Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 19/06/2012 18:24:59

Title: Why do some batteries get hot?
Post by: thedoc on 19/06/2012 18:24:59
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  
Asked by Neil S. Briscoe


                                        Visit the webpage for the podcast in which this question is answered. (http://www.thenakedscientists.com/HTML/podcasts/show/2454/)

 

Title: Why do some batteries get hot?
Post by: thedoc on 19/06/2012 18:24:59
We answered this question on the show...



 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.
Title: Re: Why do some batteries get hot?
Post by: syhprum on 19/06/2012 20:59:18
Most remote contol units use two batteries AA or AAA do you know if one or two got Hot ?.
If both got hot it was an external short but if only one it was a faulty battery.
Could you possibly measure how much current is taken being with and without buttons being pushed and can you see if there is any corrosion due to past leakage.
Title: Re: Why do some batteries get hot?
Post by: Horence on 11/07/2017 07:49:14
When this battery get hot, the way for me to fix this problem is to stop charging. Then put it in the place that it can down hot quickly. This way is just for the case when I charging camera battery and phone with 400w power inverter from bestek. Why this battery will get hot, to me is about charging time. Since I charge the battery over time, so this case will happen. In usual day, we need to pay more attention to this trouble, it is very dangerous.
Title: Re: Why do some batteries get hot?
Post by: SeanB on 13/07/2017 21:12:21
With the commonly used alkaline battery you probably have them in a housing next to each other, the only thing stopping them shorting out being the thin insulating self adhesive sleeve. Common is for this sleeve to be punctured at the edges of the cell negative, as this is a sharp edge and stresses the thin plastic sheet. Then this can either short out to the adjacent cell, or the spring terminal from the contact in the remote was not centred and slipped enough to short out the cell.

In either case there is enough current supply ability in the cell to supply a high current, generally over 15A, and this will heat up the cell and the wire quite a lot and very quickly. The older zinc carbon chemistry is a lot less able to supply high current, and as well the cell cannot supply moderate current for long periods, as the internal chemistry rapidly polarises, and this reaction is slow to dissipate as the depolarising chemicals in the mix ( an ammonia salt) are there in limited amounts. Thus you can short these cells without them getting hot, though they can only supply under 1A for the AA and AAA varieties, and only for a short time.

If you are using rechargeable cells then they have a very high current capability, certainly well over 30A for the best AA and AAA cells, but they will rapidly overheat and vent internal pressure ( and thus KOH electrolyte) via the safety vent built into the cell. They can cause a fire very easily.
Title: Re: Why do some batteries get hot?
Post by: DudleyC on 14/07/2017 18:14:22
A simple answer would be the heat generation in a complete circuit.