The batteries used on cordless tools are dry cells. If they are charged
at too high a voltage they heat up. If there is a malfunction they can
heat to the point of ignition.
Another thing the guy at the DeWalt store said was on that issue of heat. I
said that future purchases would be 18 volt rather than my present 14.4. He
said I should reconsider because the smaller batteries have fewer cells and
cool down faster after load stops, which increases battery life. The big
die young.