Whether you’re using a desktop computer or a laptop, your computer’s motherboard contains an integrated battery. 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 is tiny and only active when you’re not actually using your computer.
The motherboard’s battery is used for low-level system functions like powering the real-time clock and storing a computer’s BIOS settings. On newer computers, the battery may only be used for the clock.
What’s a BIOS?
Every computer has a Basic Input/Output System, known as a BIOS. (Newer computers actually have UEFI firmware. UEFI replaces the traditional BIOS, but largely serves the same role as the BIOS.) The BIOS is 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 located on a device – usually your hard drive, but a boot loader can also boot off a USB device or optical disc. The boot loader then loads your operating system – Windows, Linux, Mac OS X, or whatever else is installed on your computer.
The BIOS is responsible for low-level system tasks. You can enter your computer’s BIOS settings screen by pressing a key during boot. (On new computers that ship with Windows 8, UEFI is used instead, so you’ll need to access the BIOS from within Windows 8.)
The BIOS settings screen allows you to configure low-level settings for your computer’s hardware. For example, you may be able to overclock your computer’s CPU by adjusting settings like the CPU’s multiplier or front side bus (FSB). However, some motherboards may not allow you to control these settings at all. Other settings, like the computer’s boot order – the order in which the computer attempts to load operating systems from connecting hardware devices – and the system time can also be configured here.
What’s a CMOS?
CMOS stands for complementary metal-oxide-semiconductor. Traditionally, BIOS settings were stored in CMOS RAM, which was powered by a battery when the computer was powered off. The battery was necessary because the settings would be lost if all power was lost, just as the data in your computer’s RAM is lost when it loses power.
Modern computers often 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.
Why Your Motherboard Needs a Battery
So if many computers now store BIOS settings in non-volatile memory that doesn’t need a battery, why do motherboards still come with batteries? Simple: Motherboards still include a Real Time Clock (RTC). This clock runs all the time, whether the computer is powered on or not. The real time clock is essentially a quartz watch, like the ones that people wear on their wrists (or used to wear on their wrists before cell phones came along.)
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 the Battery Fails
As we all know from experience, batteries don’t last forever. Eventually, a CMOS battery will stop working. This may happen anywhere between two and ten years from when the computer (or its motherboard) was manufactured. If your computer is powered-on all the time, its battery will last much longer. If the computer is powered-off most of the time, its battery 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”, “CMOS Read Error”, or “CMOS Checksum Error” when you start the computer. You may also see more cryptic error messages, like “New CPU Installed” – the motherboard can’t remember that the CPU was installed previously, so 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.
Replacing the Battery
In these cases, you’ll need to replace the CMOS battery. On most computers, it’s a small, silver disc located on the motherboard. The exact type of battery is usually a CR2032 battery – also used in calculators, watches, and other small electronic devices.
The battery is often removable, so it can be pried out and a new battery can be inserted. (Note that you should power off the computer and be careful of static electricity when doing this sort of thing. Follow all the normal precautions for tinkering around inside of your computer.) However, on some computers, the battery may be soldered on, requiring a complete replacement of the motherboard or a repair performed by the manufacturer.
“Pulling the CMOS battery” – in other words, removing it and re-inserting it – may also be used as a troubleshooting step on computers that store their BIOS settings in CMOS RAM. For example, if a computer has a BIOS password, removing the CMOS battery and reinserting it will cause the computer to forget the BIOS password and all its other settings. (If the computer stores its password in non-volatile memory, this won’t help – although there’s probably a way to reset it using a jumper on the motherboard.)
You can also reset the BIOS settings from within the BIOS settings screen, assuming the computer is booting properly and you’re not locked out of the BIOS setup screen with a password. This option may be named “Clear CMOS” or “Reset to Defaults.”
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Image Credit: Battery on Computer Motherboard via Shutterstock, BIOS Setup Utility via Richard Masoner on Flickr, Save to CMOS and Exit via Nick Gray on Flickr, Insides of Quartz Watch via Shutterstock, CMOS Checksum Error via Gordon Joly on Flickr, Expert Putting CMOS Battery in Motherboard via Shutterstock