What a CMOS battery reset does and when you need one

A CMOS battery is a small coin-shaped battery on your motherboard that keeps your computer's date, time, and basic settings alive when the power is off. When that battery dies or loses charge, your computer forgets these settings and may fail to start properly. Resetting the CMOS battery clears all stored settings and returns them to factory defaults — this fixes boot problems, forgotten passwords, and incorrect date/time that won't update.

You'll know you need a reset if your computer won't start, shows a date stuck in 2006, asks for a BIOS password you don't remember, or displays error messages about CMOS checksum failures. A reset is also the standard troubleshooting step when a computer has been sitting unused for months and suddenly won't turn on.

The process takes 10 to 15 minutes and requires no special tools beyond a Phillips screwdriver. You do not need technical experience — the battery sits in an obvious slot and comes out by hand.

Key Takeaways

  • Shut down your computer completely, unplug the power cable, and wait at least 30 seconds before opening the case to discharge any remaining power.
  • Locate the round silver CMOS battery on the motherboard (usually near the center or edge) and note which direction the + side faces before removing it.
  • Leave the battery out for 10 to 30 seconds, then reinstall it in the same direction and close the case.
  • When you power on, your computer will show a setup screen or ask you to set the date and time — this confirms the reset worked.
  • If your computer still won't start after a reset, the problem is likely not the CMOS battery, and you may need to consult a repair technician.

Preparing your computer for safe battery removal

Before you touch anything inside the case, shut down your computer completely. Do not put it to sleep or hibernate — use the full shutdown option. Wait 10 seconds after the screen goes dark, then unplug the power cable from the back of the computer. This step is critical because the motherboard holds a small electrical charge even when powered off, and unplugging removes that charge so you don't accidentally damage components.

Wait at least 30 seconds with the power cable unplugged. This gives any residual charge time to drain completely. Some technicians recommend waiting a full minute for older computers. While you wait, locate a Phillips screwdriver — most desktop cases use Phillips head screws, though some use thumbscrews that need no tool at all.

Open the case by removing the side panel. On most desktop towers, two or four screws on the back hold the left or right panel in place. Slide the panel toward the back of the case and lift it off. Lay the case on its side on a clean, flat surface so you can see the motherboard clearly. Avoid carpeted areas if possible, as static electricity from carpet can damage components — a concrete floor or wooden table is safer.

Finding and removing the CMOS battery

The CMOS battery is a round, silver, coin-shaped object about the size of a dime or quarter. It sits in a small circular slot on the motherboard and is usually labeled "CMOS," "BATT," or "RTC BATT" on the board itself. Look near the center of the motherboard or along the edges — the exact location varies by manufacturer, but it is always visible and never hidden under other components.

Before you remove the battery, look at which direction the + (positive) side faces. Most batteries have a + symbol printed on the top. Note this direction — you will reinstall the battery the same way. If you cannot see the + marking clearly, take a photo with your phone so you have a reference.

Press down gently on the battery with your finger or a plastic tool (not metal). The battery will pop up slightly out of its slot. Lift it out by hand — it comes free easily and requires no force. If it does not pop up, you may be pressing on the wrong component; look for the round silver object and try again. Set the battery aside on a clean surface.

Waiting and reinstalling the battery

Leave the battery out of the slot for 10 to 30 seconds. This pause allows the motherboard's capacitors to fully discharge and forget the stored settings. Ten seconds is usually enough, but 30 seconds is safer if you have time. During this wait, do not touch other components or move the case around.

After the wait, pick up the CMOS battery and reinsert it into the slot, making sure the + side faces the same direction it faced before. Push down gently until you hear or feel a small click — the battery should sit flush in the slot and not stick up above the motherboard. If it does not click into place, lift it out and try again; the slot has a specific orientation and the battery will only fit one way.

Close the case by sliding the side panel back into place and reinstalling the screws. Tighten them firmly but do not over-tighten — you should feel resistance, not strain. Plug the power cable back in and press the power button.

What to expect when you power on after a reset

Your computer may take longer than usual to start — this is normal. You may see a message on the screen saying "CMOS checksum error" or "CMOS battery low" before the computer boots. Some computers show a setup screen asking you to set the date and time. Others boot directly to Windows or your operating system but display an incorrect date in the system tray.

If you see any of these signs, the reset worked. Set your date and time to the correct values (right-click the clock in Windows and select "Adjust date/time"), and the computer should operate normally from that point forward. If your computer had a BIOS password set before the reset, that password is now cleared and you can enter the BIOS without it.

If the computer still will not start or shows no signs of life after the reset, the problem is likely not the CMOS battery. Possible causes include a failed power supply, a loose RAM stick, or a motherboard failure. In that case, reseat your RAM (remove and reinstall each stick firmly) and check that all power cables are connected securely. If the computer still does not start, contact a repair technician.

Common mistakes to avoid during a CMOS reset

The most frequent error is not waiting long enough after unplugging the power cable. If you open the case when ready, residual charge on the motherboard can interfere with the reset. Always wait at least 30 seconds. Another common mistake is removing the wrong battery — some computers have a second battery elsewhere on the motherboard, but the CMOS battery is always the round silver coin-shaped one in a circular slot.

Do not force the battery out or reinstall it. If it does not pop up easily, you are pressing in the wrong spot. If it does not slide in smoothly, you may have the orientation wrong — flip it and try again. Forcing either action can crack the battery or damage the slot. Also avoid touching the gold contacts on the battery or the slot itself with your bare fingers; skin oils can cause poor contact. If you do touch them, wipe them gently with a dry cloth before reinstalling.

Finally, do not assume a CMOS reset will fix every computer problem. A reset clears BIOS settings and passwords, but it does not repair hardware failures, fix corrupted operating systems, or recover deleted files. If your computer has other issues beyond forgotten settings or boot failures, a CMOS reset alone will not solve them.

Frequently Asked Questions

How long does a CMOS battery last?

A CMOS battery typically lasts 3 to 10 years depending on the computer's age, how often it is powered on, and the quality of the battery. Older computers are more likely to have a dead CMOS battery. If your computer is more than 5 years old and shows signs of a dead battery, replacement is often the next step after a reset fails to fix the problem.

Will resetting the CMOS delete my files or programs?

No. A CMOS reset only clears the BIOS settings stored on the motherboard — it does not touch your hard drive, files, or installed programs. Everything on your computer remains intact. The only settings that change are the date, time, and any BIOS passwords or boot order preferences you had set.

What if the battery does not pop up when I press on it?

You may be pressing on the wrong component. Look for a round, flat, silver object about the size of a coin — it should be clearly visible on the motherboard. If you see multiple round objects, the CMOS battery is usually labeled on the motherboard itself. If you still cannot locate it, consult your motherboard's manual or search online for your specific motherboard model plus "CMOS battery location."

Can I replace the CMOS battery instead of just resetting it?

Yes. If the battery is dead and a reset does not fix the problem, you can buy a replacement CMOS battery (usually a CR2032 or similar coin cell) for $5 to $15 at any electronics store. Installation is identical to the reset process — remove the old battery and insert the new one in the same direction. A replacement battery solves the problem permanently if the original battery has reached the end of its life.

Do I need to turn off my computer before opening the case?

Yes, always. Never open a computer case while it is powered on or in sleep mode. Always shut down completely, unplug the power cable, and wait 30 seconds. Opening a live case risks electrical shock and can damage components if you accidentally touch a powered circuit.