If you’re using an older desktop computer or a laptop, your computer’s motherboard contains an integrated battery. But unlike a standard laptop battery, the motherboard’s battery doesn’t power your computer while you’re using it.
Quite the opposite, actually—the battery (known as “CMOS”) is tiny and only active when you’re not using your computer.
So, why is there a battery on the motherboard and what is it for? 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 volatile memory type) stored BIOS settings.
CMOS RAM requires a battery; the settings would otherwise be lost when the PC was switched off.
Modern computers don’t use CMOS RAM anymore. They store the BIOS settings in non-volatile memory, which means that the settings don’t need constant power to be saved.
Meanwhile, modern UEFI motherboards store settings on flash memory or on the computer’s hard disk drive. No battery is necessary on these systems, but you’ll often find it anyway.
What Is UEFI?
The Unified Extensible Firmware Interface (UEFI) specification was introduced to supplant BIOS. An industry-wide standard agreed upon by chip manufacturers Intel and AMD, along with Microsoft and PC manufacturers, UEFI improves on BIOS.
Due to having its roots in the 1980s IBM-compatibles era of personal computers, BIOS has some limitations. UEFI overcomes these, adding, for example, support for drives of 2.2TB or larger, 32-bit and 64-bit modes, and Secure Boot.
This last feature is a method of securing the PC. Secure Boot ensures that malware does not exploit a computer’s boot process. It does this by checking that any code executed at boot has a valid digital signature. Our in depth look at UEFI and how to disable it for dual booting explains further.
Other features of the UEFI include boot selection, overclocking, and configuring various motherboard-specific settings.
What’s a BIOS?
Instead of UEFI, older computers use a BIOS, or Basic Input/Output System, stored in a chip on your computer’s motherboard.
When your computer boots up, the BIOS starts up, performs a power-on self-test (POST), and initializes the computer’s hardware. The BIOS then passes control over to a boot loader, usually on your hard drive. (A 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 your computer’s BIOS settings screen by pressing a key during boot.
The BIOS settings screen allows you to configure low-level settings for your computer’s hardware. These differ across motherboard manufacturers, but some options are universal. An example is changing the computer’s boot order—the order in which the computer loads operating systems from connected storage.
Intel intends to replace BIOS with UEFI on all chipsets by 2020.
Why Your Motherboard Needs a Battery
So, if many computers store BIOS settings in non-volatile memory, why do motherboards still come with batteries? Simple: Motherboards still include a Real Time Clock (RTC).
Power the computer on or off—the battery runs all the time. The real time clock is essentially a quartz watch, like an old wristwatch.
When the computer is off, the battery provides power for the real time clock to run. This is how your computer always knows the correct time when you power it on.
When Is it Time to Replace Your Motherboard Battery?
As we all know from experience, batteries don’t last forever. Eventually, a CMOS battery will stop working; they typically last up to 10 years.
Regular use of your computer means the CMOS battery lasts longer. Conversely, a battery in a computer that is mostly powered-off will die sooner—it’s using the battery more, after all.
If the battery fails on an older computer that stores its BIOS settings in CMOS, you’ll see error messages like:
- CMOS Battery Failure
- ACPI BIOS Error
- CMOS Read Error
- CMOS Checksum Error
- New CPU Installed
This last one is particularly confusing at first, but the explanation is simple. Without a battery powering the BIOS, the motherboard can’t remember that the CPU was already installed previously. As such, it thinks it’s new every time you boot your computer.
On a newer computer that stores its BIOS settings in non-volatile memory, the computer may boot normally, but the computer may stop keeping track of time when it’s powered off. This can lead to connection issues and problems downloading updates, so it is worth fixing.
How to Replace the Motherboard’s CMOS Battery
To fix these problems, you’ll need to replace the CMOS battery, a small, silver disc located on the motherboard. Usually a CR2032 battery, it’s also used in calculators, watches, and other small electronic devices.
Before proceeding, you should power off your computer, remove the power cable, and if using a laptop, disconnect the battery. Take care to follow standard PC maintenance steps when opening your PC and be careful of static electricity
Note that the battery may be soldered onto the motherboard in some computers. This will require a complete replacement of the motherboard or a repair performed by the manufacturer.
Pull the CMOS Battery to Troubleshoot PC Issues
Removing and re-inserting the CMOS battery (known as “pulling”) may also be used as a troubleshooting step on older computers.
For example, if a computer has a BIOS password, removing and replacing the CMOS battery will wipe the password. Note that the other BIOS settings will also be wiped, however.
(If the computer stores its password in non-volatile memory, this won’t help. You might otherwise find a way to reset the password using a jumper on the motherboard.)
You can also reset the BIOS settings from within the BIOS, assuming the computer is booting properly. Look for an option named Clear CMOS or Reset to Defaults.
That’s Why Your Motherboard Has a Battery
So, now you know why there is a battery on your motherboard:
- On older systems, the CMOS battery retains the BIOS settings
- For more recent machines, the CMOS battery powers the PC’s clock
Replacing the CR2032 battery on your motherboard is straightforward, although in some cases they’re fixed to the motherboard. Fortunately, CR2032 batteries are easy to come by, so replacement shouldn’t be an issue.
Here’s another common battery type you’ve probably used but not know the name of: the 18650 battery.
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