What a CMOS battery does and why you might remove it

The CMOS battery is a small coin-shaped battery inside your computer that keeps your system clock running and stores basic hardware settings even when the power is off. It typically lasts three to ten years. You might need to remove it to reset your BIOS settings to factory defaults, troubleshoot startup problems, clear a forgotten BIOS password, or replace the battery when it dies.

Removing the CMOS battery is one of the simplest hardware tasks you can do yourself. It requires no special tools and takes about five minutes. The battery sits on your motherboard — the main circuit board inside your computer case — and pops out by hand.

Key Takeaways

  • The CMOS battery is a small round battery on your motherboard that keeps your system clock and BIOS settings running when the computer is off.
  • You must shut down your computer completely, unplug the power cable, and wait a few seconds before opening the case to avoid electrical damage.
  • The battery sits in a small plastic holder on the motherboard and usually pops out by pressing a small metal clip or by lifting the battery straight up.
  • After removing the battery, wait 10 to 30 seconds before reinstalling it or leaving it out to let the CMOS memory clear.
  • If you cannot locate the battery or it does not pop out easily, check your motherboard manual or look for a small round silver disc about the size of a dime.

Shut down and open your computer safely

Start by shutting down your computer completely through the operating system. Do not force a restart or hold the power button. Once the screen goes dark, wait a few seconds, then unplug the power cable from the back of the computer tower. This step is critical — even when a computer appears off, electricity can still reach the motherboard if it is plugged in.

If your computer has a power supply switch on the back (a small toggle or button), flip it to the off position as well. Wait at least 10 seconds. This gives any remaining electrical charge time to drain from the motherboard.

Open your computer case by removing the side panel. Most cases have two or three thumbscrews on the back, or a latch you slide. If you are unsure how to open your specific case, check the manual or look for a diagram on the inside of the panel itself. Set the panel aside in a safe place where it will not get scratched.

Locate the CMOS battery on the motherboard

Look at the motherboard — the large green or brown circuit board that fills most of the case. The CMOS battery is a small round silver disc, usually about the size of a dime or slightly larger. It sits in a shallow plastic holder with a small metal clip on one side. The holder is typically labeled "CMOS" or "BATT" on the motherboard itself.

The battery is usually located near the edge of the motherboard, often in the lower half or middle section. If you cannot spot it immediately, look for any round silver disc sitting in a plastic socket. If your motherboard manual is available, it will show you the exact location — you can often find the manual online by searching for your motherboard model number.

Remove the battery

Look at the plastic holder holding the battery. Most holders have a small metal clip on one side. Press or push this clip away from the battery — it should move slightly. The battery will pop up or slide out of the holder. If there is no visible clip, try gently lifting the battery straight up with your fingernail or a plastic tool. Do not use a metal screwdriver, as it can damage the motherboard.

If the battery does not move easily, stop and check your motherboard manual. Some older or specialized boards have different battery holders. Forcing it can break the plastic socket and damage your motherboard permanently.

Once the battery is out, set it aside. You now have a few options: leave the battery out for 10 to 30 seconds to clear the CMOS memory, replace it with a new battery of the same type (usually CR2032), or reinstall the same battery if you were just testing the removal.

Reinstall the battery or replace it

If you are replacing the battery, take the old one to an electronics store or pharmacy — most places that sell batteries can match it. The most common CMOS battery is a CR2032, but check the old battery to be sure. When you have the new battery, slide or press it into the holder with the positive side (marked with a + symbol) facing up. You should hear or feel a small click when it seats properly.

If you are reinstalling the same battery after clearing the CMOS, wait at least 10 seconds with the battery out before putting it back in. This ensures the CMOS memory has fully cleared. Then slide the battery back into the holder the same way it came out, positive side up.

Close the case and power on

Once the battery is back in place, close your computer case by sliding the side panel back on and tightening the thumbscrews or latches. Do not over-tighten — just snug enough that the panel does not rattle.

Plug the power cable back into the computer and flip the power supply switch to on if your case has one. Press the power button to start the computer. You may see a message on startup saying the CMOS has been reset or that the date and time are incorrect — this is normal. You can set the correct date and time in your BIOS settings or in Windows.

What to expect after removing the CMOS battery

If you removed the battery to reset your BIOS, your computer will boot with all settings returned to factory defaults. This means any custom settings you had — like boot order, overclocking, or security settings — will be gone. Your system clock will also reset to a default date, usually sometime in the early 2000s.

If you removed the battery to clear a forgotten BIOS password, the password will no longer work on your next startup. You will be able to enter the BIOS setup menu without a password. If you removed it simply to replace a dead battery, everything should work normally once the new battery is installed.

If your computer does not start after reinstalling the battery, make sure the battery is fully seated in its holder and the positive side is facing up. If it still does not start, the battery may not be the problem — there could be another hardware issue.

Frequently Asked Questions

Can I remove the CMOS battery while the computer is on?

No. Always shut down completely and unplug the power cable first. Removing the battery while the computer is powered can damage the motherboard or other components. Even if the computer appears off, electricity can still be present in the system.

How long should I leave the CMOS battery out?

Leave it out for at least 10 to 30 seconds. This gives the CMOS memory time to clear completely. Some people leave it out for a minute or longer, but 30 seconds is usually enough. The longer you wait, the more certain you can be that all settings have been erased.

What type of battery do I need to replace a dead CMOS battery?

The most common type is CR2032, but check your old battery to be certain. The battery type is printed on the surface. You can buy a replacement at any pharmacy, electronics store, or online retailer. They typically cost between two and five dollars.

Will removing the CMOS battery erase my files or operating system?

No. The CMOS battery only stores hardware settings and the system clock, not your files or operating system. Everything on your hard drive or SSD will remain unchanged. You will only lose custom BIOS settings and the correct date and time.

What if I cannot find the CMOS battery on my motherboard?

Check your motherboard manual — search online for your motherboard model number plus "manual" to find a PDF. The manual will show you exactly where the battery is located. If you still cannot find it, take a photo of your motherboard and search for your model online, or contact the motherboard manufacturer for help.