[From: firstname.lastname@example.org (Ron Bean)]
The battery maintains power to the CMOS RAM and the real-time clock when your PC is turned off. You may have a small lithium "coin" battery soldered to the motherboard, or a larger external one plugged into a connector. Some motherboards have a jumper to select either type, and a few have a NiCd battery that recharges automatically, or a lithium battery encapsulated in the clock chip. NOTE: Always write down your CMOS settings before you mess with the battery! In fact, you should write them down now anyway, in case the battery fails later. The batteries that are soldered in or encapsulated with the clock chip are supposed to last 10 years or more, but your mileage may vary. Some people find that the external type has to be replaced every couple of years. Self-recharging NiCds that get power from a disk drive cable are available as aftermarket items. A few people have tried to save money by substituting 4 alkaline AA batteries for the expensive external lithium battery, but they have to be replaced more often. If you need to replace a soldered-in battery, have a repair shop install a socket (you shouldn't attempt this yourself, unless you're experienced at soldering on expensive multi-layer circuit boards). If the battery is encapsulated in the chip, there is no way to replace it without replacing the chip-- again, consult a repair shop if it's not socketed. These chips can be "turned off" via software to extend battery life during storage, and are shipped from the factory that way.