CMOS batteries
The CMOS battery is used to store system information, such as the time and date, and should be replaced when receiving the error "System battery dead", if the computer does not keep the correct time or date, or if you are losing a dramatic amount of time over a period of time.
Before replacing the CMOS battery, you need to know what battery the computer is using. This can be done either by opening the computer and locating the battery or through the motherboard or computer documentation. We suggest visiting a local Radio Shack or other electronics store if you've got the battery but can't identify it or to purchase a new battery.