|It could be one of several types:|
The rechargable type already mentioned, usually a short, fat cylinder that kind of looked like a barrel. They always seemed to have corrosion on the ends.
A separate battery pack that attached somewhere on the chassis with velcro and connected to the motherboard with a 4-port, 2-wire connection.
There's one type that looks like an IC chip. It may be found in a 24-pin DIP socket (if you're lucky) or it may be soldered directly to the motherboard. The Dallas DS1287 was the most common. Another brand name was Odin.
Then there's the coin-type mostly used now, about the size of a nickel.
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