Science Questions

Will a torch shine with charged and dead batteries in it?

Sun, 1st Nov 2009

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Jesse, USA asked:

If I have a torch and I put three batteries in it and two are fully charged and one is fully drained, will the torch still produce light?


I think that very much depends on the chemistry of your batteries.  A battery is essentially a chemical reaction which is split into two halves and the only way Ė and letís say you got part half A and half B, the whole lotís got to happen to be driven in any way that can happen is quite passing electron through your circuit.  And eventually, the battery runs out because you run out of all the chemicals you need for the chemical reaction.  Now, basically you're saying, if we apply a large voltage and carry on pushing electrons through the battery, whatís going to happen?  That would depend on what other the chemical reactions go on to move charge through the electrolyte in the battery.  Normally, thatís often quite inefficient.  If youíve got have something like a lead acid battery which is symmetrical, it will just start up in the wrong direction.  Certainly a very simple that acid battery will Ė and so, it will turn into battery but pointing the wrong direction.  If you have other chemistries of batteries, it could cause all sorts of havoc and it will depend on the exact chemistry.  It will certainly become a very high resistance.  It will work for a bit but eventually, itís going to run out of Ė it will either stop or it could do all sorts of strange things to your battery, itís certainly going to damage a rechargeable battery.


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Jesse asked the Naked Scientists: Dear Naked Scientists, I have a question. If I have, say, a torch, and I put three batteries in it, two fully charged and one drained, will the torch still produce light? Thanks, Jesse Ithaca, NY, USA What do you think? Jesse , Mon, 2nd Nov 2009

With a traditional torch it is common to connect batteries in series. Putting a run-down battery in the torch will prevent you getting a bright output. This is because the net voltage output will be lower (the total voltage output is the sum of the battery voltages) so the current, produced from this voltage across the light bulb's impedance, will be lower. Also run-down batteries start to have a higher internal impedance than when charged so this also reduces the net current and will also waste power from the good batteries.

In some electrical items (and some more modern torches) the batteries may be in parallel. Here it is also important not to put in a run down battery with good  batteries. In this case the good batteries will try to charge the run down one. This again wastes power and will reduce the available current to power the light. It can also be hazardous because there can be a lot of heat generated.

The actual behaviour will also depend on the type of batteries used, but it is fair to say that it is generally not a good idea to mix run-down batteries with good ones. graham.d, Mon, 2nd Nov 2009

Wow! That's a new one. No isolating diodes or anything?

Sounds like a scheme invented by battery manufacturers to make us buy more batteries! Geezer, Mon, 2nd Nov 2009

Wow! That's a new one. No isolating diodes or anything?

Sounds like a scheme invented by battery manufacturers to make us buy more batteries!

Some of the high power LED torches have them in parallel. The combined voltage would be the same as a single cell but the current drain would be shared between the cells so they would last longer.

that mad man, Mon, 2nd Nov 2009

That's OK as long as all the batteries have exactly the same potential, but if they don't (as Graham.D points out) the cell with the greatest potential (voltage) will try to "charge" the cell with the lower potential. As a result, the energy in the cell with the higher potential will be dissipated without doing any useful work.

At least, that's what theory says. Perhaps the effect is minimal with modern alkaline batteries. Geezer, Wed, 4th Nov 2009

You will find that equipment manufacturers will say in the instructions (I never read them either) that you should never mix batteries of different types or mix old and new batteries. graham.d, Wed, 4th Nov 2009

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