The battery pack in an electric vehicle (EV) can produce a lot of heat, especially during rapid charging. Ideally, batteries should be operated at temperatures below 35° C. When consistently operated at higher temperatures, degradation of the charge-carrying capacity of the electrodes will reduce the battery’s capacity. If the battery cell temperatures get extremely high, it can cause more rapid degradation.
Mechanisms include separator tearing due to temperature gradients, dendrite formation, and associated separator piercing. At extremely high temperatures, electrolyte off-gassing and separator collapse present the risk of thermal runaway.
EVs use a battery thermal management system (BTMS) with a liquid or air-cooling system to prevent overheating while allowing rapid charging. Individual cells are mounted to a metallic plate, which absorbs the heat from the cells and transfers it to the cooling liquid or air.
When operated at moderately elevated temperatures of 25° to 55° C, most of the degradation occurs at the electrodes, especially the cathode. This is caused by surface films forming on the electrodes and structural changes to the electrodes themselves. These changes reduce the charge that can be stored at the electrodes and the battery’s capacity.
Further degradation may also be associated with electrolyte decomposition and the dissolution of active materials. It is, however, also interesting to note that the short-term performance of batteries increases with temperature, up to about 45° C, with batteries about to store more energy when charged at higher temperatures.
Causes of overheating include the rate of:
- Charge/discharge
- Ambient temperature
- Battery age/degradation.
Internal resistance and electrochemical reactions in the battery mean that heat is generated by charging and discharging and will increase with the current flowing through the battery.
This is a typical operating characteristic, and the BTMS should be designed to mitigate for it. Elevated ambient temperatures present two challenges for thermal management. They raise the baseline from which the temperature increases, and they reduce the cooling system’s effectiveness.
Extreme weather events are increasingly pushing operating conditions out of design parameters and increasing the risk of battery failure. When a combination of ambient temperature, battery age, and rate of charge result in overheating, the battery management system may limit the discharge rate and depth, or the charge rate, to protect the battery. This can result in an immediate and significant performance loss until the temperature reduces.
Filed Under: Batteries, FAQs