Home > Choosing a Motherboard > What is CMOS
Desktop computer motherboards have a coin-shaped lithium battery, which is technically known as a CMOS (Complementary Metal-Oxide-Semiconductor) battery. This battery powers a small piece of memory on the motherboard called CMOS memory, which is also sometimes called the CMOS chip.
In this article we'll explain in simple terms what the CMOS battery is used for (it's different on older vs newer motherboards), how to check CMOS battery health, how to replace or reset the CMOS, and more.
Related: Choosing the Best Motherboard for Gaming
In older systems, this CMOS memory/RAM was used store BIOS (Basic Input/Output System) settings, so that those settings wouldn't be lost every time you turned your PC off (the battery would preserve the settings).
Nowadays, modern motherboards store BIOS settings in a different place - within what's called non-volatile memory such as Electrically Erasable Programmable Read-Only Memory (EEPROM). Modern motherboards that use a UEFI instead of a BIOS (UEFI is basically a more modern version of a BIOS with a easier to use graphical interface) store settings on flash memory or even on a system's hard disk.
These types of memory can retain BIOS settings even when your computer is switched off and has no power, which begs the question - if BIOS settings aren't stored in CMOS memory anymore, why do motherboards still have a CMOS battery? The answer is because motherboards need a way to track time.
In modern desktop computers, the primary utility of a CMOS battery is to keep the internal system clock running even when you turn off the computer.
This clock is technically called a Real-time Clock (RTC), which is present in any electronic devices that need to maintain accurate data about the time of day. The CMOS battery runs the RTC on a motherboard, so without a CMOS battery the internal clock of your computer would reset every time you shut it down.
The location of the CMOS battery on a motherboard can vary depending on the make and model, but it's not hard to find. Simply look for the small, round metal battery:
The lifespan of a CMOS battery varies on the quality of the battery, usage patterns, and the environment your computer lives in. For example, frequent power outages, exposure to extreme temperatures, or physical damage can shorten its lifespan.
On the flipside, using your computer more often can actually extend the longevity of the CMOS battery - a system that is always turned off will use the battery more often (since it has to keep the system clock running when the PC is off).
But in general, a typical CMOS battery can last up to 10 years, but it is possible for it to last longer or shorter than this period. Over time, the battery will gradually lose its charge, and as a result, the system may start to experience issues.
There are a few ways to check if the CMOS battery on a motherboard is healthy and working correctly, or if it's dying/dead and needs replacing, including:
Here are the simple steps required to replace a CMOS battery on a desktop motheboard:
Clearing your computer's CMOS may be necessary in certain situations to reset the BIOS or UEFI settings (if you have a system that stores BIOS settings in the CMOS).
For example, if you're troubleshooting a PC build that has issues with booting or with its display, or if your BIOS data has become corrupted somehow, it may be a good idea to clear the CMOS to reset BIOS/UEFI settings back to their defaults.
Depending on whether you have access to the BIOS or UEFI menu, there are 3 main ways to clear the CMOS on a desktop computer:
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