I recently purchased charger which came with a set of Nickel Metal Hydride AA batteries. I notice the batteries are rated at 1.2 volts, whereas regular alkaline batteries are 1.5 volts. Why the difference in voltage? And will some devices have inferior performance with NiMH due to the lower voltage?
Technically, what you're talking about is a "cell," not a "battery," although common usage IS certainly to call all such things "batteries." But this is one case where the difference is important.
A "cell" is the smallest unit of an electrochemical source that will produce power - i.e., it is the basic unit of construction for "batteries" (which are arrays of multiple cells, generally in series), and it has one each "positive" plate and "negative" plate. The voltage you get across a single cell depends on the particular chemistry in question. For instance, the lead-acid batteries you have in your car provide about 2.1 volts per cell, so a six cell battery actually has a nominal unloaded voltage, at full charge, of about 12.6 volts. The good old carbon-zinc "dry" cells that used to be the norm in flashlights and such, as well as the more recently developed "alkaline" types, provide 1.5 volts/cell. And, as you've discovered, nickel metal hydride (NiMH) cells give you about 1.2 volts/cell, nominal.
Some devices intended for the carbon-zinc or standard alkaline types may, in fact, have trouble with the lower cell voltage; hopefully, they'll tell you this in the operator's manual, and advise that you not use the NiMH or other lower-voltage types. And others will have no problem at all.
Bob M.
Different chemicals result in different emf (voltage). No two batteries of different chemical composition have exactly the same voltage. Many people call cells with anywhere between 1 and 2 volts "one and a half" volt batteries, since for so many decades carbon-zinc was the only game in town, and we got so used to it.
Indeed, I had a camera that would not work with NiMH cells because it didn't have quite enough emf.
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