The CMOS battery is a small coin cell on the motherboard that keeps BIOS settings and the system clock alive when the PC is unplugged. A dead one causes the date and time to reset and custom BIOS settings to revert to default every time you power on. Replacing it is a simple swap with a standard CR2032 battery.
- The CMOS battery keeps BIOS settings and the clock alive when unplugged.
- A dead battery causes the date to reset and settings to revert.
- It’s usually a CR2032, the same type used in many watches.
- Replacing it takes a few minutes with the PC powered off.
What the CMOS Battery Actually Does
CMOS stands for the type of memory chip that stores your BIOS settings, boot order, and system clock. That chip needs constant tiny power to hold its data, and the CMOS battery supplies it whenever the PC is unplugged from the wall. Without it, the motherboard would forget everything and reset to factory defaults every time it lost main power.
Signs Your CMOS Battery Is Dying
- The system clock resets to an old or default date every time you boot
- Custom BIOS settings, such as boot order or fan curves, keep reverting
- The PC occasionally fails to boot until you enter the BIOS and save settings again
- You see a CMOS checksum error message on startup
Will a Dead CMOS Battery Stop the PC From Turning On?
Usually not by itself. A dead CMOS battery causes settings and clock resets, not a failure to power on. If the PC won’t turn on at all, look at other causes or check the power supply and connections first.
How to Replace the CMOS Battery
- Power off the PC, unplug it, and hold the power button for a few seconds to drain residual power.
- Open the case and locate the small, round, silver battery on the motherboard, often near the PCIe slots.
- Gently push the small retaining clip beside it, and the battery should pop up or slide out.
- Note the battery type, almost always a CR2032, and buy an exact replacement from a source such as Duracell.
- Insert the new battery with the same orientation, positive side facing the same way as the old one.
- Close the case, reconnect power, and boot into the BIOS to reset the date, time, and any custom settings.
How to Reset the BIOS Using the CMOS Battery
Removing the CMOS battery for a few minutes is also a manual way to reset the BIOS to factory defaults, useful if a bad overclock or setting is preventing boot. Power off, unplug, remove the battery, wait 5 to 10 minutes, then reinstall it and boot normally. Many boards, including higher-end models from ASUS ROG, also include a dedicated Clear CMOS button or jumper as a faster alternative, listed in the motherboard manual.
Note: resetting the CMOS does not erase files or programs. It only resets BIOS and firmware settings back to default, which may include disabling a previously enabled XMP profile that then needs re-enabling.
Confirm Your Motherboard Model First
Check what motherboard do I have to confirm your exact model before ordering a replacement battery, and check how to update your BIOS if a reset also means reconfiguring firmware settings.
Do Laptops Have a CMOS Battery?
Most do, though it’s usually smaller and harder to access, often requiring you to open the back panel or remove other components first. Some modern laptops use a rechargeable cell instead of a replaceable coin battery.
On the CPU/RAM/GPU side: how to update CPU BIOS.
Frequently Asked Questions
How long does a CMOS battery last?
Typically several years, often 3 to 10 depending on usage, since the battery only needs to supply tiny amounts of power while the PC is unplugged.
Can a dead CMOS battery cause a boot loop?
It’s uncommon but possible if corrupted settings prevent a stable boot. Resetting or replacing the battery, then reconfiguring the BIOS, often resolves it.
Do I need to reinstall Windows after replacing the CMOS battery?
No. Replacing it only affects BIOS settings and the clock, not your operating system or files.
Is every CMOS battery a CR2032?
The vast majority of desktop motherboards use a CR2032, but always check your specific board’s manual to confirm before buying a replacement.
