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Temperature Operating Limits
All batteries depend for their action on an electrochemical process whether charging or discharging and we know that these chemical reactions are in some way dependent on temperature. Nominal battery performance is usually specified for working temperatures somewhere in the + 20°C to +30°C range however the actual performance can deviate substantially from this if the battery is operated at higher or lower temperatures. See Temperature Characteristics for typical performance graphs.
Arrhenius Law tells us that the rate at which a chemical reaction proceeds, increases exponentially as temperature rises (See Battery Life). This allows more instantaneous power to be extracted from the battery at higher temperatures. At the same time higher temperatures improve electron or ion mobility reducing the cell's internal impedance and increasing its capacity.
At the upper end of the scale the high temperatures may also initiate unwanted or irreversible chemical reactions and / or loss of electrolyte which can cause permanent damage or complete failure of the battery. This in turn sets an upper temperature operating limit for the battery.
At the lower end of the scale the electrolyte may freeze, setting a limit to low temperature performance. But well below the freezing point of the electrolyte, battery performance starts to deteriorate as the rate of chemical reaction is reduced. Even though a battery may be specified to work down to -20°C or -30°C the performance at 0°C and below may be seriously impaired.
Note also that the lower temperature working limit of a battery may be dependent on its State of Charge. In a Lead Acid battery for instance, as the battery is discharged the Sulphuric Acid electrolyte becomes increasingly diluted with water and its freezing point increases accordingly.
Thus the battery must be kept within a limited operating temperature range so that both charge capacity and cycle life can be optimised. A practical system may therefore need both heating and cooling to keep it not just within the battery manufacturer's specified working limits, but within a more limited range to achieve optimal performance.
Thermal management however is not just about keeping within these limits. The battery is subject to several simultaneous internal and external thermal effects which must be kept within control.
Heat Sources and Sinks
Electrical Heating (Joule Heating)
The operation of any battery generates heat due to the I2R losses as current flows through the internal resistance of the battery whether it is being charged or discharged. This is also known as Joule heating. In the case of discharging, the total energy within the system is fixed and the temperature rise will be limited by the available energy. However this can still cause very high localised temperatures even in low power batteries. No such automatic limit applies while charging as there is nothing to stop the user continuing to pump electrical energy into the battery after it has become fully charged. This can be a very risky situation.
Battery designers strive to keep the internal resistance of the cells as low as possible to minimise the heat losses or heat generation within the battery but even with cell resistances as low as 1milliOhm the heating can be substantial. See Effects of Internal Impedance for examples.
Thermochemical Heating and Cooling
In addition to Joule heating the chemical reactions which take place in the cells may be exothermic, adding to the heat generated or they may be endothermic, absorbing heat during the process of the chemical action. Overheating is therefore more likely to be a problem with exothermic reactions in which the chemical reaction reinforces the heat generated by the current flow rather than with endothermic reactions where the chemical action counteracts it. In secondary batteries, because the chemical reactions are reversible, chemistries which are exothermic during charging will be endothermic during discharging and vice versa. So there's no escaping the problem. In most situations the Joule heating will exceed the endothermic cooling effect so precautions still need to be taken.
Lead acid batteries are exothermic during charging and VRLA batteries are prone to thermal runaway (See below). NiMH cells are also exothermic during charging and as they approach full charge, the cell temperature can rise dramatically. Consequently, chargers for NiMH cells must be designed to sense this temperature rise and cut off the charger to prevent damage to the cells. By contrast Nickel based batteries with alkaline electrolytes (NiCads) and Lithium batteries are endothermic during charging. Nevertheless thermal runaway is still possible during charging with these batteries if they are subject to overcharging.
The thermochemistry of Lithium cells is slightly more complex, depending on the state of intercalation of the Lithium ions into the crystal lattice. During charging the reaction is initially endothermic then moving to slightly slightly exothermic during most of the charging cycle. During discharge the reaction is the reverse, initially exothermic then moving to slightly endothermic for most of the discharge cycle. In common with the other chemistries, the Joule heating effect is greater than the thermochemical effect so long as the cells remain within their design limits.
External Thermal Effects
The thermal condition of the battery is also dependent on its environment. If its temperature is above the ambient temperature it will lose heat through conduction, convection and radiation. If the ambient temperature is higher, the battery will gain heat from its surroundings. When the ambient temperature is very high the thermal management system has to work very hard to keep the temperature under control. A single cell may work very well at room temperature on its own, but if it is part of a battery pack surrounded by similar cells all generating heat, even if it is carrying the same load, it could well exceed its temperature limits.
Temperature - The Accelerator
The net result of the thermo-electrical and thermo-chemical effects possibly augmented by the environmental conditions is usually a rise in temperature and as we noted above this will cause an exponential increase in the rate at which a chemical reaction proceeds. We also know that if the temperature rise is excessive a lot of nasty things can happen
- The active chemicals expand causing the cell to swell
- Mechanical distortion of the cell components may result in short circuits or open circuits
- Irreversible chemical reactions can occur which cause a permanent reduction in the active chemicals and hence the capacity of the cell
- Prolonged operation at high temperature can cause cracking in plastic parts of the cell
- The temperature rise causes the chemical reaction to speed up increasing the temperature even more and could lead to thermal runaway
- Gases may be given off
- Pressure builds up inside the cell
- The cell may eventually rupture or explode
- Toxic or inflammable chemicals may be released
- Law suits will follow
Thermal Capacity - The Conflict
It is ironic that as battery engineers strive to cram more and more energy into ever smaller volumes, the applications engineer has increasing difficulty to get it out again. The great strength of new technology batteries is unfortunately also the source of their greatest weakness.
The thermal capacity of an object defines its ability to absorb heat. In simple terms for a given amount of heat, the bigger and heavier the object is, the smaller will be the temperature rise caused by the heat.
For many years lead acid batteries have been one of the few power sources available for high power applications. Because of their bulk and weight, temperature rise during operation has not been a major problem. But in the quest for smaller, lighter batteries with higher power and energy densities, the unavoidable consequence is that the thermal capacity of the battery will be decreased. This in turn means that for a given power output, the temperature rise will be higher.
(This assumes a similar internal impedance and similar thermochemical properties which might not necessarily be the case.) The result is that heat dissipation is a major engineering challenge for high energy density batteries used in high power applications. Cell designers have developed innovative cell construction techniques to get the heat out of the cell. Battery pack designers must find equally innovative solutions to get the heat out of the pack.
EV and HEV Battery Thermal Considerations
Similar conflicts occur with EV and HEV batteries. The EV battery is large with good heat disipation possibilities by convection and conduction and subject to a low temperature rise due to its high thermal capacity. On the other hand the HEV battery which must handle the same power is less than one tenth of the size with a low thermal capacity and low heat dissipation properties which means it will be subject to a much higher temperature rise.
Taking into account the need to keep the cells operating within their allowable temperature range (See Cycle Life in the section on Lithium Battery Failures) the EV battery is more likely to encounter problems to keep it warm at the low end of the temperature range while the HEV battery is more likely to have overheating problems in high temperature environments even though they both dissipate the same amount of heat.
In the case of the EV, at very low ambient temperatures, self heating (I2R heating) by the current flow during operation will most likely be insufficient to raise the temperature to the desired operating levels because of the battery's bulk and external heaters may be required to raise the temperature. This could be provided by diverting some of the battery capacity for heating purposes. On the other hand, the same heat generation in the HEV battery working in high temperature environments could send it into thermal runaway and forced cooling must be provided.
See also EV, HEV and PHEV Specifications in the Traction Battery section
The operating temperature which is reached in a battery is the result of the ambient temperature augmented by the heat generated by the battery. If a battery is subject to excessive currents the possibility of thermal runaway arises resulting in catastrophic destruction of the battery. This occurs when the rate of heat generation within the battery exceeds its heat dissipation capacity. There are several conditions which can bring this about:
- Initially the thermal I2R losses of the charging current flowing through the cell heat up the electrolyte, but the resistance of the electrolyte decreases with temperature, so this will in turn result in a higher current driving the temperature still higher, reinforcing the reaction till a runaway condition is reached.
- During charging the charging current induces an exothermic chemical reaction of the chemicals in the cell which reinforces the heat generated by the charging current.
- Or during discharging the heat produced by the exothermic chemical action generating the current reinforces the resistive heating due to the current flow within the cell.
- The ambient temperature is excessive.
- Inadequate cooling
Unless some protective measures are in place the consequences of the thermal runaway could be meltdown of the cell or a build up of pressure resulting an explosion or fire depending on the cell chemistry and construction. See more details in the section on Lithium Battery Failures.
The thermal management system must keep all of these factors under control.
Thermal runaway can occur during the charging of valve regulated lead acid batteries where gassing is inhibited and the recombination adds to the temperature rise. This does not apply to flooded lead acid batteries because the electrolyte boils off.
Low temperature operating conditions are relatively easy to cope with. In the simplest case there is usually enough energy in the battery to power self heating elements which gradually bring the battery up to a more efficient operating temperature when the heaters can be switched off. In some cases it is enough to keep the battery on its recharging cycle when it is not in use. In more complex cases for example with high temperature batteries such as the Zebra battery running at temperatures well above normal ambient temperatures some external heating may be required to bring the battery up to its operating temperature on start up and special thermal insulation may be needed to maintain the temperature for as long as possible after it has been switched off.
For low power batteries the normal protection circuits are sufficient to keep the battery within its recommended operating temperature limits. High power circuits however need special attention to thermal management.
- Protection From Overheating -
In most cases this simply involves monitoring the temperature and interrupting the current path if the temperature when the temperature limits are reached using conventional protection circuits. While this will prevent damage to the battery from overheating it can however cut off the battery before its current carrying limit is reached seriously limiting its performance.
- Dissipation of Surplus Heat Generated -
Removing heat from the battery allows higher currents to be carried before the temperature limits are reached. Heat flows out of the battery by convection, conduction and radiation and the pack designer's task is to maximise these natural flows by keeping the ambient temperature low, by providing a solid, good heat conducting path from the battery (using metallic cooling rods or plates between the cells if necessary), by maximising its surface area, by providing good natural air flow through or around the pack and by mounting it on a conductive surface.
- Uniform Heat Distribution -
Even though the battery thermal design may be more than sufficient to dissipate the total heat generated by the battery, there could still be localised hot spots within the battery pack which can exceed the specified temperature limits. This can be a problem with the cells in the middle of a multi cell pack which will be surrounded by warm or hot cells compared with the outer cells in the pack which are facing a cooler environment.
A temperature gradient across the battery pack can seriously affect the life of the battery. Arrhenius' Law indicates that for every 10°C increase in temperature, the chemical reaction rate approximately doubles. This puts an unbalanced stress on the cells in the battery and also exacerbates any age related deterioration of the cells. See also Interactions Between Cells and Cell Balancing.
Separating the cells to avoid this problem adds to the volume of the pack. Thermal imaging may be needed to identify potential problem areas.
Passive dissipation can be improved still further by mounting the cells in a block of thermally conductive material which acts as a heat sink . Heat transfer from the cells can be maximised if a phase change material (PCM) is used for this purpose since it also absorbs the latent heat of the phase change as it changes from the solid to the liquid state. Once in the liquid state, convection also comes into play increasing the potential for heat flow and for equalising the temperature across the battery pack. High conductivity graphite sponge materials saturated by wax which absorbs extra heat when the temperature reaches the melting point are available for this application.
- Minimum Addition to the Weight -
For very high power applications, such as traction batteries used in EVs and HEVs, natural cooling may be insufficient to maintain a safe working temperature and forced cooling may be required. This should be the last resort as it complicates the battery design, adds weight to the battery and consumes power. If forced cooling is unavoidable however, the first choice would normally be forced air cooling using a fan or fans. This is relatively simple and inexpensive but the thermal capacity of the thermal fluid, air, which is intended to carry the heat away is relatively low limiting its effectiveness. In the worst case liquid cooling may be required.
For very high cooling rates working fluids with a higher thermal capacity are required. Water is normally the first choice because it is inexpensive but other fluids such as ethylene glycol (anti freeze) which have a better thermal capacity may be used. The weight of the coolant, the pumps to circulate it, the cooling jackets around the cells, the pipework and manifolds to carry and distribute the coolant and a radiator or heat exchanger to cool it, all add dramatically to the total weight, complexity and cost of the battery. These penalties could well outweigh the gains expected to be achieved by using high energy density battery chemistries.
In some applications, such as electric vehicles as noted above, there is the opportunity to use the waste heat for heating the passenger compartment and most automotive systems include some form of integrating the battery thermal management with the vehicle climate controls. This is however only beneficial during cold weather. In hot climates the high ambient temperature places an added burden on the battery thermal management.