CMOS Battery Dying? The Signs and How to Replace It in a Desktop PC
Every motherboard has a small battery that keeps the real-time clock running, and on many boards it also preserves BIOS settings, while the PC is unplugged. These batteries last a long time, usually several years, but they eventually die. When that happens, the symptoms can look strangely serious. This guide explains how to tell when it is the battery, how to replace it, and what to set up again afterward.
What the CMOS battery actually does
The name comes from the small CMOS memory chip that once stored BIOS settings. On modern boards those settings usually live in flash memory, but the coin cell still powers the real-time clock and, on many boards, keeps the settings area alive when the PC has no power at all. While the PC is plugged into a live outlet, standby power from the PSU usually takes over. That is why a dead battery often only shows up after a power outage or after the machine is unplugged.
Signs the battery is dying
- The clock is wrong after the PC has been unplugged, often reset to a default date years in the past.
- BIOS settings revert to defaults. Memory profiles (XMP/EXPO), fan curves, and boot order disappear after a power cut.
- Boot messages such as "CMOS checksum error", "CMOS battery low", or "date/time not set, press F1 to continue".
- HTTPS certificate errors in the browser. A clock that is badly wrong can make secure sites fail to load until the date is fixed.
- The PC takes an extra boot cycle after a power loss because it is retraining memory with default settings.
BIOS settings can also reset after a failed boot with unstable overclock or memory settings, because many boards fall back to defaults automatically. A BIOS update also clears settings. If the clock keeps time while unplugged, the battery is probably fine.
Confirm it before you replace it
- Set the correct time in the BIOS, save, and boot into the operating system.
- Shut down and unplug the PC (or turn off the PSU switch) for an hour or so.
- Power it back on and check the BIOS clock. If it reset or drifted badly, the battery is the likely cause.
- Optional: test the cell with a multimeter. A fresh CR2032 reads a little above 3V. A battery that reads well below its rated voltage is near the end. The exact cutoff where a board starts complaining varies.
Which battery you need
Most desktop motherboards use a CR2032 3V lithium coin cell in an exposed holder. A few boards mount it vertically, or put it under an M.2 heatsink or a graphics card, and some small boards use a wired cell with a connector. Check the board manual's layout diagram to find it and confirm the type.
| Type | Where you see it | Notes |
|---|---|---|
| CR2032 in a socket | Most ATX and micro-ATX desktop boards | Easy to swap. Buy from a reputable brand. |
| CR2032 with leads and a connector | Some mini-ITX boards, small-form-factor PCs, and laptops | Buy the wired version with the matching connector, or a replacement part made for that system. |
| Wired cell in a heat-shrink sleeve | Many laptops and some prebuilt systems | Use the manufacturer's part number when you can. |
| No separate cell | Some laptops and mini PCs | The clock runs from the main battery. Follow the service guide. |
Coin cells are dangerous if swallowed, particularly for children and pets. Keep spare and used cells out of reach, and recycle used ones according to local guidance. Do not throw them loose into a drawer or the trash.
How to replace it in a desktop
- Write down your important BIOS settings before you start, or take photos of each screen: memory profile, boot order, fan curves, virtualization, Secure Boot and TPM settings, and anything else you changed.
- Shut down, turn off the PSU switch, and unplug the power cord. Press the case power button a few times to drain any leftover standby power.
- Ground yourself by touching bare metal on the case, or use an anti-static wrist strap.
- Find the battery. You may need to remove the graphics card or an M.2 heatsink to reach it.
- Release the old cell. Most holders have a small metal latch. Press it gently with a fingernail or a plastic tool, and the cell pops up. Avoid levering against the board with metal screwdrivers.
- Insert the new cell with the correct side facing up (usually the "+" side with the text), and press until it clicks in.
- Reassemble, plug in, and power on. Expect the board to load defaults, and possibly to take longer on the first boot while it trains memory.
What to set up again afterward
- Date and time in the BIOS. The operating system usually syncs it once you are online.
- Memory profile (XMP/EXPO), or your RAM runs at default speeds.
- Boot order and boot mode (UEFI vs CSM), especially on multi-drive or dual-boot systems.
- Secure Boot and TPM (fTPM/PTT), if Windows 11 or BitLocker relies on them.
- SATA/NVMe mode, if you changed it from the default (for example RAID).
- Fan curves and any custom power settings.
Changing firmware security settings can trigger a BitLocker recovery prompt on the next boot. Make sure you have your recovery key, which is stored in your Microsoft account for many personal devices, before you start.
Many boards can save BIOS profiles to a USB drive. If yours can, save your settings now. Then the next reset, whether from a battery or a BIOS update, only takes a minute to fix. Our BIOS update guide covers profiles and the update process.
Laptops and mini PCs
Replacing the clock battery in a laptop or mini PC is a different job. The cell is usually wired, sometimes tucked under other components, and occasionally only reachable after removing the main battery or board. Find the service manual for your exact model, disconnect the main battery first, and order the correct part. If the laptop is under warranty, or the cell is buried, a repair shop is the sensible choice.
A dead CMOS battery will not stop a PC from working
A PC with a dead clock battery still runs. You will just keep fixing the clock and settings after every power loss. That means a dead CMOS cell is not the cause of a PC that will not turn on at all. If that is your problem, start with the no-POST troubleshooting checklist. The one related trick is the deliberate CMOS clear (using the jumper, the button, or by removing the battery for a few minutes), which can rescue a board stuck on bad settings.
Frequently asked questions
How long does a CMOS battery last?
Usually several years, often longer on a PC that stays plugged into live power, because standby power takes over the load. A PC that spends a lot of time fully unplugged drains it faster.
Can I use a CR2025 instead of a CR2032?
They share a diameter and voltage, but a CR2025 is thinner and has less capacity, so it may not make firm contact in the holder. Use the type your board specifies.
Will replacing the CMOS battery delete my files?
No. Your files are on the drives. Only firmware settings such as time, memory profile, and boot order are reset.
Is it safe to replace the CMOS battery with the PC plugged in?
Turn off the PSU and unplug the PC first. Working on a board with standby power present risks shorts and damage.
The takeaway
If your PC forgets the time or its BIOS settings every time it loses power, the coin cell is almost certainly the cause, and it is one of the cheapest fixes in PC hardware. Confirm it with the unplug test, photograph your BIOS settings, swap in the cell type your manual specifies, then set your memory profile, boot order, and security settings again. The whole job takes about ten minutes.