CMOS and BIOS battery

In short: The CMOS is a small, low-power memory on the motherboard that holds BIOS settings and the time. The BIOS battery (usually a CR2032 coin cell) powers it while the computer is off.

In more detail: The name comes from the construction (complementary metal-oxide-semiconductor), which needs hardly any power. The settings (boot order, date, time, fan curves) are stored in it. If the battery is empty they are lost and the computer starts with factory settings.

In Depth

• Typical signs of an empty battery: Date and time jump back to an old value, messages such as “CMOS checksum error” or “CMOS battery failure”, settings must always be redone, the computer finds no boot device at startup.
• Lifetime: About 3 to 10 years.
• Replacing: Switch off the computer, unplug the power cord, carefully release the battery from its holder, insert the same coin cell (CR2032, 3 V). Then redo the settings.
• Clearing CMOS: Move the “CLR_CMOS” jumper, press a small button or briefly remove the battery. Helps if the computer does not start because of a wrong setting (for example after overclocking).
• Modern UEFI systems store settings in NVRAM flash, but the clock and a few data still run on the battery.
• Security: A BIOS/UEFI password can be reset this way. Whoever can open the computer can bypass it. That is why chassis locks and disk encryption matter.
• Real-time clock (RTC): The clock also keeps running on this battery.

See also: BIOS, POST (power-on self-test), Motherboard/mainboard