THERMAL RUNAWAY.
Batteries are capable of performing to their rated capacities when the temperature conditions and charging rates are within the values specified. In the event that these are exceeded “thermal runaway” can occur, a condition which causes violent gassing, boiling of the electrolyte and finally melting of the plates and casing, with consequent danger to the aircraft structure and jeopardy of the electrical system.
Since batteries have low thermal capacity heat can be dissipated and this results in lowering of the effective internal resistance. Thus, when associated with constant voltage charging, a battery will draw a higher charging current and thereby set up the “runaway” condition of ever-increasing charging currents and temperatures.
In some aircraft, particularly those employing nickel-cadmium batteries, temperature-sensing devices are located within the batteries to provide a warning of high battery temperatures and to prevent overcharging by disconnecting the batteries from the charging source at a predetermined temperature
Aircraft Electrical Systems.
E H J Pallett.
ISBN. 0-582-98819-5.