What makes batteries heat up?

I own a digital camera that uses standard AA batteries. I went to a family affair earlier in the week and put some spare AA batteries in my pocket in case the batteries in the camera ran out.

For whatever reason, I never ended up taking the batteries out of my pocket and have been carrying them around for a few days, moving them (along with all the other junk in my pockets) from pants to pants.

This morning, as I was sitting at my desk, I realize that my pocket suddenly feels very hot. Of course, I right away figure out that it’s the batteries. Sure enough, of the four I was carrying, two were VERY hot to the touch. Needless to say, I threw them out.

However, that began to make me wonder - what was it that suddenly caused them to heat up like that? If it helps, the only other things in that pocket were my Palm pilot (in a leather case), some coins and some dollar bills. Could the coins coming in contact with the batteries make them heat up like that? If not, what else could it be?

Zev Steinhardt

It does sound like they were shorted out; in some designs of battery, the entire outer case is part of the negative terminal; if the insulation on this was broken, then a coin could short it to the positive terminal.

Yes, it could be the coins in combination with fauly insulation on the batteries, or maybe th ciohs somehow got perfectly balanced on both ends of a couple of batteries.