Why do some batteries get hot?

17 June 2012

Question

Hi Chris

I just picked up the remote for my hifi to discover it was hot. It was, of course, the batteries generating this amount of heat. Naturally, the remote would not work, so I removed the batteries and replaced them with new ones.

My question is:

Why do the electrolytes in some batteries get hot when the battery dies, whereas other batteries just die without generating heat? What chemistry suddenly takes place, which was not taking place before, that generates this heat?

Thanks as always.

Regards
Neil

Answer

Dave - Charging batteries isn't 100% efficient and similarly, discharging batteries isn't 100% efficient. The way electronic engineers like to think about it is that the battery has a resistance, so if you draw a current from that battery then you're pushing that current through a certain resistance and so, it will heat up. If you short out a battery, basically taking a wire from under the battery and connecting it to the other end of the battery then there's no resistance anywhere else - there's nowhere else the energy to go so all that energy will get dumped into the internal resistance of the battery and it will get very, very warm. One thing which could've caused it is that your remote control somehow shorted itself out. That could've been in the remote control itself, but if you changed a battery and it worked fine, then probably not - but it could also be inside the battery. Something could have even gone wrong with bits of metal touching inside the battery.

Chris - That's good. So it sounds like, if nothing else is to blame, probably the battery has developed an internal short and shorted itself out, and it's dissipating all the energy into itself, making itself get hot.

Dave - Yeah, certainly in a remote control because the remote control draws so little current, that there shouldn't be enough power being dissipated anywhere else.

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