What a CMOS battery does and why it fails

Your computer's CMOS battery is a small coin-shaped battery (usually CR2032) that sits on your motherboard and keeps your system clock running and your BIOS settings saved when the computer is powered off. When it dies, your computer will forget the time, date, and any custom settings you made in BIOS — and it may fail to start until you replace it.

CMOS batteries typically last three to ten years depending on how often your computer runs. You'll know it's failing when your system clock resets to an earlier date every time you restart, or when you see CMOS checksum error messages on startup. The battery itself doesn't wear out from use — it simply loses its charge over time.

Replacing it is one of the easiest hardware repairs you can do yourself, and you don't need special tools or technical knowledge. The whole process takes about ten minutes.

Key Takeaways

  • A CMOS battery keeps your computer's clock and BIOS settings alive when power is off, and lasts three to ten years before needing replacement.
  • You'll need a CR2032 coin battery (about $3 to $8), a Phillips screwdriver, and an anti-static wrist strap to protect your motherboard from static discharge.
  • The battery sits in a small circular socket on the motherboard and pops out with gentle pressure — you don't need to solder or use tools to remove it.
  • After replacement, your computer may ask you to confirm the date and time on first startup, and you'll need to re-enter any custom BIOS settings you had before.

Gather your tools and the right replacement battery

You need three things: a CR2032 battery (the standard CMOS battery for most computers), a Phillips screwdriver, and an anti-static wrist strap. The battery costs $3 to $8 at any pharmacy, electronics store, or online retailer. The wrist strap (also called an ESD strap) costs $5 to $15 and connects to a grounded surface to prevent static electricity from damaging your motherboard — it's not strictly required for a single battery swap, but it's cheap insurance.

Before you buy, check your motherboard manual or search online for your specific motherboard model to confirm it uses a CR2032. Most desktop computers do, but some older or specialized boards use different sizes. If you can't find the manual, you can open the case and look at the battery itself — the size is printed on it.

Do not attempt this repair on a laptop unless you are comfortable disassembling one. Laptop CMOS batteries are often soldered to the motherboard or require removing the entire bottom panel, and the risk of damage is much higher.

Power down and open your computer safely

Shut down your computer completely and unplug the power cable from the back of the case. Wait at least 30 seconds — this drains residual power from the motherboard and reduces the risk of static damage or accidental short circuits.

Put on your anti-static wrist strap and clip the other end to a metal part of the case (usually a screw or the metal frame). This grounds you and prevents static electricity from your body from reaching the motherboard.

Open your case using the Phillips screwdriver. Most desktop cases have two or four screws on the back panel. Remove the side panel that faces the motherboard — you're looking for the one that gives you a clear view of the circuit board inside.

Locate and remove the CMOS battery

Look for a small circular battery holder on the motherboard. It's usually silver or gold-colored and about the size of a dime. The battery sits in a socket with a small metal clip on one side. You'll find it somewhere in the middle or lower section of the board, often near the RAM slots or power connectors.

If you can't spot it, check your motherboard manual — it will show you exactly where it is. You can also search online for "[your motherboard model] CMOS battery location" and look at photos.

Gently push the metal clip away from the battery. The battery will pop up slightly. Lift it out with your fingers — it's not glued or soldered, just sitting in the socket. If it doesn't pop out easily, don't force it. Push the clip a bit more firmly and try again.

Install the new battery and close the case

Take your new CR2032 battery and check the orientation. The flat side with the number printed on it should face up (toward you). Slide the battery into the socket with the flat side up, then gently press down until the metal clip snaps back into place. You'll hear or feel a small click when it's seated correctly.

Close your case and screw the side panel back on. Plug the power cable back in and start your computer normally.

What to expect after you restart

Your computer may display a CMOS checksum error or ask you to confirm the date and time on the first startup. This is normal — the BIOS is simply noticing that the battery was replaced. Press the key it tells you to (usually Delete or F2) to enter BIOS setup, confirm the date and time are correct, and save your changes. Your computer will then restart normally.

If you had custom BIOS settings before — like overclocking, boot order, or security features — you'll need to re-enter them now. The BIOS resets to factory defaults when the battery dies, so write down any custom settings before you start if you remember them. If you don't remember them, the factory defaults are usually fine for everyday use.

After this first restart, your computer should work normally. The clock will keep accurate time, and your settings will stay saved even when the power is off.

Frequently Asked Questions

Can I replace the CMOS battery while the computer is in sleep mode?

No. Always shut down completely and unplug the power cable. Sleep mode leaves power flowing to the motherboard, and touching components while power is present can damage them or cause injury.

What if the battery doesn't pop out when I push the clip?

Push the clip more firmly — it may need more pressure than you expect. If it still won't budge, check that you're pushing the clip in the right direction (away from the battery, not toward it). If the battery is truly stuck, stop and consult your motherboard manual rather than risk breaking the socket.

Do I need to back up my BIOS settings before replacing the battery?

Most motherboards don't have a built-in backup feature for BIOS settings. If you have custom settings you want to keep, write them down before you start. After you replace the battery, you can re-enter them manually in BIOS setup.

Will replacing the CMOS battery erase my files or programs?

No. The CMOS battery only stores the system clock and BIOS settings. Your hard drive and all your files remain untouched. The only thing you'll lose is the custom BIOS settings, which reset to factory defaults.

How do I know if my computer actually needs a new CMOS battery?

The most common sign is the system clock resetting to an earlier date every time you restart. You may also see "CMOS battery low" or "CMOS checksum error" messages on startup. If your clock is fine and you see no error messages, your battery is probably still good.