Why Does My Motherboard Have A Battery?

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battery on computer motherboard   Why Does My Motherboard Have A Battery?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.)

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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.

bios setup utility memory timing parameters   Why Does My Motherboard Have A Battery?

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.

save to cmos and exit   Why Does My Motherboard Have A Battery?

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.)

insides of quartz watch with battery   Why Does My Motherboard Have A Battery?

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.

cmos checksum error   Why Does My Motherboard Have A Battery?

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.

putting cmos battery in motherboard   Why Does My Motherboard Have A Battery?

“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.”

Do you have any other questions? Leave a comment or ask your fellow readers on MakeUseOf Answers!

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

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24 Comments - Write a Comment



WOW. I saw the battery in there when I was cleaning my pc. I thought about it for a minute, and wondered what it was for. It’s funny that I see this post now and enlightened me. I love makeuseof.com.



“CMOS stands for complimentary metal-oxide-semiconductor.” Actually, it’s “complementary.” “Complimentary” means giving out a compliment, praise, or approval; “complementary” means it complements, or completes, something. The two words have different meanings, but are often confused for one another.

Alberto Lerma

Usually when someone say something like this I show up this picture: http://www.flickr.com/photos/8311021@N03/3856828826/

But in this case you’re totally right, if you’re trying to explain (and publish) something YOU SHOULD (at least) write it correctly.

Chris Hoffman

Sorry, you’re right. It’s easy to miss such little typos while writing such a long piece, however. It’s fixed!



I think it should be ROM, not RAM.. ROM (Read Onlu Memory) and RAM (Random Access Memory) are “quite” different. We could only write data in ROM one time only, But in RAM we could store, wirte and read data manytime.. You should correct it.. But thanks for the info anyway.. :)

Craig Friday

but if it was ROM it could not be changed
you can change your settings so it is RAM

Chris Hoffman

Well, the BIOS itself is stored in ROM, and the settings are stored in RAM.


prasanth vikkath

Ya I know lol


Paul G. Williams

This is really good to know, thanks.



my laptop does not show correct time……everytime i have to reset using internet time sync after turning on laptop………….is it related to cmos battery ??????????
help me


If your laptop has a motherboard battery, that’s likely the cause. Have you spent a large amount of time NOT using your laptop?


i’ve been using my laptop regularly….and i am not sure about whether my laptop has any motherboard battery its HP G 62 ,I had purchased it two years ago

Chris Hoffman

Yes, it’s quite likely a battery problem.

Juan Pedro Secondo

Some laptops were designed to be used on 110 volts 50 Hz. If you’re using 220 volts 60 Hz that might change the clocking too.

Your computer has a difference in time every time you start up or is it reset like 01/01/2000 for example. When the time is completely reset then the culprit is the battery.


i assume there is no such frequency problem

Date shown by my laptop is close to actual date but time shown is always wrong…….so i guess its not battery problem

Juan Pedro Secondo

The other culprit could be overclocking the CPU. That’s forcing the CPU to work at a faster frequency than it was designed.


thanx for your reply……………would you please tell me about how to check whether its overclocked or not



I’ve used the troubleshooting technique you mentioned earlier. – “Pulling the CMOS battery” – in other words, removing it and re-inserting it – may also be used as a troubleshooting step…”

It actually worked in resetting the BIOS settings.

My motherboard battery took 8 years to run out off juice. However, until now, I still don’t bother changing it. It won’t cause significant problems to your OS anyways. It is quite annoying to constantly see the error messages each time I cold boot though.


Piseth Mao

Then from now on I should not leave my Windows Laptop off for so long!



Had a case recently where a computer wouldn’t boot. Went to manufacturer screen with options and then zip! Suspected power or hard drive – hard drive fine, pulled CMOS battery and replaced problem solved. Lots of the tech sites were talking about dead motherboard and other heavy stuff! (PC is HP DC7600) HAve had a few minor faults with this all of which people have said would be fatal. Usually fixed fairly cheaply :)


Keith Swartz

Great read! Thanks for the write!


Ron Lister

I’ve had to change those before and had to move the jumper to the other pin to reset the CMOS too.


Roystan Ang

My Asus Rampage II Extreme is not remembering the BIOS settings I have saved. It resets everytime even though I have replaced with new batteries and still no luck. Any possible fix? Thanks in advance =)

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