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Q: My primary use for rechargeable batteries is in my digital camera. The salesman pointed out to me that NiMH (nickel metal-hydride) batteries are available in varying "strengths" that determine their power and endurance. Is it correct that the mAh (milliamperes per hour) number is an indication of power and that the higher number is better? Are cameras ever limited to what mAh to use and is there a need to match the mAh number when combining batteries in an appliance? Do batteries lose intensity with each charging and is there a point at which they will no longer accept charge -- and how would I distinguish that condition as opposed to a problem with the camera itself?

-- DONNA GRAJCAR, Mount Washington

TECHMAN: Yes, batteries can be complicated.

While mAh can give you a good idea of how much use you might get out of a set of batteries, in reality, the units don't specifically measure that. They measure current and specifically how much current a given battery can supply over time.

There isn't any general prohibition on particular mAh ratings, and that variance is generally acceptable; after all, most NiMH cells are already pushing at only 1.2 volts compared to their alkaline brethren's 1.5, and most digital cameras won't be harmed by using lower mAh ratings -- you can use alkaline batteries in many digital cameras, which generally are rated at less than 800 mAh while most NiMH batteries are rated between 1600 and 2000 mAh. But then again, that's also part of the reason alkaline batteries last such a worthlessly short period in digital cameras.

An 1800 mAh NiMH battery can theoretically supply 1800 milliamperes for an hour, or 18 milliamperes for 100 hours. But if we wanted to know how much energy is in a battery we would measure in Watt-hours or volts X amperes over time. Then we'd factor in the fact that as a battery loses its charge through use, the voltage output drops progressively, and as the voltage drops more current is drawn from the battery, changing the rate at which the cell is depleted.

Even all of that doesn't tell us much about how much energy can be delivered at any point, only how much total accessible electrical energy is in the battery. What all of that means is that no one available number can tell us how much use we'll get out of a given battery -- and, of course, that's even less than half of the whole story because some battery profiles work better in high-drain digital products -- such as your camera -- and some better in lower-drain older technologies -- such as flashlights.

The best bet for most digital camera users is NiMH cells nearing 2000 mAh. They are a little more expensive than the lower mAh batteries normally packaged with a charger or camera but worth it in terms of the extra shots these cells will deliver between charges. You do need to take care that the batteries you put in an appliance are all the same rating and make, also taking care that when installing or charging them they have the same level of depletion.

As to differentiating between failure of a battery to charge and a problem with the camera itself -- the best bet is always to try a fresh pack of batteries. The "memory effect," or tendency of batteries to lose capacity through repeated use, was mainly true for older Ni-Cad (nickel-cadmium) batteries, though it does affect NiMH cells, because, with use, all nickel-based cells will form inert crystals that impede capacity. While "exercise" or repeated full discharge/recharge cycles can sometime recondition cells, most NiMH batteries aren't good for much more than 300 cycles.