[Jesse]

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?

[graham.d]

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.

[Geezer]

(and some more modern torches) the batteries may be in parallel.

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!

[that mad man]

(and some more modern torches) the batteries may be in parallel.

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.

[Geezer]

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'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.

[graham.d]

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.

[thedoc]

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