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This battery is usually a non-rechargeable lithium coin cell, or sometimes a rechargeable nickel battery in a plastic sleeve. Either way it may fail after maybe 10 years or more causing the computer to lose its time and date on loss of power. If you're lucky, it will be on a flying lead connected to the motherboard via a miniature plug and socket, or it might be a coin cell in a socket. Otherwise it could be soldered in and a certain level of disassembly may be required to access it. If you can't get hold of an identical replacement, provided it's the same voltage and the same chemistry it should be fine. If you need to improvise at all, make sure you get a "tagged" battery having tags spot-welded to it which you can solder to. It's not possible to solder directly to a battery of any type without damaging it.