Why does my motherboard have a battery?

If you are using an old computer then your motherboard contains a built-in round battery. But unlike a standard laptop battery, this small motherboard battery is not your computer's main source of power. What exactly is it used for?


The battery on the motherboard (known as "CMOS battery") is a small round battery that only supplies power when you are not using your computer. So why is there a battery on the motherboard and why? How long does a CMOS battery last? Let's find out.

What is a CMOS battery?
CMOS stands for Complementary Metal-Oxide-Semiconductor. In the early days of personal computers, CMOS RAM (a type of cache) stored BIOS settings.


CMOS RAM required a battery, otherwise the settings would be lost each time the computer was shut down. This is why this particular battery is called a CMOS Battery. Modern computers do not use CMOS RAM. They store the BIOS settings on an EEPROM circuit or in flash memory, which means that the settings do not need constant power to be stored permanently. Although you do not need a battery in these systems, the small CMOS battery is still present in many machines.

What is UEFI?
The Unified Extensible Firmware Interface (UEFI) was introduced in 2007 to replace the BIOS (Basic Input / Output System). It is an industry standard agreed upon by chip makers (Intel and AMD), software makers (Microsoft) and PC makers to improve the BIOS. In a few words UEFI is the evolution of the BIOS.

Due to the limitations of the BIOS and its incompatibility with the new high-speed processors, the manufacturers agreed to the UEFI standard. UEFI overcomes these limitations by adding, for example, support for 2,2 TB or larger drives, 32-bit and 64-bit functions, faster boot, better graphical interface, its own drivers and operating system-independent applications, and finally secure Boot.


This last function is a method of computer security. Secure Boot ensures that no malware exploits the computer boot process. It does this by checking that any code executed at startup has a valid digital signature.

Other features of UEFI include boot option, overclocking and setting of various motherboard features.

What is the BIOS?
Instead of UEFI, older computers use the BIOS or a basic input / output system stored on a chip on your computer motherboard.

When your computer starts, the BIOS starts, performs a power-on self-test (POST), and initializes the computer hardware. The BIOS then transfers control to a boot loader, usually located on your hard drive. (The boot loader can also boot from a USB device or optical disc.)


The boot loader then loads your operating system - Windows, Linux, macOS or whatever. The BIOS is responsible for low-level system tasks. You can enter the BIOS settings screen of your computer by pressing a specific key when starting your machine.

The BIOS settings screen lets you make low-level settings for your computer hardware. These differ from motherboard manufacturers, but some options are the same. An example is changing the computer's boot order - the order in which a computer loads operating systems from connected storage.

Because your motherboard has one more battery
So, although many computers store BIOS settings in a flash drive, why do motherboards still come with a battery? Because motherboards simply contain another Real Time Clock (RTC) that needs power to maintain its data.

Whether your computer is turned off or on, the small CMOS battery is running continuously. Your computer's internal clock is essentially a quartz clock, just like an old wristwatch.

When the computer is turned off, the battery provides power to measure time. This is how your computer always knows the right time to turn it on and does not show you a date 10 years ago.

When is it time to replace your motherboard battery?
As we all know from experience, batteries do not last forever. At some point the CMOS battery will stop working. They usually last up to 10 years.

Regular use of your computer means that the CMOS battery lasts longer, since it is rechargeable. Conversely, a battery in a computer that is normally shut down will die earlier, as it uses the battery more and loses charge cycles.

If you have an even older computer with BIOS cached and the CMOS battery is exhausted, then you will see error messages such as:

  • CMOS Battery Failure
  • ACPI BIOS Error
  • CMOS Read Error
  • CMOS Checksum Error
  • New CPU Installed

The latter, although it seems strange, the explanation is simple. Without a battery that powers the BIOS, the motherboard cannot remember that the CPU was already installed. Therefore, it thinks it is new every time you start your computer.

On a newer computer that stores the BIOS settings in a flash drive, but with the CMOS battery unlocked, the computer will boot normally, but will start counting down the time from the date it was built. For example it will tell you that it is now January 1, 2007. This can lead to connection problems and problems receiving application updates, so it is worthwhile to change its battery, because otherwise you have to start at the right time and chronology.

How to replace the motherboard CMOS battery
To fix these problems, you will need to replace the CMOS battery, a small, silver disc on the motherboard. It is usually a CR2032 battery, also used in calculators, watches and other small electronic devices.


Before proceeding, you must turn off your computer, unplug the power cord, and if you are using a laptop, unplug the battery. Then press your computer's power button for 10 seconds to discharge all the internal capacitors. Be sure to follow the standard computer maintenance steps when opening it and paying attention to static electricity.

Note that on some computers the battery may be stuck to the motherboard. This will require a complete replacement of the motherboard or repair by the manufacturer.

If all goes well, simply remove the battery and replace it with a new one. Be sure to wear disposable plastic gloves to avoid short-circuiting the new battery with your bare hands. The technique of the above photo is not correct. Pay attention to its polarity. Do not place it upside down. For goodness sake, take a picture beforehand, so you know it was in its normal position.

Remove the CMOS battery to troubleshoot the PC
Removing and reconnecting the CMOS battery to older computers can be used as a troubleshooting step.

For example, if a computer has a BIOS code, removing and replacing the CMOS battery will clear the password. Note that your other personal BIOS settings will also be deleted. If your computer stores the password in a fixed memory, this will not help you. Logically you will find a different way to reset the password, using a jumper on the motherboard.

In conclusion:
On older systems, the CMOS battery retains the BIOS settings.
For the latest machines, the CMOS battery powers the computer clock.
Replacing the CR2032 battery on your motherboard is a simple task.

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