How to Replace a CMOS Battery and Reset the BIOS

Published 2026-10-16 · LuxuriousComputers Repair

Every PC motherboard has a small coin battery, usually a CR2032, that keeps the real-time clock running and preserves BIOS settings when the computer is unplugged. These batteries typically last several years. When one runs down, the symptoms can look alarming but the fix is simple and inexpensive.

Signs the CMOS battery is dying

• The date and time are wrong every time you start the PC, often reset to a date years in the past.
• Messages at startup such as CMOS checksum error, CMOS battery failure, or Press F1 to continue.
• BIOS settings, such as boot order or a memory profile, revert to defaults after the PC is unplugged.
• Websites show certificate or security errors because the clock is wrong.
• The problem is worse after the PC has been unplugged or the power strip switched off.

If the clock is only wrong while Windows is running, that is usually a time sync setting rather than the battery. Open Settings > Time & language > Date & time, turn on Set time automatically, and select Sync now.

Before you open the case

• Write down any BIOS settings you changed. Replacing the battery resets them. Common ones are boot order, memory XMP or EXPO profiles, fan curves, and virtualization settings.
• Check for BitLocker. If the drive is encrypted, a BIOS reset can trigger a request for the BitLocker recovery key at the next boot. Find it in your Microsoft account under your devices before you start.
• Get the right battery. Most desktop boards use a CR2032. Check the old battery's marking to be sure.

Replacing the battery in a desktop

1. Shut down Windows, switch off the power supply using the switch on its back, and unplug the power cord.
2. Press the power button once to drain remaining power.
3. Remove the side panel. Touch a bare metal part of the case to discharge static.
4. Find the coin battery on the motherboard. On some boards it is partly hidden under the graphics card, which you may need to remove first.
5. Press the small metal tab beside the battery; it should pop up. Do not pry against the motherboard with a metal screwdriver.
6. Insert the new battery with the positive side, marked +, facing up, and press until it clicks.
7. Close the case, reconnect power, and start the PC.

Laptops are different

In many laptops the CMOS battery is a small cell wrapped in plastic with a wire and connector, mounted under the keyboard or bottom cover. Replacing it often means removing the bottom panel and disconnecting the main battery first. In some models it is soldered to the board. Check the service manual for your exact model before starting.

After replacing it: set up the BIOS

The first start will likely show a CMOS error or go straight to BIOS setup. That is expected.

1. Enter BIOS setup, usually with Delete or F2 at startup.
2. Set the correct date and time.
3. Check that the boot mode is UEFI and the boot order starts with Windows Boot Manager.
4. Re-enable any settings you wrote down, such as XMP.
5. Save and exit, usually with F10.

Once Windows starts, let it sync the time automatically.

Resetting the BIOS without replacing the battery

Clearing the CMOS restores factory BIOS settings. It is useful if the PC will not boot after an overclock, a memory profile change, or a forgotten BIOS setting. Methods, from easiest:

• From inside BIOS setup: choose Load Optimized Defaults or Load Setup Defaults, then save.
• Clear CMOS button: many newer desktop boards have a button on the rear panel or the board itself. Press it with the PC powered off and unplugged.
• Jumper: a two- or three-pin header labeled CLR_CMOS, CLRTC, or JBAT1. With the PC unplugged, short the pins as the manual describes for several seconds.
• Remove the battery: take the coin battery out for several minutes with the PC unplugged, then reinstall it.

A BIOS password is a different matter. On many laptops, a supervisor password is stored separately and is not cleared by removing the battery. That needs the manufacturer's process.

If problems continue with a new battery

If the clock still resets with a fresh battery, check that the battery is the correct type and fully seated. Persistent problems can point to a motherboard fault, which is less common but possible on older boards.

Help in Marion

If you would rather not open the computer, or the battery is inside a laptop that needs disassembly, LuxuriousComputers in Marion can replace the battery, restore your BIOS settings, and make sure the system boots correctly.