Question for EE (electrical engineering) folks

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Can anyone who is an electrical engineer or who is a EE major tell me, in fairly non-technical terms, why rechargeable AAA and AA batteries are 1.2 volts instead of the 1.5 volts like their non-rechargeable counterparts? I know a little physics (two semesters of the non-calculus based physics), some chemistry (had a chemistry minor), and a bit of math (one semester of calculus which involved more application and proofs of how the derivatives were formed).

I am not an electrical engineer, and I may be wrong, but I've heard it is either impossible or too expensive/difficult to make a battery which can be recharged so it will discharge a precise voltage. In other words, when you put it in a charger and then use it, it may output a slightly different voltage than intended. There's a certain amount of "wiggle room" in the end result. Thus, I believe it's essentially a design decision - if the precise output may vary, to keep it low enough to avoid any possibility of causing harm to the devices the batteries are used in. (Too low a voltage may cause problems in a few cases, but is not likely except under incredibly unusual circumstances to result in damage - even a slight overvoltage will damage some delicate components.)

For that matter, batteries may be officially rated at 1.5 volts - but I've tested brand new batteries whose output fell as low as 1.3 volts... And the voltmeter I used was accurate enough I have no reason to doubt it. I've never tested onewhich put out too high a voltage - again, I presume manufacturers are more careful to avoid the scenario which would result in their product causing damage.

The voltage depends on the reactivity of the materials used to make the battery. You can look up the relative reactivity on a galvanic potentials chart. As an EE, we usually try to minimize the potentials in our circuits to minimize galvanic corrosion. Lithium is very reactive, hence the high voltage of lithium batteries. You have the usual engineering tradeoff in that matching the exact voltage may be secondary to how much energy is stored--folks want their batteries to last.

It might help to study the carbon zinc battery, even if it isn't used very much these days.

http://en.wikipedia.org/wiki/Zinc%E2%80 ... on_battery

It is the electrodes that primarily determine the battery voltage--in this case a graphite rod and the zinc case. Similarly, NiCds use nickel and cadmium electrodes.

Lots of folks use to take these apart to learn about batteries. You shouldn't do that with modern batteries--they have so much energy in them that they can actually burn up.