Mbits/sec and a maximum charging current of 500mA.

Power always flows from the host to the device, but data can flow in both directions. For this reason the USB host connector is mechanically different from the USB device connector and thus USB cables have different connectors at each end. This prevents any 5 Volt connection from an external USB source from being applied to the host computer and thus from possibly damaging the host machine.

Subsequent upgrades increased the standard data rates to 5 Gigabits/sec and the available current to 900 mA. However the popularity of the USB connection has led to a lot of non standard variants paricularly the use of the USB connector to provide a pure power source without the associated data connection. In such cases the USB port may simply incorporate a voltage regulator to provide the 5 Volts from a 12 Volt automotive power rail or a rectifier and regulator to provide the 5 Volts DC from the 110 Volts or 240 Volts AC mains supply with output currents up to 2100 mA. In both cases the device accepting the power has to provide the necessary charge control. Mains powered USB power supplies, often known as "dumb" USB chargers, may be incorporated into the body of the mains plugs or into separate USB receptacles in wall mounted AC power socket outlets.

See more about USB connections in the section on battery Data Buses.

·     Inductive Charging

Inductive charging does not refer to the charging process of the battery itself. It refers to the design of the charger. Essentially the input side of charger, the part connected to the AC mains power, is constructed from a transformer which is split into two parts. The primary winding of the transformer is housed in a unit connected to the AC mains supply, while the secondary winding of the transformer is housed in the same sealed unit which contains the battery, along with the rest of the conventional charger electronics. This allows the battery to be charged without a physical connection to the mains and without exposing any contacts which could cause an electric shock to the user.


A low power example is the electric toothbrush. The toothbrush and the charging base form the two-part transformer, with the primary induction coil contained in the base and the secondary induction coil and the electronics contained in the toothbrush. When the toothbrush is placed into the base, the complete transformer is created and the induced current in the secondary coil charges the battery. In use, the appliance is completely separated from the mains power and since the battery unit is contained in a sealed compartment the toothbrush can be safely immersed in water.


The technique is also used to charge medical battery implants.


A high power example is a charging system used for EVs. Similar to the toothbrush in concept but on a larger scale, it is also a non-contact system. An induction coil in the electric vehicle picks up current from an induction coil in the floor of the garage and charges the vehicle overnight. To optimise system efficiency, the air gap between the static coil and the pickup coil can be reduced by lowering the pickup coil during charging and the vehicle must be precisely placed over the charging unit.

A similar system has been used for electric buses which pick up current from induction coils embedded beneath each bus stop thus enabling the range of the bus to be extended or conversely, smaller batteries can be specified for the same itinerary. One other advantage of this system is that if the battery charge is constantly topped up, the depth of discharge can be minimised and this leads to a longer cycle life. As shown in the section on Battery Life, the cycle life increases exponentially as the depth of discharge is reduced.

A simpler and less expensive alternative to this opportunity charging is for the vehicle to make a conductive coupling with electric contacts on an overhead gantry at each bus stop.

Proposals have also been made to install a grid of inductive charging coils under the surface along the length of public roadways to allow vehicles to pick up charge as they drive along however no practical examples have yet been installed.


·    Electric Vehicle Charging Stations

For details about the specialised, high power chargers used for EVs, see the section about Electric Vehicle Charging Infrastructure.


Charger Power Sources

When specifying a charger it is also necessary to specify the source from which the charger derives its power, its availability and its voltage and power range. Efficiency losses in the charger should also be taken into account, particularly for high power chargers where the magnitude of the losses can be significant. Some examples are given below.


Controlled Charging

Easy to accommodate and manage.

·  AC Mains

Many portable low power chargers for small electrical appliances such as computers and mobile phones are required to operate in international markets. They therefore have auto sensing of the mains voltage and in special cases the mains frequency with automatic

switching to the appropriate input circuit. Higher power applications may need special arrangements. Single phase mains power is typically limited to about 3 KW. Three phase power may be required for charging high capacity batteries (over 20 KWh capacity) such as those used in electric vehicles which may require charging rates of greater than 3 KW to achieve reasonable charging times.

·  Regulated DCBatterySupply

May be provided by special purpose installations such as mobile generating equipment for custom applications.

·  Special Chargers

Portable sources such as solar panels.


Opportunity Charging

Opportunitycharging is charging the battery whenever power is available or between partial discharges rather than waiting for the battery to be completely discharged. It is used with batteries in cycle service, and in applications when energy is available only intermittently.

It can be subject to wide variations in energy availability and wide variations in power levels. Special control electronics are needed to protect the battery from overvoltage. By avoiding complete discharge of the battery, cycle life can be increased.

Availability affects the battery specification as well as the charger.

Typical applications are:

·  Onboard vehicle chargers (Alternators, Regenerative braking)

·  Inductive chargers (on vehicle route stopping points)

·  Solar power

·  Wind power


Mechanical charging

This is only applicable to specific cell chemistries. It is nor a charger technology in the normal sense of the word. Mechanical charging is used in some high power batteries such as Flow Batteries and Zinc Air batteries. Zinc air batteries are recharged by replacing the zinc electrodes. Flow batteries can be recharged by replacing the electrolyte.


Mechanical charging can be carried out in minutes. This is much quicker than the the long charging time associated with the conventional reversible cell electrochemistry which could take several hours. Zinc air batteries have therefore been used to power electric buses to overcome the problem of excessive charging times.


Charger Performance

The battery type and the application in which it is used set performance requirements which the charger must meet.

·  Output Voltage Purity

The charger should deliver a clean regulated voltage output with tight limits on spikes, ripple, noise and radio frequency interference (RFI) all of which could cause problems for the battery or the circuits in which it is used. For high power applications, the charging performance may be limited by the design of the charger.

· Efficiency

When charging high power batteries, the energy loss in the charger can add significantly to the charging times and to the operating costs of the application. Typical charger efficiencies are around 90%, hence the need for efficient designs.

· Inrush Current

When a charger is initially switched on to an empty battery the inrush current could be considerably higher than the maximum specified charging current. The charger must therefore be dimensioned either to deliver or limit this current pulse.

· Power Factor

This could also be an important consideration for high power chargers.




Battery Characteristics

Lithium is the lightest of metals and it floats on water. It also has the greatest electrochemical potential which makes it one of the most reactive of metals. These properties give Lithium the potential to achieve very high energy and power densities in high power battery applications such as automotive and standby power.


Many variations of the basic Lithium chemistry have been developed to optimise the cells for specific applications or perhaps in some cases to get around the patents on the original technology. Lithium metal reacts violently with water and can ignite into flame. Early commercial cells with metallic lithium cathodes were considered unsafe in certain circumstances, however modern cells don't use free Lithium but instead the Lithium is combined with other elements into more benign compounds which do not react with water.

The typical Lithium-ion cells use Carbon for its anode and Lithium Cobalt dioxide or a Lithium Manganese compound as the cathode. The electrolyte is usually based on a Lithium salt in an organic solvent.


Lithium batteries have now taken their place as the rechargeable battery of choice for portable consumer electronics equipment. Though they were expensive when introduced, volume production has brought the prices down.


See more details below.


Cell Nomenclature

There is considerable confusion about naming standards for cells with different systems used in Europe, the USA and Japan as well as manufacturers ' own standards.

One convention is two letters followed by a series if numbers.

The first letter represents the cell chemistry. The second letter represents the shape of the cell.

The numbers represent the dimensions of the cell in millimetres. For cylindrical cells the first two digits are the diameter and the remaining digits the length. For prismatic cells the first two digits represent the thickness, the second pair the height and the last pair the width.

Because of the plethora of "standards" the only safe course in identifying a cell is to consult the manufacturers' data sheets.


Common Primary Cells

See Battery Case Sizes for dimensions of common primary cells.

Cylindrical Cells

LC18650 is a common Li-ion cell in a Cylindrical can   Size (diameter18mm height 65.0mm)

See Cylindrical Cell Sizes for a listing of typical cylindrical cell sizes and capacities

Prismatic Cells

LP083448 is a Li-ion cell in a Prismatic can   Dimensions( thickness 8mm height 48mm width 34 mm)

See Prismatic Cell Sizes for a listing of typical prismatic cell sizes and capacities.


See Power Cell Sizes for examples of high power prismatic cells. (High power cylindrical cells are also available)


See also Battery Pack Design


Cell Balancing

Cell Balancing


In multi-cell batteries, because of the larger number of cells used, we can expect that they will be subject to a higher failure rate than single cell batteries. The more cells used, the greater the opportunities to fail and the worse the reliability.

Batteries such as those used for EV and HEV applications are made up from long strings of cells in series in order to achieve higher operating voltages of 200 to 300 Volts or more are particularly vulnerable. The problems can be compounded if parallel packs of cells are required to achieve the desired capacity or power levels. With a battery made up from n cells, the failure rate for the battery will be n times the failure rate of the individual cells.


All cells are not created equal

The potential failure rate is even worse than this however due to the possibility of interactions between the cells. Because of production tolerances, uneven temperature distribution and differences in the ageing characteristics of particular cells, it is possible that individual cells in a series chain could become overstressed leading to premature failure of the cell. During the charging cycle, if there is a degraded cell in the chain with a diminished capacity, there is a danger that once it has reached its full charge it will be subject to overcharging until the rest of the cells in the chain reach their full charge. The result is temperature and pressure build up and possible damage to the cell. With every charge - discharge cycle the weaker cells will get weaker until the battery fails. During discharging, the weakest cell will have the greatest depth of discharge and will tend to fail before the others. It is even possible for the voltage on the weaker cells to be reversed as they become fully discharged before the rest of the cells also resulting in early failure of the cell. Various methods of cell balancing have been developed to address this problem by equalising the stress on the cells.


Self Balancing

Unbalanced ageing is less of a problem with parallel chains which tend to be self balancing since the parallel connection holds all the cells at the same voltage and at the same time allows charge to move beween cells whether or not an external voltage is applied. There can however be problems with this cell configuration if a short circuit occurs in one of the cells since the rest of the parallel cells will discharge through the failed cell exacerbating the problem.


See Interactions Between Cells for more details.


The problems caused by these cell to cell differences are exaggerated when the cells are subject to the rapid charge and discharge cycles (microcycles) found in HEV applications.

While Lithium batteries are more tolerant of micro cycles they are less tolerant of the problems caused by cell to cell differences.


Because Lead acid and NiMH cells can withstand a level of over-voltage without sustaining permanent damage, a degree of cell balancing or charge equalisation can occur naturally with these technologies simply by prolonging the charging time since the fully charged cells will release energy by gassing until the weaker cells reach their full charge. This is not possible with Lithium cells which can not tolerate over-voltages. Although the problem is reduced with Lead acid NiMH batteries and some other cell chemistries, it is not completely eliminated and solutions must be found for most multicell applications.







No matter what battery management techniques are used, the failure rate or cycle life of a multicell battery will always be worse than the quoted failure rate or cycle life of the single cells used to make up the battery.








Once a cell has failed, the entire battery must be replaced and the consequences are extremely costly. Replacing individual failed cells does not solve the problem since the characteristics of a fresh cell would be quite different from the aged cells in the chain and failure would soon occur once more. Some degree of refurbishment is possible by cannibalising batteries of similar age and usage but it can never achieve the level of cell matching and reliability possible with new cells.


Equalisation is intended to prevent large long term unbalance rather than small short term deviations.


Cell selection

The first approach to solving this problem should be to avoid it if possible through cell selection. Batteries should be constructed from matched cells, preferably from the same manufacturing batch. Testing can be employed to classify and select cells into groups with tighter tolerance spreads to minimise variability within groups.


Large versus small cells

The high energy storage capacities needed for traction and other high power battery applications can be provided by using large high capacity cells or with large numbers of small cells connected in parallel to give the same capacity as the larger cells. In both cases the large cells, or the parallel blocks of small cells, must be connected in series to provide the required high battery voltage.


·                     Using large cells keeps the interconnections between cells to a minimum allowing simpler monitoring and control electronics and lower assembly costs. Until electric vehicles conquer a substantial percentage of the transportation market, the large cells they need will continue to be made in relatively small quantities, often with semi-automatic or manual production methods, resulting in high costs, wide process variability and the consequent wide performance tolerance spreads. When the cells are used in a serial chain, cell balancing is essential to equalise the stress on the cells, caused by these manufacturing variances, to avoid premature cell failures.


There are also safety issues associated with large capacity cells. A single 200 AmpHour Lithium Cobalt cell typically used in EV applications stores 2,664,000 Joules of energy. If a cell fails or is short circuited or damaged in an accident, this energy is suddenly released, often resulting in an explosion and an intense fire, known euphemistically as an “event” in the battery industry. When such an event occurs in a battery pack there is a strong likelihood that the fire and pressure damage resulting from a cell failure will cause neighbouring cells to fail in a similar way, ultimately affecting all of the cells in the pack with disastrous consequences.


·                     Using small cells connected in parallel to provide the same voltage and capacity as the larger cells results in many more interconnections, greater assembly costs and possibly more complex control electronics. Small, cylindrical, 2 or 3 AmpHour cells, such as the industry standard 18650 used in consumer electronics applications, are however made in volumes of hundreds of millions per year in much better controlled production facilities without manual intervention on highly automated equipment. The upside is that unit costs are consequently very low and reliability is much higher. When large numbers of cells are connected in a parallel block, the performance of the block will tend towards the process average of the component cells and the self balancing effect will tend to keep it there. The parallel blocks will still need to be connected in series to provide the higher battery voltage but the tolerance spread of the blocks in the series chain will be less than the tolerance spread of the alternative large capacity cells, leaving the cell balancing function with less work to do.


On the safety front, the more reliable low capacity cells are much less likely to fail and if a failure does occur, the stored energy released by any cell is only one hundredth of the energy released by a 200 AmpHour cell. This lower energy release is much easier to contain and the likelihood of the event propagating through the pack is much reduced or eliminated. This is perhaps the most important advantage of designs using lower capacity cells.


See also What a Joule can do


Pack construction

Another important avoidance action is to ensure at all times an even temperature distribution across all cells in the battery. Note that in an EV or HEV passenger car application, the ambient temperature in the engine compartment, the passenger compartment and the boot or trunk can be significantly different and dispersing the cells throughout the vehicle to spread the mechanical load can give rise to unbalanced thermal operating conditions. On the other hand, if the cells are concentrated in one large block, the outer cells in contact with ambient air may run cooler than the inner cells which are surrounded by warmer cells unless steps are taken to provide an air (or other coolant) flow to remove heat from the hotter cells. After cell selection, equalising the temperature across the battery pack should be the first design consideration in order to minimise the need for cell balancing. See also Thermal Management (Uniform heat distribution)


Cell equalisation

To provide a dynamic solution to this problem which takes into account the ageing and operating conditions of the cells, the BMS may incorporate a Cell Balancing scheme to prevent individual cells from becoming overstressed. These systems monitor the State of Charge (SOC) of each cell, or for less critical, low cost applications, simply the voltage across, each cell in the chain. Switching circuits then control the charge applied to each individual cell in the chain during the charging process to equalise the charge on all the cells in the pack. In automotive applications the system must be designed to cope with the repetitive high energy charging pulses such as those from regenerative braking as well as the normal trickle charging process.


Several Cell Balancing schemes have been proposed and there are trade-offs between the charging times, efficiency losses and the cost of components.


Active balancing

Active cell balancing methods remove charge from one or more high cells and deliver the charge to one or more low cells. Since it is impractical to provide independent charging for all the individual cells simultaneously, the balancing charge must be applied sequentially. Taking into account the charging times for each cell, the equalisation process is also very time consuming with charging times measured in hours. Some active cell balancing schemes are designed to halt the charging of the fully charged cells and continue charging the weaker cells till they reach full charge thus maximising the battery's charge capacity.

·                     Charge Shuttle (Flying Capacitor) Charge Distribution

With this method a capacitor is switched sequentially across each cell in the series chain. The capacitor averages the charge level on the cells by picking up charge from the cells with higher than average voltage and dumping the charge into cells with lower than average voltage. Alternatively the process can be speeded up by programming the capacitor to repeatedly transfer charge from the highest voltage cell to the lowest voltage cell. Efficiency is reduced as the cell voltage differences are reduced. The method is fairly complex with expensive electronics.

·                     Inductive Shuttle Charge Distribution

This method uses a transformer with its primary winding connected across the battery and a secondary winding which can be switched across individual cells. It is used to take pulses of energy as required from the full battery, rather than small charge differences from a single cell, to top up the remaining cells. It averages the charge level as with the Flying Capacitor but avoids the problem of small voltage differences in cell voltage and is consequently much faster. This system obviously needs well balanced secondary transformer windings otherwise it will contribute to the problem.

Passive balancing

Dissipative techniques find the cells with the highest charge in the pack, indicated by the higher cell voltage, and remove excess energy through a bypass resistor until the voltage or charge matches the voltage on the weaker cells. Some passive balancing schemes stop charging altogether when the first cell is fully charged, then discharge the fully charged cells into a load until they reach the same charge level as the weaker cells. Other schemes are designed continue charging till all the cells are fully charged but to limit the voltage which can be applied to individual cells and to bypass the cells when this voltage has been reached.

This method levels downwards and because it uses low bypass currents, equalisation times are very long. Pack performance determined by the weakest cell and is lossy due to wasted energy in the bypass resistors which could drain the battery if operated continuously. It is however the lowest cost option.

Charge Shunting

The voltage on all cells levelled upwards to the rated voltage of a good cell. Once the rated voltage on a cell has been reached, the full current bypasses fully charged cells until the weaker cells reach full voltage. This is fast and allows maximum energy storage however it needs expensive high current switches and high power dissipating resistors.

Charge limiting

A crude way of protecting the battery from the effects of cell imbalances is to simply switch off the charger when the first cell reaches the voltage which represents its fully charged state (4.2 Volts for most Lithium cells) and to disconnect the battery when the lowest cell voltage reaches its cut off point of 2 Volts during discharging. This will unfortunately terminate the charging before all of the cells have reached their full charge or cut off the power prematurely during discharge leaving unused capacity in the good cells. It thus reduces the effective capacity of the battery. Without the benefits of cell balancing, cycle life could also be reduced, however for well matched cells operating in an even temperature environment, the effect of these compromises could be acceptable.


All of these balancing techniques depend on being able to determine the state of charge of the individual cells in the chain. Several methods for determining the state of charge are described on the SOC page.

The simplest of these methods uses the cell voltage as an indication of the state of charge. The main advantage of this method is that it prevents overcharging of individual cells, however it can be prone to error. A cell may reach its cut off voltage before the others in the chain, not because it is fully charged but because its internal impedance is higher than the other cells. In this case the cell will actually have a lower charge than the other cells. It will thus be subject to greater stress during discharge and repeated cycling will eventually provoke failure of the cell.


More precise methods use Coulomb counting and take account of the temperature and age of the cell as well as the cell voltage.


Redox Shuttle (Chemical Cell Balancing)

In Lead acid batteries, overcharging causes gassing which coincidentally balances the cells. The Redox Shuttle is an attempt to provide chemical overcharge protection in Lithium cells using an equivalent method thus avoiding the need for electronic cell balancing. A chemical additive which undergoes reversible chemical action absorbing excess charge above a preset voltage is added to the electrolyte . The chemical reaction is reversed as voltage falls below the preset level.


For batteries with less than 10 cells, where low initial cost is the main objective, or where the cost of replacing a failed battery is not considered prohibitive, cell balancing is sometimes dispensed with altogether and long cycle life is achieved by restricting the permitted DOD. This avoids the cost and complexity of the cell balancing electronics but the trade off is inefficient use of cell capacity.


Whether or not the battery employs cell balancing, it should always incorporate fail safe cell protection circuits.



Battery Storage

Battery Storage


The optimum storage conditions for batteries depend on the active chemicals used in the cells. During storage the cells are subject to both self discharge and possible decomposition of the chemical contents. Over time solvents in the electrolyte may permeate through the seals causing the electrolyte to dry out and lose its effectiveness. In all cases these processes are accelerated by heat and it is wise to store the cells in a cool, benign environment to maximise their shelf life. The glove compartment of a car does not qualify as a suitable storage location since temperatures may exceed 60°C shortening dramatically the life of the battery. (See Battery Life)

For cells with the same nominal cell chemistry, individual manufacturers may add different additives to optimise their cell performance for a particular parameter and this may affect the behaviour of the cells during storage. It is possible to make some general recommendation about storage but the best guidance for storage is to consult the manufacturers' specifications and recommendations for their products. Some general guidelines for some common cell chemistries follow:



The possible storage temperature range for Lithium-Ion batteries is is -20°C to 60°C but for prolonged storage period -20°C to 25°C is recommended and 15°C is ideal. Cells should be stored with a partial charge of between 30% and 50%. Although the cells can be stored fully discharged the cell voltage should not drop below 2.0 Volts per cell and cells should be topped up to prevent over-discharge. The maximum voltage should not exceed 4.1 Volts


If secondary cells must be for a prolonged period the state of charge should be checked regularly and provision should be made for recharging the cells before the cell voltage drops below the recommended minimum after which the cells suffer irreparable deterioration. ( This is particularly true for battery packs which may have associated electronics which add to the self discharge drain on the cells)


Battery Safety

Battery Safety


Batteries have the potential to be dangerous if they are not carefully designed or if they are abused. Cell manufacturers are conscious of these dangers and design safety measures into the cells. Likewise, pack manufacturers incorporate safety devices into the pack designs to protect the battery from out of tolerance operating conditions and where possible from abuse. While they try to make the battery foolproof, it has often been explained how difficult this is because fools can be so ingenious. Once the battery has left the factory its fate is in the hands of the user. It is usual to provide "Instructions For Use" with battery products which alert the end user to potential dangers from abuse of the battery. Unfortunately there will always be perverse fools who regard these instructions as a challenge.



Subjecting a battery to abuse or conditions for which it was never designed can result in uncontrolled and dangerous failure of the battery. This may include explosion, fire and the emission of toxic fumes.


We are helped in assessing what hazards to protect against, and the degree of protection required, by the publication of national standards. Some of these are listed in the section on Standards. Typical safety test requirements are outlined in the section on Testing.


"Designed in" safety measures

These are not something that the battery applications engineer can control but they could influence the choice of cells to be specified for a particular application.

Cell chemistry - In the quest for ever higher energy and power densities cell makers have utilised ever more reactive chemical mixes, but these highly reactive properties which are needed to provide the higher energy densities are likely to increase the risk of danger in case of cell failures. For safety reasons the cell maker may compromise on the maximum power by using a less reactive chemical mix or by introducing some form of chemical retardant in order to reduce the risk of fire or explosion if a cell suffers physical damage. As an example, the original Lithium-ion cells used cathodes consisting of Lithium Cobalt oxides and these provide maximum power, however Lithium Manganese oxides and Lithium phosphate cells which have slightly lower power ratings are now the preferred choice for many applications because they are inherently safer if damaged. Lithium titanate anodes do not depend on an SEI layer for stability and are inherently safer, though at the cost of lower energy density.

Electrolytes - Chemical inhibitors are often added to electrolytes to make them self extinguishing or flame retarding in case of abuse of the cell which could lead to fire.

Cell construction - Low power cells have relatively simple mechanical structures which have undergone many years of development and cell failures caused by poor mechanical design a very rare. For high power cells however, thermal design can be a source of weakness. Getting the excess heat out of the cell can be a problem and poor designs can result in localised hotspots within the cell which can cause cell failure. Good thermal performance for high power cells requires substantial thermal conduction paths.

Separators - If for any reason a cell overheats, this can cause the separators, which are typically made of plastic, to distort or melt. In the worst case this could lead to a short circuit between the electrodes with even more serious consequences. Internal short circuits can also occur due to dendrite or crystal growth on the electrodes. External circuits can not protect against an internal short circuit and various separators have been designed to avoid this problem. These include

Rigid separators which do not distort even under extreme temperature conditions.

Flexible ceramic powder coated plastic which prevents contact between the electrodes, resists penetration by impurities, reduces shrinkage at high temperatures and impedes the propagation of a short circuit across the separator.

"Shut down separators" with special plastic formulations, similar to a resettable fuse, whose impedance suddenly increases when certain temperature limits are reached. The melting plastic in the shut down separator closes up its pores thus avoiding a short circuit but the action is not reversible.

Once an internal short occurs, there is not much that can be done by external measures to protect the battery. Such an occurrence can be detected by a sudden drop in the cell voltage and this can be used to trigger a cut off device to isolate the battery from the charger or the load. While it does not solve the problem at least it prevents external events from making it any worse. Fortunately an internal short circuit is a rare occurrence.

The likelihood of an internal short circuit occurring can be minimised by keeping the cell temperature within limits and this should be the user's first line of defence.

Robust packaging - As with cell construction this is unlikely to be a source of problems.

Circuit Interrupt Device (CID) - Some cells also incorporate a CID which interrupts the current if the internal gas pressure in the cell exceeds specified limits.

Safety vents - If other safety devices fail and a cell is allowed to overheat, chemical reactions can result in gassing and the active materials will also expand due to the temperature rise. This can cause a dangerous build up of pressure inside the cell which could result in rupture of the case or even an explosion. Safety vents are needed as a final safety precaution to release the pressure before it reaches a dangerous level. Automatic release guard vents prevent the absorption of external air into the cell but allow controlled release of excess internal pressure to avoid leakage and prevent uncontrolled rupture of the cell case.

Keyed and shrouded connectors or terminals - These are designed to protect the operator, to prevent accidental short circuits and the connection of incorrect loads or chargers to the battery. See Battery Pack Design (External connections)

All the designed-in safety precautions can be worthless if the manufacturing processes are not controlled properly. Burrs on the electrodes, misaligned or out of tolerance components, contaminated electrode coatings or electrolytes can all cause short circuits or penetration of the separator. A short circuit caused by a microscopic metallic particle may simply cause cause local overheating or an elevated self discharge rate due to a relatively high impedance current path between the electrodes, but a direct short circuit due to penetration of the separator by a burr on the electrode can lead to excessive overheating and eventually thermal runaway of the cell.


External Safety Devices

Protecting the cell from out of limits operating conditions, either from the loads imposed by the intended application or abuse by the user or from unsuitable charging regimes, is the job of the battery pack designer.


Heat is the biggest killer of batteries and this is most likely to be due to unsuitable charging methods or procedures. But chargers are not the only culprits. Overloading the cell during discharge also causes overheating. Many safety devices are therefore based on sensing the cell temperature and isolating the cell from its load or from the charger if the temperature reaches dangerous levels.

See the section on Chargers for more information about safe charging.

Heat damages a cell no matter what its source and a cell will suffer the same damage by being placed in a high ambient temperature environment as it would from improper use. There is no practical way to protect the cell from this kind of abuse. (Sounding an alarm could be a possibility)


Apart from damage from overheating, a battery may be damaged from excessive currents and from over and under voltage. Suitable protection methods and how they are implemented are described in detail in the section Battery Protection Methods


Short Circuits and their Consequences (What can a Joule do?)

Short circuiting a capacitor or a battery is definitely not recommended as the destructive power unleashed is often seriously under estimated.


As an example, a 0.1Farad capacitor charged to 14 Volts will store 10 Joules of energy (E = ½ CV2 ). This may not seem very much, it is only 10 Watt seconds, but it is enough to punch a hole through aluminium foil creating a lot of sparks. 30 Joules is enough to weld a wire to a ball bearing. This is because the discharge period is very short, almost instantaneous, resulting in a power transfer of hundreds of watts.


Batteries store even more energy. For comparison, a fully charged 3.6 Volt, 1000 mAh mobile phone battery has a low internal impedance and contains 12,960 Joules of energy. Short circuiting these cells can cause extremely high currents and temperatures within the cell resulting in the breakdown of the chemical compounds from which it is made. This in turn can cause the rapid build up of pressure within the cell resulting in its catastrophic failure, with unpredictable consequences including the uncontrolled rupture of the cell or even fire.

By the same token, a single, fully charged 200 Ah, 3.6 Volt Lithium Ion automotive cell (or the similar capacity from any other cell chemistry) contains 2,592,000 Joules of energy. Don't wait around to see what happens if you drop a wrench on the terminals!!




Battery (and User) Protection System

The diagram below summarises the types of problems which can occur in Lithium energy cells and their consequences together with the actions which may be taken by the Battery Management System (BMS) to address the problems and the results of the actions.


Cell Protection Mechanisms



See also Why Batteries Fail


Multi Level Battery Safety Plan

The responsibility for battery safety starts at the cell maker's premises and continues through to the design of the battery application. A multi-level safety plan should include consideration of at least the following components.

Begin with intrinsically safe cell chemistry

Designed in safety measures (See above)

Supplier and production audit

Cell design audit

Manufacturer's technical capability

Staff (Engineering, Management)

Facilities (Materials analysis capability)

Manufacturer's quality systems.

Process controls. (In place and being implemented)

Cell level safety devices

CID (Circuit Interrupt Device)

Shut down separator

Pressure vent

External circuit devices

PTC resistors (Low power only)


Cell and battery isolation to prevent event propagation

Electrical (Contactors)

Physical (Separation, barriers)

BMS Software

Monitoring of all key indicators coupled to control actions.

(Cooling, Power disconnect)

BMS Hardware

Fail safe back-up hardware switch off in case of software failure. Set to slightly higher limits than the software controls.

Battery switch off in case the low voltage BMS power supply or other system component fails.


Use multiple low capacity cells which release less energy in case of an event.

Design in physical barriers to heat and flame propagation between cells.


Automotive High Voltage High Capacity Batteries

Concerns are often expressed about the safety of high power automotive batteries if they are damaged or crushed in an accident. Such batteries are normally subject to stringent safety testing before they may be approved for use and a range of International Standards has been developed for this purpose.

Nevertheless batteries in general present a lower hazard in the case of an accident than a full tank of petrol.

The dangers don't just come from the chemical content of the batteries. High capacity batteries store an immense amount of energy which can cause enormous damage if the battery is short circuited.


See also the trade-off between High and Low Capacity Cells and the consequences for safety.




Handling Instructions


User Safety Precautions

These are intended to protect the user as well as the battery. Detailed recommendations for handling and using batteries are given in the section on User Safety Instructions


MSDS Material Safety Data Sheets

Material Safety Data Sheets are designed to provide safety information about any physical or chemical hazard associated with a particular product and procedures for handling or working with hazardous material content. They are intended for employers, employees and emergency services responsible for dealing with fire or medical emergencies.

MSDS's are specific to individual products or classes of products and include information such as the chemical composition of each of the chemicals used and physical data ( melting point , boiling point , flash point etc.) as well as the reactivity, toxicity, flammability, health effects, recommended first aid, storage, disposal, protective equipment and procedures to follow in case of a fire, spill or leak.

In the case of batteries, the information is usually provided by the cell manufacturer since they control the contents of the product.

See MSDS for an example of a typical data sheet for Lithium cells used in mobile phones.


See also Battery Death for dangerous operating practices which could damage the battery and Electric Shocks for an outline of potential hazards to the user when working with batteries.




Battery Abbreviations


$                      Dollar in US currency

18650             Li-ion cylindrical cell format measuring 18mm times 65mm

A                    Ampere (electrical)

AC                  Alternating current

ADAC            Allgemeiner Deutscher Automobil-Club (German automobile club)

AFC                Alkaline fuel cell

AGM              Absorbent Glass Mat (battery)

AGV               Automatic Guided Vehicle

Ah                  Ampere-hour

APU                Auxiliary Power Unit

BAPCO           Business Applications Performance Corporation

Bar                 Unit ofpressure; 1 bar = 100kPa; 1 bar = 14.503psi

bbl                  Measurements of liquid, 1 barrel = 42 US gallons (35 Imperial gallons), 159 liters

BCG                The Boston Consulting Group

BCI                 Battery Council International

BMS               Battery management system

BMW              Bavarian Engine Works (Bayerische Motoren Werke) 

BTU                British Thermal Unit; 1 BTU = 1,054 joules; 1 BTU = 0.29Wh

C                     Celsius, Centigrade (temperature) 

cal                  Calorie; 1cal = 4.18 joules; 1cal = 4.18 watt/s; 1,000 joules = 0.277Wh

CARB            California Air Resources Board

CCA                Cold cranking amps at –18°C (0°F). The norms differ as follows:

                        BCI discharges battery at CCA-rate for 30s; battery at or above 7.2V passes

                        IEC discharges battery at CCA-rate for 60s; battery at or above 8.4V passes

                        DIN discharges battery at CCA-rate for 30s and 150s; battery at or above 9V and 6V respectively passes

CCCV             Constant current constant voltage (charge method)

CCV                Closed circuit voltage (battery under charge or discharge)

CDMA            Code Division Multiple Access (cell phones)

CEC                Certificate of Equivalent Competency (International regulations)

CID                 Circuit interrupt device

CIPA               Camera and Imaging Products Association

CL                   Current limiting (as in charging a battery)

CNG                Compressed natural gas

CNT                Carbon nanotube

CPU                Central processing unit

Co                   Cobalt (metal)

COC                Certificate of Competency

CO2                 Carbon dioxide

CPR                Cardiopulmonary resuscitation

C-rate             Discharge rate of a battery

DC                   Direct current

DGP                Dangerous Goods Panel

DIN                 Deutsches Institut für Normung (German Institute for Standardization)

DLC                Double-layer capacitor

DMFC             Direct Methanol Fuel Cell

DoD                 Depth of discharge

DOE                Department of Energy (US)

DOT                 Department of Transportation (US)

DSP                 Digital signal processor

dT/dt                Delta Temperature over delta time (charge method)

EBM                Electronic battery monitor

ect.                 Et cetera. Latin: And so forth

EDTA              Crystalline acid

EIS                  Electrochemical Impedance Spectroscopy

ELC                 Equivalent lithium content

EMF                Electromagnetic field

EMF                Electromotive force

EPA                Environmental Protection Agency (US)

EV                   Electric vehicle

F                      Fahrenheit (temperature)

f                      Farad (unit of capacitance)

FAA                Federal Aviation Administration

FC                   Fuel cell

FCVT              FreedomCAR & Vehicle Technologies (US Department of Energy)

Foot/’             Foot (dimension) 1’= 12”; 1’ = 0.3048m; 1’times 3.28 = 1m

g                     Gram; 1g = 0.035oz; 1g times 28.35 = 1oz

GSM               Global System for Mobile Communications (cell phones)

h                     Hour (time)

HEV                Hybrid electric vehicle

hp                   Horsepower (power) 1hp = 745.7 watts

Hz                   Hertz (electrical frequency)

I                       Current(electrical)

i.e.                  Id est. Latin: That is

IATA               International Air Transport Association

IC                    Integrated circuit (chip)

IC                    Internal combustion (engine)

ICAO              International Civil Aviation Organization

IEC                 International Electrochemical Commission

Inch/“             Inch; 1”= 25.4mm; 1” = 0.0254 meter; 1”times 39.3 = 1m

IPF                  Interfacial protective film

IPP                  IECaircraft battery rating (0.3/15s power discharge)

IPR                  Aircraft battery rating according to IEC (15s power discharge)

IS                     Intrinsic safety (used on batteries)

J                      Joule, 1J = 1A at 1V for 1s = 1 watt/s; 1J = 0.238calorie/s

kg                    Kilogram; 1kg = 0.45 pound; 1kg times 2.2 = 1 pound

kJ                    Kilo-Joule; 1kJ = 0.277Wh

km                   Kilometer; 1km = 0.621 miles; 1km times 1.60 = 1 mile

kN                    Kilo-Newton (law of motion) 1N = 1kg m/s2

kPa                  Kilo-Pascal (pressure); 1kPa = 0.01 bar; 1kPa = 0.145psi

kW                  Kilowatt (electrical energy); 1kWh = 3.6MJ; 1MJ = 860kcal = 238cal/s 

kWh                Kilowatt-hour (electrical power)

L                      Inductance (electrical coil)

lb                     Pound (weight, from Roman libra) 1 lb times 0.45 = 1kg

LCD                Liquid crystal display

LCO                Lithium cobalt oxide

LED                Light emitting diode

LFP                 Lithium-iron-phosphate

LFPT               Low frequency pulse train (method to test a battery)

LiCoO2            Lithium-ion-cobalt-oxide

LiFePO4          Lithium-iron-phosphate-oxide

Li-ion               Lithium-ion battery (short form)

LiMn2O4          Lithium-ion-manganese-oxide

LiNiCoAlO2     Lithium-ion-nickel-cobalt-aluminum-oxide

LiNiMnCoO2   Lithium-on-nickel-manganese-cobalt-oxide

Li5Ti5O13       Lithium-titanate-oxide

L/km                Liter per kilometer

LMO                Lithium-manganese-oxide

LTO                 Lithium-titanate

m                     Meter (dimension) 1m = 3.28 feet; 1m times 0.30 = 1 foot

mAh                Milliampere-hours

MCFC              Molten carbonate fuel cell

Microfarad [µF]     Capacitor rating, one-millionth 10-6 of a farad)

Min                 Minute (time)

mm                  Millimeter (dimension) 1mm = 0.039”; 1mm times 25.4 = 1”

Mn                  Manganese(chemical element used in batteries)

mpa                 Mega-Pascall unit of pressure

Mpg                Miles per gallon

ms                   Millisecond, one-thousand of a second (10-3).

MW                 Megawatt (power)

N                      Newton (law of motion) 1N = 1kg m/s(force required to accelerate 1kg at 1m/s)

NaS                 Sodium-sulfur (battery)

NASA              National Aeronautics and Space Administration

NCA                Lithium-ion battery with nickel, cobalt, aluminum cathode

NCV                Net calorific value (1 food calorie = 1.16 watt-hour)

NDV                Negative delta V (full-charge detection)

NG                   Natural gas, consumption measured in joules (1,000 joules = 0.277Wh)

NiCd                Nickel-cadmium (battery)

NiFe               Nickel-iron (battery)

NiH                 Nickel-hydrogen battery

NiMH              Nickel-metal-hydride (battery)

NiZn                Nickel-zinc (battery)

NMC               Lithium-ion with nickel, manganese, cobalt cathode

NRC               National Research Council

NTC                Negative temperature coefficient

OCV                Open circuit voltage

OEM               Original equipment manufacturer

Oz                    Ounce; 1 oz = 28 grams; 1 oz times 0.035 = 1 gram 

PAFC              Phosphoric acid fuel cell

PC                   Personal computer

PEM                 Proton exchange membrane (fuel cell), also PEMFC

PEMFC            Proton exchange membrane fuel cell, also PEM

pf                     Pico-farad (capacitor rating, one-trillionth 10-12 of a farad)

pf                     Power factor (ratio of real power to the apparent power on AC)

PHEV              Plug-in hybrid electric vehicle

PRBA              Portable Rechargeable Battery Association

psi                   Pound per square inch (pressure) 1psi = 0.145kPa; 1psi times 6.89 = 1kPa

PTC                 Positive temperature coefficient

PTC                 Over-voltage protection (batteries, motors, speakers)

QA                   Quality assurance

Qi                    Standard on inductive charging by Wireless Power Consortium (WPC)

Q-Mag™        Quantum magnetic battery analysis (Cadex trademark)

R                     Resistor (electrical)

RBRC             Rechargeable Battery Recycling Corporation

RC                   Remote control (hobbyist)

RC                   Reserve capacity of starter battery. Conversion formula:. RC divided by 2 plus 16 = Ah

R&D                Research and development

RPM               Revolution per minute

s                      Second (time)

SAE                 Society of Automotive Engineers, founded early in 1900 by US auto manufacturers

SBS                 Smart Battery System

SEI                  Solid electrolyte interphase (Li-ion)

SG                   Specific gravity (acid density of electrolyte)

SLA                 Sealed lead acid (battery)

SLI                  Starter-light-ignition (battery), also knows as starter battery

SMBus           System Management Bus (smart battery)

SoC                 State-of-charge

SoF                  State-of-function

SOFC              Solid oxide fuel cell

SoH                 State-of-health

UL                    United Laboratories (product safety testing and certification)

UPS                 Uninterruptible power supply

USB                 Universal Serial Bus (data)

V                      Voltage (electrical)

VA                   Volt-ampere (similar to watt with true current flow in a reactive load)

VAC                Voltage with alternating current (grid)

VL                   Voltage limiting (as in charging a battery)

VRLA              Valve regulated lead acid (battery)

W                     Watt (electrical energy; voltage times current = watts)

Wh                   Watt-hour (electrical power; watts times h = Wh); 1Wh = 860 cal/h = 0.238cal/s

Wh/kg             Watt-hour per kilogram (measurement of specific energy)

Wh/km            Watt-hour per kilometer
Wh/l                 Watt-hour per litter (measured in energy density)

Wi-Fi               Wireless fidelity (network)

W/kg               Watt per kilogram (measurement of specific power)

WPC               Wireless Power Consortium

WW                World War

Z                      Impedance (reactance-based resistance, frequency dependent)

ZEBRA            Zeolite Battery Research Africa Project (battery)


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