CMOS battery
From Conservapedia
In a computer, a CMOS battery is tied to the motherboard, the coordinating mainframe of any PC or Mac computer. The battery is responsible for retaining system memory, and especially date and time information, when the system is powered down. Should the CMOS battery fail, the BIOS (Basic Input-Output System), which coordinates baseline system processes, will likely wipe, as well.
In short, do not remove your CMOS battery.
