Technology Explained

Why Does My Motherboard Have a Battery?

Christian Cawley Updated 18-12-2019

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.

Advertisement

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.

A typical CR2032 motherboard battery

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 How to Disable UEFI Secure Boot to Dual Boot Any System UEFI can interfere with installing a second OS. Here's how to disable UEFI Secure Boot and dual boot any operating systems you like. Read More 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.

Motherboard CMOS batteries retain BIOS settings on old systems
Image credit: Toniperis/Wikimedia Commons

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 How to Change the Boot Order on Your PC (So You Can Boot From USB) Learn how to change the boot order of your computer. It's important for troubleshooting issues and tweaking settings, and it's a lot easier than you think. Read More —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 Your Windows 10 Time Is Wrong? Here's How to Fix the Clock When your Windows 10 time is wrong or keeps changing, the fix could be easy. Don't let your computer clock go wrong again. Read More 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 What Is Static Electricity? And How to Get Rid of It Concerned about static electricity? If you're building a PC, you should be. Learn how to get rid of static electricity today. Read More

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 The Best 18650 Battery and How to Avoid Buying Fakes The 18650 is a type of rechargeable lithium-ion battery. Looking for the best 18650 battery? Here's what you should know about them and how to avoid dangerous fakes. Read More .

Image Credit: amphoto/Depositphotos

Explore more about: Battery Life, BIOS, UEFI.

Whatsapp Pinterest

Enjoyed this article? Stay informed by joining our newsletter!

Enter your Email

Leave a Reply

Your email address will not be published. Required fields are marked *

  1. Ahmed
    November 30, 2018 at 6:46 pm

    "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"
    Are you sure?

  2. Mike Walsh
    September 22, 2017 at 7:19 pm

    I don't really agree with your statement that soldered-on batteries necessitate a motherboard change, or a repair performed by the manufacturer.

    I've got a 15-yr old Dell laptop; an original Inspiron 1100, from 2002. The CMOS battery was actually rechargeable, although soldered to the underside of the motherboard. It only expired last year, believe it or not; I sourced a replacement battery from a specialist battery supplier in London, in the UK; performed a complete strip down (easier on laptops of that vintage; you may recall they were a lot more spacious inside..!); removed the old battery, soldered the new one on, and re-assembled it.

    Should last me another decade, at least..!

  3. hana
    April 3, 2017 at 4:36 am

    i replace a new cmos battery, change date and time but after shutdown time and date never change

  4. mike
    February 24, 2017 at 6:11 am

    I always see these pictures of someone holding a "new" uninstalled battery. Has anyone actually tried to pry it out of that snap connector? I tried, its almost impossible. So far I have a one in two success rate, the other attempt broke the motherboard. It would be nice to see an article on how to actually get the battery out of the holder.

    • Dr. Zook
      November 14, 2018 at 10:41 am

      There is a clip on one side of the battery compartment that holds the battery in place. Pull the clip back a little bit and the battery will pop out.

    • Dr. Zook
      November 14, 2018 at 10:43 am

      The battery compartment has a clip on one side. Pull the clip outwards (away from the centre of the battery) a little bit and the battery should pop out all by itself.

  5. Dylan LaRose
    October 29, 2016 at 2:16 am

    can you hook up a light that also uses 3vs to the battery also?

  6. Roja
    October 20, 2016 at 5:41 pm

    It's very useful to me. Thanks a lot. Now I have a clear idea about bios.

  7. sunil
    February 5, 2016 at 3:21 am

    batter of my pc is dead the time doesn't work.my computer shuts down itself frequently. is it because this battery case?

  8. Sam
    January 17, 2015 at 11:11 am

    Thank you!

  9. Roystan Ang
    March 6, 2013 at 7:10 pm

    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 =)

  10. Ron Lister
    February 22, 2013 at 4:55 am

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

  11. Keith Swartz
    February 22, 2013 at 1:01 am

    Great read! Thanks for the write!

  12. Bob
    February 21, 2013 at 6:25 pm

    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 :)

  13. Piseth Mao
    February 21, 2013 at 7:46 am

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

  14. Kirby
    February 21, 2013 at 7:32 am

    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.

  15. raj
    February 21, 2013 at 6:00 am

    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

    • Kirby
      February 21, 2013 at 7:34 am

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

      • raj
        February 23, 2013 at 8:48 pm

        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
          February 28, 2013 at 6:15 pm

          Yes, it's quite likely a battery problem.

    • Juan Pedro Secondo
      February 21, 2013 at 5:28 pm

      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.

      • raj
        February 23, 2013 at 8:54 pm

        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
          February 25, 2013 at 4:10 pm

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

        • raj
          February 25, 2013 at 6:46 pm

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

  16. Paul G. Williams
    February 21, 2013 at 4:40 am

    This is really good to know, thanks.

  17. prasanth vikkath
    February 21, 2013 at 4:03 am

    Ya I know lol

  18. Ivan
    February 20, 2013 at 11:45 pm

    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
      February 21, 2013 at 4:24 am

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

      • Ivan
        February 21, 2013 at 8:42 am

        You should check this: http://en.wikipedia.org/wiki/BIOS :)

        • Chris Hoffman
          February 28, 2013 at 6:14 pm

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

  19. Doc
    February 20, 2013 at 10:27 pm

    "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
      February 21, 2013 at 5:34 am

      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
      February 28, 2013 at 6:13 pm

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

  20. Anonymous
    February 20, 2013 at 10:17 pm

    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.