What a CMOS battery does and why it fails

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 computer is powered off. When it dies, your computer will forget the time, date, and sometimes boot settings every time you restart — you might also see error messages about the system date being wrong.

CMOS batteries typically last three to ten years depending on how often your computer runs and how warm it gets inside the case. You'll know it's failing when your clock resets to an old date after shutdown, or when you see a "CMOS checksum error" or "CMOS battery low" message during startup. The battery itself costs between $3 and $15, and replacing it is a straightforward task that takes about 15 minutes.

Key Takeaways

  • A CMOS battery looks like a small silver coin (usually CR2032 size) and sits on the motherboard near the center or edge of the board.
  • Before you open the case, shut down your computer completely, unplug the power cable, and wait 30 seconds to discharge any residual electricity.
  • The battery usually clips into a socket or holder — gently pry it up with a plastic tool or your fingernail rather than forcing it.
  • After you install the new battery, your computer will boot normally, though you may need to reset the time and date in your system settings.
  • If error messages continue after battery replacement, the motherboard itself may have a problem and you should consult a repair technician.

Gather your tools and materials

You need very little to replace a CMOS battery. Have a replacement battery on hand before you open the case — the most common type is a CR2032, which you can find at any pharmacy, electronics store, or online retailer for a few dollars. Check your computer's manual or search the model number online to confirm the exact battery type if you're unsure.

For tools, you'll need a Phillips head screwdriver to open the case and a plastic pry tool or old credit card to gently lift the battery out of its holder. A static wrist strap is optional but helpful if you're working on a carpeted floor — it prevents accidental electrical discharge that could damage components. If you don't have a wrist strap, straightforward touch a metal part of the case occasionally to ground yourself.

Power down and open the case safely

Shut down your computer completely through the operating system, then unplug the power cable from the back of the tower or the wall outlet. Wait at least 30 seconds — this allows the motherboard to fully discharge any stored electricity that could damage components or harm you. Do not rely on sleep mode or hibernation; the computer must be completely off and unplugged.

Locate the two or four screws that hold the side panel of your case in place. These are usually on the back edge of the case. Remove the screws and set them aside in a small container so you don't lose them. Slide or lift the panel off gently — some cases require you to slide the panel toward the front first before it lifts away.

Find and remove the old CMOS battery

Look at the motherboard — it's the large green or brown circuit board that fills most of the case. The CMOS battery is a small, round, silver coin about the size of a dime or slightly larger. It's usually mounted in a plastic holder or socket somewhere on the motherboard, most often near the center, toward the bottom edge, or near the power connectors on the right side. If you can't spot it when ready, consult your motherboard manual or search for your specific motherboard model online to see a labeled diagram.

Once you've located the battery, look at how it sits in the holder. Most CMOS batteries clip into a socket with a small metal tab or spring on one side. Use your plastic pry tool or fingernail to gently lift the edge of the battery away from the holder — do not force it or pry from the center, as this can damage the socket. The battery should pop up slightly. Lift it out by the edge and set it aside.

Install the new battery

Before you insert the new battery, check the orientation. The positive side (marked with a + symbol) should face upward, toward you, when the battery is in the holder. Slide the new battery into the socket at a slight angle, then press down gently until you hear or feel a small click. The metal clip or spring should snap back over the edge of the battery to hold it in place.

Do not force the battery down — if it doesn't seat smoothly, remove it and check that you're inserting it in the correct direction and that the socket isn't blocked by dust or debris. A gentle push is all that's needed. Once the battery is find, you're done with the internal work.

Close the case and power on

Slide the side panel back onto the case and find it with the screws you removed earlier. Tighten them by hand — you don't need to use force, just enough so the panel doesn't rattle. Plug the power cable back in and press the power button to start your computer.

Your computer should boot normally. You may see a message about the system date being incorrect or a CMOS checksum error — this is normal and means the battery replacement worked. Once the system is fully loaded, open your system settings and reset the date and time to the correct values. On Windows, right-click the clock in the taskbar and select "Adjust date/time." On Mac, go to System Settings > General > Date & Time.

What to do if problems continue

If your computer still shows CMOS errors after you've installed a new battery and restarted, the problem may not be the battery itself. Dust or corrosion inside the socket can prevent good contact, or the motherboard may have developed a fault. Try removing the new battery, cleaning the socket gently with a dry cotton swab, and reinstalling the battery to may support a clean connection.

If error messages persist after cleaning and reinstalling, or if your computer won't boot at all, the motherboard may need professional repair. Contact a local computer repair shop or the manufacturer's support line for your specific model. Do not attempt further troubleshooting on your own, as opening the case repeatedly increases the risk of accidental damage.

Frequently Asked Questions

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

No. The computer must be completely shut down and unplugged from the wall. Sleep mode leaves power flowing to the motherboard, which can damage components or cause injury. Always power off fully and wait 30 seconds before opening the case.

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

Search online for your motherboard model number plus "CMOS battery location" — most manufacturers publish diagrams showing exactly where it sits. If your computer is very old or uses a non-standard design, a repair technician can locate it for you.

Do I need to reset my BIOS settings after replacing the battery?

Not usually. Your BIOS settings should remain intact because they're stored separately from the CMOS battery. The battery only powers the clock and a small amount of temporary storage. If you notice boot problems or missing settings after replacement, you may need to reset the BIOS, but this is rare.

Is it safe to touch the motherboard while replacing the battery?

Yes, as long as the computer is unplugged and you've waited 30 seconds. Avoid touching the gold connectors or the chips themselves, but touching the green circuit board or plastic components is safe. If you're on a carpeted floor, touch a metal part of the case occasionally to discharge any static buildup.

What if the old battery is leaking or corroded?

If you see white or blue crusty material around the battery socket, the battery has leaked. Clean it gently with a dry cotton swab before installing the new battery. If corrosion is heavy or the socket looks damaged, do not attempt further replacement — take the computer to a repair shop, as the motherboard may need professional cleaning or replacement.