Temperature termination and the thermal characteristics of NiCd and NiMH batteries

The temperature of a battery under charge is a characteristic parameter that can be used to control the charge to a Ni-Cd or a Ni-MH battery. As the battery reaches full charge, the internal cell temperature rises dramatically. Detecting this temperature increase can be used as an effective method of terminating charge to a battery. Temperature termination is an excellent approach assuming constant current cannot be used, and providing that any problems involved with sensing temperature accurately and reliably are taken into account. Nickel-cadmium and nickel-metal hydride batteries heat differently during charge, but both chemistries display a rapid increase in temperature as they enter overcharge. The rapid increase in temperature can be used to signal the onset of overcharge. However, every sustained rise of 5°C in temperature at high charge rates will cut battery lifetimes in half as a result of subjecting a battery to repeated overcharging. Therefore, the temperature rise must be kept as small as possible. Thermistors can be used as a temperature sensor in such circumstances