Purpose of the CMOS Battery

A computer’s motherboard is made up of many essential components including the complementary metal-oxide semiconductor (CMOS) battery. Without this battery, a computer cannot properly function or even boot at all. While a CMOS battery is designed to last up to 10 years, it is possible for it to weaken and suffer failure, which will become evident in the computer’s performance.

The CMOS battery looks like a watch battery on your computer's motherboard.


In 1982, the CMOS 65C02 processor was first introduced by Western Design Center, Inc. Computers using the Intel 80286 processor and beyond began to require a small CMOS battery on the motherboard to help provide power to the CMOS chip at all times, even when the computer was not turned on. All modern computer motherboards still require a CMOS battery.


The CMOS battery most often resembles a large watch battery. It is usually a silver coin cell lithium battery, often similar in size to a nickel. This battery is installed into a circular slot on the computer’s motherboard and provides power to the CMOS semiconductor chip that stores important system-settings information.


The function of the CMOS battery is to provide continuous power to the CMOS chip on the computer’s motherboard. The CMOS chip stores essential system information and settings such as date and time, hard disk type, boot drive order, floppy drive type and BIOS settings. The CMOS chip requires the power source of the battery to sustain and preserve these settings.

Signs of Failure

A CMOS battery can begin to weaken and fail over time, depending on the environment the computer is exposed to. Some symptoms of a weakening battery are consistent changes in system date and time, system component drivers requiring re-installation every time the computer is booted or a computer that shuts itself down consistently without a reason. The BIOS may provide low-battery warnings on the start-up screen. The computer may also report error messages such as "CMOS Checksum Error," "CMOS Read Error" or "CMOS Battery Failure."


If your computer is reporting CMOS errors, first attempt to recharge the CMOS battery by leaving your computer running for a full 24 hours. Often, this will clear up a CMOS battery issue. But if recharging does not fix the issue, you may need to replace the CMOS battery. You will have to open your computer, find the battery on the motherboard and record the information etched on the battery to find a matched replacement for it. You can easily replace the coin cell battery by gently pulling the old battery out of its container with your fingers and inserting a new one in its place.