when the cells get too hot, they leak hydrogen gas (the H in NiMH) which when mixed with the right concentration of air explodes. I've had a few cells vent on my when they get too hot. I immediately put them somewhere away from anything flamable & let them cool down. Luckily I've never yet had one explode. I have had a Lipo cell catch fire in one of my boats after the ESC shorted out which in turn shorted out the Lipo. that wasn't pretty either.