What Is a CMOS Battery?

By Rebecca O'Brien

The CMOS battery is a small battery that powers certain components on a computer's motherboard while the computer is turned off. The battery is a 3 volt lithium button cell and has a lifespan of up to ten years. Without a CMOS battery, many important computer systems would not be able to function.


CMOS batteries are usually CR2032 batteries. They are small, silver circles that sit on the computer's motherboard.


The CMOS battery maintains the system clock and the memory that stores the BIOS while the computer is powered off.


A CMOS battery offers warning signs before it dies. The most common problem is a system time that changes each time the computer starts.

Expert Insight

If a computer has been left off for an extremely long period of time and will not boot, the CMOS battery may need to be replaced.

Fun Fact

Even though the CMOS battery is integral to the function of the computer, it is the easiest and most inexpensive component to replace.