Science Questions

How efficient are rechargeable batteries?

Sat, 16th Jun 2012

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Question

Gabrielle Keighley asked:

Hi, Naked Scientists.

 

My name is Gabrielle Keighley and my dad has a question that he was wondering that you could answer please. He recently bought some rechargeable batteries for some of our toys and as he was doing the first charge, we wondered about the efficiency of recharging batteries. Is it an efficient process regarding the amount of power or energy going into the battery as compared to the amount of power that is stored and used?

 

Cheers from the Land of Oz.

 

We love the show. Thank you.

Answer

That will rather depend on what you call an efficient process! 

Certainly, not all of the energy which you use to charge a battery will come out of the battery in the end.  You can feel that by feeling the battery - whilst it is being charged, it is getting warmed.  So there must be energy being wasted.

If you look at the efficiency of charging standard, nickel cadmium or nickel metal hydride battery, the efficiency is about 60 to 70%, so you're wasting 30 or 40% of the energy you're putting into the battery itself, and you're probably also wasting some more energy in the charger because that's not going to be 100% efficient either.  So, you might be talking about half the energy you're using actually ending up in that battery.

That doesn’t sound very good, but if you're going to use something with a battery, you need some power and you need to get it from somewhere.  If you compare that to using a throw-away battery, that's going to be – I would’ve thought - only a few percent efficient.  Only 1 or 2% efficient because you've got to get materials to make a battery, you've got to refine them, you've got to put them all into a case.  And so, [60% efficiency] doesn’t sound very good, but it’s far, far better than the alternatives.

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If you use the batteries within a few days after recharging, you get back most of the energy you put into them. It's not 100% because the batteries get warm while charging, and that warmth is lost to the surroundings.

If you don't use them for a month or two, they will lose all their charge due to internal leakage. So that makes them 0% efficient. You should not use rechargeables for smoke detectors because they will need to be recharged every month or two, whereas most non-rechargeables can power a smoke detector for at least a year.

If you store a bunch of batteries for emergencies, they should be non-rechargeable.

Hopefully, someone will soon invent a rechargeable battery that doesn't have to be recharged so often. Phractality, Sun, 1st Apr 2012

How does the charge retention of reachargable batteries comprae with capacitors?, there was some worry after the war that the capacitors in unexploded bombs and mines might retain their charge for several years, was this misguided ?. syhprum, Sun, 1st Apr 2012

There are quite a few kinds of rechargeable batteries now designed to be able to hold their charge for months, and they come charged rather than flat too, though they still do lose charge over time and won't necessarily perform well straight out of the packaging.

Capacitors do seem to hold their charge for a long time - a flashgun put away for months may be able to release a full flash after all that time even if the batteries were taken out of it when it was stored, but I don't know what would happen if it was left for years. Perhaps someone will try it and answer in this thread... David Cooper, Sun, 1st Apr 2012

Electrolytic capacitors don't hold their charge as well as paper or mica capacitors. I think the latter can hold a charge for many years, provided the voltage is not sufficient to ionize the surrounding air. Phractality, Sun, 1st Apr 2012

Some types of capacitors can even "self-charge", which can be very dangerous. Geezer, Mon, 2nd Apr 2012

The types of general-purpose rechargeables you use in toys and things are normally NiMH, and are somewhere around 80% efficient (you get back about 80% of what you put in) - assuming you have a good quality charger which terminates the charge when it should. If you put excess charge in then you won't get that energy back AND it'll prematurely wear-out the batteries. techmind, Wed, 4th Apr 2012

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