What are a CMOS battery and its functions?

What is a CMOS battery? What are its functions?

CMOS (complementary metal-oxide-semiconductor) is a chip on the motherboard that contains BIOS configuration, date, time and other information that the computer needs during startup.

Normally, this information would be lost when the computer is shut down or loses power for some reason (dead battery, power failure, etc.).

The CMOS battery supplies power to the CMOS chip, even when the computer is shut down.

This battery is usually a watch type battery that can supply power for a year or more.

One of the most common signs that your CMOS battery is failing is your computer being unable to keep time and date even after being reset in the BIOS.

Sometimes you will see error messages during boot such as, “CMOS checksum error” or “CMOS read error.

The CMOS battery in a computer is used to keep and maintain the real-time clock RTC, which is the system clock and power the memory chip that contains the system configuration file.

The system configuration files contain all initial settings as configured for the proper computer operation.

PC motherboards have a small “CMOS chip” on them that is powered by a battery even when the PC is off, in fact, even if it’s unplugged.

Sometimes the “CMOS battery” is a watch battery on the motherboard, sometimes a camera or other battery stuck to the case and wired to the board.

“CMOS” in this context is kind of an antiquated misnomer in that it refers to the chip technology used for the “chip” – Complementary Metal Oxide Semiconductor.

Basically, it’s a very low power chip technology – so the battery lasts a long time – which is good because some motherboard vendors seem to consider their boards disposable in that the batteries are not replaceable!

What does the “CMOS chip” do?

It keeps track of the motherboard configuration data. Have you ever booted up to the configuration screen where it describes the amount of memory and disk controller etc? That information is kept in the chip.

It also contains an RTC (Real Time Clock) – a wristwatch so that your computer knows what time it is when it boots up.


CMOS stands for Complementary Metal-Oxide Semiconductor and CMOS battery is also known as Non-Volatile RAM (NVRAM). Basically, CMOS is a battery-powered chip typically used in computers. If we talk specifically then CMOS is used by

  1. Digital logic circuits
  2. Static RAM (SRAM)
  3. Microprocessors
  4. Microcontrollers

Basically, CMOS battery provides power for Real-Time Clock whether the device is turned on or off.

Additionally, if this is removed and re-inserted into the device, then the device BIOS password and all its other settings will get removed.

  • The CMOS is a small memory to store configuration details and BIOS settings needed to initialize the system.
  • The configuration details are date/time, disk drive, etc. It is located in a small area in Motherboard and a battery is also attached that looks like a coin.
  • Friends, It is traditionally called CMOS RAM.
  • It is also referred to as Non-Volatile BIOS memory.
  • The CMOS battery is an RTC(Real Time Clock) because it is responsible for showing the correct date and time. CMOS stands for Complimentary Metal Oxide Semiconductor.

What are a CMOS battery and its functions in 2020?

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