As a twenty year memory serves me, thermal runaways can happen with nickel-cadmium batteries. As I recall, the battery resistance drops to near zero, sucks everything out of the generator and starts getting very hot. There may also be other failures in the voltage control circuitry.. .. .This situation can happen where all current must pass through the battery -- systems where you can disconnect the battery offer an extra degree of safety.. .. .In the article I read on a thermal runaway incident, the a/c eventually caught fire on the ground.