I remember when I got to my first squadron and was going through plane captain school. We were warned that the NiCad battery onboard the aircraft could just arbitrarily go into "thermal runaway" and ultimately self-ignite. Over my career, I've found several that were bulged due to some malfunction. (meaning they were starting to go into thermal runaway) They were quickly removed and isolated. If they were remotely warm, we had been instructed to place them in a 55 gallon barrel of water. It doesn't help that a good majority of the batteries that power these new vehicles are made in China.