How efficient are rechargeable batteries?

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.