1. What it does
The CMOS battery holds the real-time clock, the boot order and the BIOS settings while the machine is off. It also keeps the network card’s identity so the computer can find your internet connection.
2. What it takes
Almost every desktop and laptop motherboard uses a CR2032. Some older machines take a CR2025 and a few take a half-size coin cell, so check the shape of the holder before you buy.
3. The signs it is flat
The clock resets to a date you have never seen every time you switch on, the machine asks for the BIOS password or setup screen, and the fans spin up loudly before the picture appears.
4. It is a five-minute job
Unplug the machine, press the power button to drain it, and open the case. The battery is a coin cell held in a clip or a socket. Take a photo of its position first, then lift it out with a plastic tool.
5. Put the date back afterwards
A new battery won’t fix a clock that still forgets the time, because the settings are lost the moment the old cell comes out. Note the date and time before you start, and set them again once the new battery is in.