What a CMOS battery does and why it fails

Your computer has a small coin-shaped battery on the motherboard called a CMOS battery. It keeps your system clock running and stores basic settings like the date, time, and boot order even when the computer is completely powered off. When this battery dies — usually after three to ten years — your computer forgets these settings every time you shut it down.

You'll notice the problem when you turn on your computer and the clock has reset to an old date, or your keyboard and mouse stop working until you restart. The battery doesn't power your computer during normal use; it only maintains settings during power-off. Replacing it is a straightforward task that takes about ten minutes and costs between five and fifteen dollars for the battery itself.

Key Takeaways

  • A CMOS battery is a small coin-shaped cell on your motherboard that keeps your system clock and settings alive when the computer is off.
  • You can replace it yourself by opening the case, locating the battery (usually near the center of the motherboard), and swapping in a new CR2032 battery.
  • The battery type is almost always CR2032, but check your motherboard manual or look at the old battery to confirm the exact model.
  • After replacement, your computer may ask you to reset the date and time, and you may need to re-enter BIOS settings like boot order.

Gather what you need before opening the case

Before you start, turn off your computer completely and unplug the power cable from the back. Wait at least thirty seconds. This drains any remaining power and protects both you and the motherboard from electrical damage.

You'll need a Phillips head screwdriver (the one with a cross-shaped tip) to open the case, and a replacement CR2032 battery. You can buy CR2032 batteries at any pharmacy, grocery store, or electronics retailer for a few dollars. If you're unsure whether your computer uses a CR2032, look at the old battery once you open the case — the model number is printed right on it.

Ground yourself before touching the motherboard by touching a metal part of the case or wearing an anti-static wrist strap. This prevents static electricity from damaging components. You don't need expensive equipment; just touch the metal frame of the case with your bare hand before you reach inside.

Open the case and locate the battery

Remove the screws from the side panel of your computer case. Most desktop cases have four to six screws along one edge. Keep them in a small container so you don't lose them. Slide the panel off — it may take a gentle tug if it's been sealed for a while.

Look at the motherboard, which is the large green circuit board inside. The CMOS battery is a round, silver, coin-shaped cell about the size of a dime. It's usually mounted in a plastic holder with a small clip. On most motherboards, you'll find it somewhere in the middle or lower-middle area, but the exact location varies by model. If you can't spot it when ready, check your motherboard manual or search online for your specific motherboard model plus "CMOS battery location."

The battery holder has a small metal clip on one side. This clip holds the battery in place and also helps you remove it. You don't need to disconnect any cables or remove the motherboard itself.

Remove the old battery and install the new one

Gently push the metal clip away from the battery. The clip will spring back slightly, and the battery will pop up just enough for you to grab it. If the clip doesn't move easily, don't force it — you might bend it. Use a plastic spoon or the edge of a plastic card to gently pry the clip back if needed.

Once the battery pops up, remove it by hand and set it aside. Check the model number on the old battery to confirm you have the right replacement. The new battery should be identical.

Insert the new CR2032 battery into the holder with the positive side (marked with a plus sign) facing up. The battery will sit flat in the holder. Push it down gently until the metal clip snaps back over it. You should hear or feel a small click. The battery is now find.

Close the case and power on your computer

Slide the side panel back onto the case and screw it closed with the Phillips head screwdriver. Make sure all screws are tight but don't over-tighten them — you only need them snug enough that the panel doesn't rattle.

Plug the power cable back in and turn on your computer. It will start normally, but you may see a message asking you to reset the date and time. This is expected. Enter the correct date and time when prompted.

You may also need to re-enter settings in the BIOS (the system setup screen that appears before Windows or your operating system loads). This includes things like boot order or hard drive settings. If you see a message about BIOS settings being reset, restart your computer and press the key shown on screen — usually Delete, F2, or F12 — to enter BIOS and restore your preferences.

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

If your clock still resets or settings don't stick after replacing the battery, the problem may not be the battery itself. A failing motherboard, a loose power connector, or a corrupted BIOS can cause similar symptoms.

Try reseating the battery: remove it again, wait ten seconds, and reinstall it. Make sure the clip is fully engaged. If the problem continues, check that your power supply is working correctly by listening for fan noise when you turn on the computer. If the computer doesn't start at all or the fans don't spin, the issue is likely the power supply, not the CMOS battery.

If you're uncomfortable troubleshooting further, a local computer repair shop can diagnose the problem for a small fee. Bring the old battery with you so they can confirm the model and rule out a straightforward replacement issue.

Frequently Asked Questions

Can I damage my computer by replacing the CMOS battery myself?

The risk is very low if you follow the steps: power off completely, unplug the cable, ground yourself, and avoid touching other components. The battery holder is designed for user replacement. The only real danger is static electricity, which you prevent by touching the metal case before you start.

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

Laptop motherboards sometimes hide the battery under a shield or bury it deeper in the case. Look up your specific laptop model online — many manufacturers have video guides for battery replacement. If it's not easily accessible, a repair shop can replace it faster than you can disassemble the laptop.

Do I need to back up my data before replacing the CMOS battery?

No. The CMOS battery only stores the system clock and BIOS settings, not your files or programs. Your data on the hard drive is completely separate and unaffected by battery replacement.

How often do CMOS batteries need to be replaced?

Most CMOS batteries last between three and ten years depending on how often your computer is powered off. If you leave your computer on constantly, the battery lasts longer. Once it fails, you'll notice the clock resetting, and that's your signal to replace it.

Is a CR2032 battery the same as the one in my watch or remote?

Yes. CR2032 is a standard battery size used in watches, calculators, car key fobs, and many other devices. You can buy it anywhere batteries are sold. Just make sure you're buying a CR2032 and not a different model number.