Dead CMOS battery can reset PC clock and BIOS settings, Engadget says

The CMOS battery acts as a backup power supply for the motherboard. It allows the computer to track the date and time even when it is unplugged, and it powers the small memory chip that stores boot-related BIOS or UEFI settings. Most desktop computers use a standard CR2032 coin cell, the same type used in Apple AirTags. Some use a CR2025 coin cell, though that is less common. Because the battery only powers a real-time clock and memory chip, it can last several years or even up to a decade.

A common sign of failure is that the computer can no longer keep accurate time and date between power cycles, especially after being unplugged from the wall. The system may reset to a default value such as January 1, 2000, or January 1, 1970, on every boot. An incorrect date and time can stop websites from loading because modern HTTPS connections rely on security certificates with narrow validity periods. It can also add confusion from inaccurate "last updated" dates on the user's own files.

Many modern operating systems now fetch the correct date and time from an online server at boot, so a failing CMOS battery may go unnoticed until it is completely dead. When that happens, the computer's BIOS or UEFI will show a warning on boot and revert to factory settings every time it starts. System-level options such as boot order and fan configurations are then set to default each time. This does not affect apps or files. To preserve motherboard settings between sessions with a dead battery, Engadget suggests using sleep mode instead of rebooting and avoiding unplugging the computer from the wall.

Many laptops use their main battery rather than a dedicated CMOS cell to maintain the internal clock, so the problem is much more likely to appear on a desktop PC. Most modern laptops do not offer easily user-replaceable components anyway.

Replacing the battery requires a new coin cell and a few minutes. Most computer cases are toolless, so a screwdriver is often unnecessary, but Engadget keeps a flathead screwdriver on hand because it makes prying the existing cell out of the motherboard easier. The process is to power off the PC, remove the side panel and locate the silver coin-shaped battery on the motherboard. If a graphics card or other bulky hardware is installed, it may need to be temporarily removed when the battery is behind it. The battery is typically held down by a clip or latch, which can be gently released with a flathead screwdriver or any flat object. After installing the new battery, the side panel goes back on. For a mini PC or non-standard motherboard, Engadget recommends reading the computer's manual before starting.

After replacement, the user should enter the BIOS or UEFI settings and adjust them to the previous configuration. Engadget notes that gamers or anyone using heavy software should re-enable Intel XMP or AMD EXPO to ensure RAM performs as it should. Users may also want to tweak the motherboard's built-in CPU overclocking features, though increased performance comes at the expense of added power draw and heat.