What a CMOS Battery Does and Why It Fails

The CMOS battery is a small coin-shaped battery on your computer's motherboard that keeps your system clock running and stores basic hardware settings even when the computer is powered off. When this battery dies, your computer will lose the time and date, forget boot settings, or fail to start until you reset the BIOS. Replacing it is a straightforward 10-minute job that requires no special tools beyond a Phillips screwdriver.

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 old date every time you restart, or when you see a "CMOS checksum error" message on startup. The battery itself costs between $3 and $8.

Key Takeaways

  • Turn off your computer completely and unplug the power cable before opening the case to avoid electrical damage.
  • The CMOS battery looks like a large watch battery (usually CR2032) and sits in a small round socket on the motherboard.
  • Push the metal clip holding the battery outward gently, and the battery will pop out—do not force it or pry with metal tools.
  • Insert the new battery with the positive side facing up, push it down until the clip snaps back into place, and reassemble your computer.
  • When you restart, your BIOS settings will reset to factory defaults, so you may need to adjust boot order or other settings afterward.

Gather Your Materials and Prepare Your Workspace

You need a Phillips screwdriver (size #2 is standard), the replacement CMOS battery (almost always a CR2032), and a clean, flat surface to work on. Lay down a static-dissipative mat or a cotton towel to prevent accidental damage from static electricity. Take a photo of your motherboard before you start so you can remember where the battery socket is located.

Before you touch anything inside the case, power down your computer completely—not sleep mode, but a full shutdown. Unplug the power cable from the back of the tower and wait 30 seconds. This discharges any remaining electrical charge in the system. If your case has a power button on the back, flip that switch to the off position as well.

Open the Computer Case and Locate the Battery

Most desktop cases have two side panels held by thumbscrews or Phillips screws. Remove the screws on the left side panel (when facing the front of the case) and slide the panel toward the back, then lift it away. You may need to disconnect a front-panel power button cable if your case has one—straightforward pull the connector straight out.

The CMOS battery is a round, silver, coin-shaped cell about the size of a large watch battery. It sits in a small circular socket on the motherboard, usually near the edge of the board or close to the CPU. Look for a small metal clip on one side of the socket. If you cannot find it when ready, consult your motherboard manual or search for your specific motherboard model online—the manual will show you exactly where it is.

Remove the Old Battery Safely

Do not use a metal tool to pry the battery out. Instead, gently push the metal clip that holds the battery toward the outside edge of the socket. The battery will spring upward slightly as the clip releases. Once it pops up, use your fingernail or a plastic spoon handle to lift it out completely. If the battery is stuck, wait a few more seconds and try again—forcing it can damage the socket.

Set the old battery aside. You may want to dispose of it at an electronics recycling center, as some areas have regulations about battery disposal, though most CMOS batteries are safe for household trash.

Install the New Battery and Close the Case

Before you insert the new battery, check that the positive side (marked with a + symbol) is facing upward. Slide the battery into the socket at a slight angle, then press it down firmly until you hear or feel a small click as the metal clip snaps back into place. The battery should sit flush with the socket—if it's sticking up, it is not fully seated.

Reconnect any cables you removed from the case, then slide the side panel back on and find it with screws. Plug the power cable back in and power on your computer. When it starts, you may see a BIOS setup message or a clock reset warning. This is normal and expected.

Reset Your BIOS Settings After Replacement

After you replace the CMOS battery, your computer's BIOS will reset to factory defaults. This means your system clock will show an incorrect date and time, and any custom boot settings you had configured will be lost. Restart your computer and enter the BIOS setup menu by pressing the key shown on the startup screen—usually Delete, F2, or F12, depending on your motherboard manufacturer.

Set the correct date and time, then adjust your boot order if needed so your computer boots from the correct drive. Save and exit the BIOS. Your system should now run normally. If you had a password protecting the BIOS, you may need to re-enter it, as that setting is also reset.

Troubleshooting If the Battery Does Not Seat Properly

If the battery will not click into place or keeps popping out, the socket clip may be bent or damaged. Try gently straightening the clip with a plastic tool—never use metal, as it can cause electrical damage. If the clip is visibly broken, the motherboard socket itself may need replacement, which requires professional repair.

If you installed the battery correctly but your computer still shows a clock error after restart, the new battery may be defective. Remove it, check that you bought the correct model (CR2032), and try a different battery from a different package or store. Occasionally a battery arrives dead from the manufacturer.

Frequently Asked Questions

How do I know which battery to buy?

The vast majority of desktop computers use a CR2032 battery. Check your motherboard manual or search your motherboard model number online to confirm. The battery package will clearly label it as CR2032, and you can find them at any pharmacy, electronics store, or online retailer for $3 to $8.

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

No. Always power down completely and unplug the power cable. Sleep mode leaves electrical current flowing through parts of the motherboard, and touching components while powered can damage them or harm you.

Will replacing the CMOS battery erase my files or programs?

No. The CMOS battery only stores the system clock and basic hardware settings. Your hard drive, files, and installed programs are completely unaffected. You may need to reset your BIOS settings, but your data is safe.

What if I see a CMOS battery error message but the battery looks fine?

The battery may be dead even if it looks undamaged. Remove it and test it with a battery tester if you have one, or straightforward replace it with a new one. A battery that has lost its charge will not show visible signs of failure.

Do I need to ground myself before touching the motherboard?

It is a good practice to wear an anti-static wrist strap or touch a metal part of the case frame before you touch the motherboard. This prevents static electricity from your body from damaging sensitive components. If you do not have a wrist strap, straightforward avoid working on a carpeted surface and touch the case frame occasionally while working.