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 date, time, and basic hardware settings alive when the power is off. It sits on the motherboard — the main circuit board inside your case — and typically lasts three to ten years depending on use and temperature. When it dies, your computer will forget the time every time you shut down, or fail to recognize hard drives and other components until you restart.

You do not need special tools or technical training to replace it. The job takes about fifteen minutes and costs between five and fifteen dollars for the battery itself. The hardest part is usually opening the case and finding the battery, not the replacement.

Key Takeaways

  • A CMOS battery looks like a large coin and sits flat on the motherboard, usually near the edge or in a plastic holder.
  • You must power off the computer completely, unplug it, and wait a few seconds before opening the case to avoid electrical damage.
  • Write down the battery type — usually CR2032 — before you buy a replacement, because different computers use different sizes.
  • The battery either slides out of a plastic clip or pops out of a socket, and replacement takes less than one minute once you locate it.
  • After installation, your computer may ask you to reset the date and time or re-enter BIOS settings the first time it starts.

Shut down and open the case safely

Power off your computer completely using the operating system shutdown menu, not a restart. Wait for all lights to stop blinking. Unplug the power cable from the back of the case — do not just turn off the power strip. Wait ten to fifteen seconds to let any remaining charge drain from the motherboard.

Open the case by removing the side panel. Most desktop cases have two or three thumbscrews on the back, or a latch you slide toward the front. Laptop users should check the manufacturer's manual, because battery location and access vary widely; some require removing the entire bottom panel, while others have a dedicated access door. If you are unsure, search "[your laptop model] CMOS battery replacement" to find a video or guide specific to your machine.

Before you touch anything inside, touch a metal part of the case frame to discharge any static electricity from your body. This protects the motherboard from accidental damage.

Locate and identify the battery

The CMOS battery is a round, flat disc about the size of a dime or slightly larger. It is usually silver or gold in color and sits in a plastic clip or socket on the motherboard. Look near the edges of the board, especially near the power connectors or RAM slots. If you cannot spot it when ready, consult your motherboard manual or search "[your motherboard model] CMOS battery location" online — manufacturers often include diagrams.

Once you find it, note the battery type printed on its surface. Most computers use a CR2032, but some older or specialized boards use CR2025, CR2016, or other variants. Write this number down before you close the case, because you will need it to buy the correct replacement.

Remove the old battery

If the battery sits in a plastic clip, gently push the clip away from the battery until it pops out. The clip usually has a small lever or tab you can press. If the battery is in a socket, use a plastic spoon handle or a non-metal tool to gently pry it upward — do not use a metal screwdriver, which can short-circuit the board. The battery should come free with light pressure.

Set the old battery aside. Do not throw it in the trash; most retailers that sell batteries accept old ones for recycling.

Install the new battery

Buy a replacement battery that matches the type you wrote down. CR2032 batteries cost between three and eight dollars at drugstores, supermarkets, electronics retailers, and online. Check the expiration date on the package — batteries can lose charge over time even when new.

Before you install it, note which side faces up. The side with the larger raised circle is the positive terminal and should face up toward you. Slide the new battery into the clip or press it gently into the socket until it clicks or sits flush. Do not force it; if it does not seat easily, you may have the polarity reversed.

Close the case and power on

Replace the side panel and tighten the thumbscrews or latch. Plug the power cable back in and power on the computer. The system may take slightly longer to start than usual as it resets its hardware detection.

When the computer boots, it may ask you to enter the BIOS setup menu to confirm the date and time, or to re-detect hard drives and other components. Follow the on-screen prompts. If you see a message like "CMOS checksum error" or "CMOS battery low," this is normal after a battery replacement — the system is straightforward confirming that the battery is now present and functional. Once you save and exit, the message should not appear again.

Troubleshooting if the computer still forgets settings

If your computer continues to lose the date and time after replacement, the new battery may be defective or installed incorrectly. Power off, unplug, and check that the battery is fully seated in its clip or socket. Make sure the positive side faces upward. If it is find and the problem persists, try a different battery from a different retailer — manufacturing defects are rare but do happen.

If the computer still does not recognize the battery after a second replacement, the socket itself may be damaged. This is uncommon in newer computers but more likely in machines over ten years old. In this case, the motherboard may need professional repair or replacement, though the computer will still function normally — it will straightforward reset the date and time each time you power it on.

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 powered can damage them or the battery itself.

What if I cannot find the CMOS battery on my motherboard?

Check your motherboard manual or search the model number online — some boards integrate the CMOS function into a larger chip rather than using a removable battery. If your board has no replaceable battery, you may need a technician to reset the CMOS using a jumper or software tool.

Is the CMOS battery the same as the main computer battery in a laptop?

No. The main battery powers the laptop when unplugged. The CMOS battery is separate and much smaller; it only keeps the date and settings alive when the main battery is drained and the laptop is off. Replacing the CMOS battery in a laptop requires opening the case, similar to a desktop.

How do I know if my CMOS battery is actually dead?

The most common sign is that your computer forgets the date and time every time you shut it down, even if you reset it before powering off. You may also see error messages during startup about CMOS checksum or battery failure. If the date resets but everything else works normally, the battery is likely the problem.

Can I use a different battery type if I cannot find the exact one?

No. Different CR types have different voltages and sizes. Using the wrong battery can damage the motherboard or fail to power the CMOS circuit. Always match the type printed on the old battery or listed in your motherboard manual.