CMOS Battery

Your PC has to keep certain settings when it’s turned off and its power cord is unplugged.
These settings include the following:

1. Date
2. Time
3. Hard drive configuration
4. Memory

Your PC keeps these settings in a special memory chip called the Complimentary Metal Oxide Semiconductor (CMOS) chip.

Actually, CMOS (usually pronounced see-moss) is a type of memory chip; it is the parameter memory for the BIOS. But that doesn’t translate into an easy-to-say acronym.

So because it’s the most important CMOS chip in the computer, it has come to be called the CMOS.

To keep its settings, the memory must have power constantly. When you shut off a computer,
anything that is left in main memory is lost forever.

To prevent CMOS from losing its information (and it’s rather important that it doesn’t), motherboard manufacturers include a small battery called the CMOS battery to power the CMOS memory.

The batteries come in different shapes and sizes, but they all perform the same function. Most CMOS batteries look like either large watch batteries or small, cylindrical batteries.