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
? 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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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
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
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
Battery Temperature Characteristics
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.
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.
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
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.
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 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 .
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.
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 /?r?s/, /?r??/,/?ro?z/, or /?ro?h?z/, 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.
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.
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.
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.
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.
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.
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.
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.
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:  a built-in temperature switch called PTC that protects against high current surges,  a circuit interrupt device (CID) that opens the electrical path if an over-charge raises the internal cell pressure to 1000 kPa (145psi), and 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.
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.