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.

What are a CMOS battery and its functions in 2023?

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.

What is a CMOS battery, and how do I remove one?

If your computer does not start correctly or displays the wrong date and time every time you log on, your CMOS battery is likely drained. Even though it sounds complicated, replacing your CMOS battery is a simple matter. But before that, it is essential to understand what exactly CMOS battery is and what role it plays.

What is the BIOS?

It is essential to understand the meaning of a computer’s BIOS basic input/output system before moving on to CMOS. BIOS is firmware built into your computer. It performs essential but straightforward functions on your computer, especially during startup.

It initializes input devices on your computers, such as the keyboard and mouse. Additionally, it performs a POST-power-on self-test that checks the computer hardware before booting the operating system. It also configures the CMOSwhich forms the non-volatile memory of the BIOS; that is, it retains information even when the computer is turned off.

It is only rudimentary information about BIOS: if you want to learn more, there are many other BIOS-related concepts.

What are a CMOS battery and its functions in 2022?

CMOS stands for “complementary metal-oxide-semiconductor,” In a nutshell, it stores information such as time, date, and system hardware configuration.

The CMOS battery keeps the CMOS and BIOS powered even when the computer is unplugged. It is because the CMOS saves the date and time in real-time. In addition, the hardware configuration and system settings must be accessible as soon as the computer starts. If the CMOS battery runs out, your computer loses all of this information, and the BIOS reverts to its default settings.

Most CMOS batteries last about 2-10 years. It is a wide range because some factors influence it. The CMOS battery is never used when your computer is plugged in. In laptops, the CMOS battery can also charge itself whenever plugged in.

That doesn’t mean your CMOS battery will drain in a few weeks if you don’t plug in your computer. Most CMOS batteries will last at least a few years, even if you don’t plug them into your computer at all.

When to replace the CMOS battery

Your computer will start to operate abnormally when the CMOS battery is drained. Here are some of the telltale signs:

  1. Incorrect date or time: The most common symptom of a drained CMOS battery is the wrong date and time on your computer. Usually, the date and time will reset to a time in the past.
  2. Input devices begin to malfunction: Since the BIOS is responsible for managing devices such as the keyboard and mouse during startup, they may malfunction if the CMOS battery is drained. The mouse cursor may become inaccurate, or there may be a delay in movement. The keyboard and mouse might also stop working altogether, as the BIOS acts as a middleman.
  3. Missing driver: It is also possible that recently installed drivers are missing due to a faulty CMOS battery. These drivers will likely be related to input/output devices connected to your computer.
  4. Power on/off issues: While there could be many reasons why your computer won’t boot at all, one of them could be a failing CMOS battery. Alternatively, your computer may also stop responding to the shutdown button.
  5. One beep/error message: Constant beeping and CMOS-related error messages can also indicate a failing CMOS battery. Error messages include CMOS checksum error, CMOS read error, CMOS battery failure, and low system battery voltage.

How to remove the CMOS battery

Removing the CMOS battery is a relatively simple process, even if you’ve never handled computer components before. Just follow the steps below.

Before handling PC components, get rid of any static discharge you may have in your body. You can do this by simply touching the metal part of your computer case.

How to remove the CMOS battery on a desktop computer

  1. Remove the PC case cover to reveal the parts housed inside. It differs case by case, so refer to the manual. Usually, you will have to unscrew a few bolts and slide the cover off.
  2. Make sure there is enough light to see the motherboard. Remove any dust that may have accumulated.
  3. Now look for a round battery on the motherboard. It should be the size of a coin.
  4. Carefully remove the battery from its housing. Remember to note the positive +ve and opposing -ve sides of the battery. Usually, the positive side is facing up.
  5. If you are replacing the CMOS battery, insert the new battery now, ensuring it is in the same orientation as the old one.
  6. Replace the cover on your computer cabinet.

How to remove the CMOS battery on a laptop

  1. The CMOS battery removal process is a bit more complicated on a laptop due to its compact size and extra screws. Usually, it’s best to hire a professional.
  2. Flip the laptop over, unscrew the bolts and remove the case to reveal the internal components. Ensure the computer is turned off and disconnected from the charging cable.
  3. First, remove the battery from the computer, then find the CMOS battery on the motherboard. It should look exactly like a desktop computer.
  4. Use caution, remove the CMOS battery.
  5. Remove the new battery in the same direction as the previous one.
  6. Reinsert the laptop battery and replace the laptop case.
  7. Tighten all case bolts and turn on your laptop to ensure it is working correctly.

CMOS demystified

The CMOS isn’t hard to figure out, and as the sections above show, replacing the CMOS battery is also a breeze. Additionally, knowledge of other essential PC components can also come in handy.


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 2022?

What are a CMOS battery and its functions in 2022?

The CMOS battery powers your laptop’s BIOS firmware, which is responsible for booting up your computer and configuring data flow. You can tell if your CMOS battery has died if your laptop has difficulty booting up, if drivers disappear, and if your laptop’s date and time are incorrect.

What happens when the CMOS battery fails?

When the CMOS battery starts to lose power and strength, the BIOS starts to lose data and the computer can begin to not work correctly. Some of the signs of a failing CMOS battery: … (CMOS checksum and read error) Drivers may stop working, it can trigger driver blue screens and crashes.

How do I know if my CMOS battery is bad?

Signs of a CMOS Battery Failure

  1. All of the settings like drive type, FDD, NUMs lock, etc., in the computer setup, will be changed. …
  2. Your computer will show a message such as “Booting Error, unable to detect disk drive.”
  3. The computer may be shutting down and will not allow you to perform any task.

Does the CMOS battery need to be replaced?

Replacing a computer’s CMOS battery isn’t difficult, but since CMOS batteries last so long it’s often not even necessary.

There are several reasons the CMOS battery needs to be replaced in your computer. This page provides information on why you would need a replacement and the steps to fix it.

Why do I need to replace my CMOS battery?

  • Computer is losing its time or date settings.
  • An error or alert message, like the following, is displayed when the computer boots.
  • CMOS Read Error
  • CMOS Checksum Error
  • System battery voltage is low
  • CMOS Battery Failure

If any of the issues or messages above are encountered, you likely need to replace the CMOS battery. The process to replace it is detailed in the following sections.

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What are a CMOS battery and its functions in 2022?