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 the 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 the BIOS — the firmware that runs before your operating system loads.

CMOS batteries typically last three to ten years depending on how often your computer runs and how old it is. You'll know it's failing when your system clock resets to an earlier date every time you restart, or when you see error messages about CMOS checksum failures on startup.

Key Takeaways

  • A CMOS battery failure shows up as your computer forgetting the time and date, or losing BIOS settings after shutdown.
  • The battery is a small coin-shaped cell (usually CR2032) that costs $3 to $8 and takes five to ten minutes to replace.
  • You must power off the computer completely, unplug it, and wait a few seconds before opening the case to avoid damaging components.
  • The battery clips into a socket on the motherboard and pops out by gently prying the clip or pressing the battery itself.
  • After replacement, your computer will reset to factory BIOS defaults, so you may need to re-enter custom settings like boot order or overclocking values.

What you need before you start

Gather a CR2032 battery (the most common type, though some older systems use CR2025 or CR2016 — check your motherboard manual or the old battery itself), an anti-static wrist strap, and a Phillips head screwdriver. The wrist strap connects to a bare metal part of your case and prevents static electricity from damaging the motherboard.

If you don't have a wrist strap, touch the metal frame of the case frequently while working to discharge static. Avoid working on carpet or fabric surfaces where static builds up easily. A well-lit workspace and a small container for screws will save you time and frustration.

Powering down and opening the case safely

Shut down your operating system normally, then turn off the power supply switch on the back of the case (or unplug the power cable if there is no switch). Wait at least 30 seconds — this drains residual power from the motherboard. Then unplug the power cable from the wall outlet.

Open the case by removing the side panel. Most cases have two or three thumbscrews or Phillips screws on the back. Lay the case on its side on a flat surface so you can see the motherboard clearly. Attach your anti-static wrist strap to the metal frame of the case now, before you touch any components.

Locating and removing the old battery

Look for a round, silver coin-shaped battery sitting in a circular socket on the motherboard. It's usually near the edge of the board and often labeled "CMOS" or "BATT" on the silkscreen (the printed text on the board). The socket has a small metal clip on one side that holds the battery in place.

To remove the battery, gently push the clip away from the battery with a plastic tool (a plastic spoon handle works) or your fingernail. The battery will pop up slightly. You can then lift it out by hand. Some sockets have a small lever instead of a clip — pull the lever away from the battery to release it. Do not force anything; if the battery won't budge, you're likely pushing the wrong direction.

Installing the new battery and reassembling

Before inserting the new battery, check that the positive side (marked with a + symbol) faces up toward you. Slide the new battery into the socket with the positive side up. The metal clip will snap back into place automatically, or you may need to push the lever back to find it. You should hear or feel a small click.

Close the case by replacing the side panel and tightening the screws. Plug the power cable back in and turn on the computer. During startup, you may see a message about CMOS settings being reset to defaults — this is normal. Your BIOS has returned to factory settings, so if you had customized anything (boot order, overclocking, RAM timings), you'll need to re-enter those settings.

What to do if the battery doesn't fix the problem

If your computer still loses the time and date after replacing the battery, the problem may be elsewhere. Check that the new battery is installed correctly — the positive side should face up and the clip should be fully seated. Power off, wait 30 seconds, and power back on to test again.

If the problem persists, the motherboard itself may have a failing capacitor or charging circuit. This is rare but possible on older boards. You can also try clearing the CMOS by removing the battery for five minutes, then reinstalling it — this forces a complete reset. If none of these steps work, the motherboard may need professional service or replacement.

Frequently Asked Questions

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

No. You must power off completely and unplug the computer. Sleep mode leaves power flowing to the motherboard, and touching components while powered can damage them or cause injury. Wait at least 30 seconds after unplugging to let the board discharge fully.

What if I can't find the CMOS battery on my motherboard?

Check your motherboard manual — search the model number online if you don't have the physical manual. Some laptops and compact systems have the battery soldered directly to the board instead of in a removable socket, which requires professional service. Desktop motherboards almost always have a removable coin battery.

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

If you've customized BIOS settings like boot order or RAM timings, write them down before you start. The battery replacement will reset everything to factory defaults, and you'll need to re-enter those settings manually. If you haven't changed anything, you won't lose anything important.

Is it safe to touch the motherboard while wearing the anti-static strap?

Yes, that's the whole point of the strap. It grounds you to the case, preventing static discharge from damaging components. Make sure the strap is actually connected to bare metal on the case, not to painted or plastic surfaces. If you don't have a strap, touch the metal frame of the case every few seconds instead.

How long does a new CMOS battery last?

Typically three to ten years, depending on how often your computer runs and how old the motherboard is. Older boards may drain the battery faster. You'll know it's failing again when the clock resets after shutdown, just like before.