Why Do Motherboards Have a Battery?

Every computer has a big chunky power supply and a thick heavy cable running to the wall to provide all the power it needs. But did you ever wonder why does a motherboard comes with a little lithium coin battery despite of all the power your PC is getting?

What is CMOS?

So the battery is there to power something called a CMOS chip. CMOS stands for complementary metal oxide semiconductor. It may sound complicated but all you need to know is that it’s a common way of manufacturing computer chips used in camera sensors to analog digital converters to amplifiers.

But inside a PC, CMOS has historically been shorthand for the chip that contained your BIOS settings. 

What is BIOS?

The BIOS or basic input output system is responsible for controlling low level settings and initializing your hardware when your computer boots up. 

You’re probably familiar with it as you may have accessed it by smashing the delete or F1 key while your PC is booting up getting some frustration out in the process.

The CMOS chip that keeps these settings was traditionally volatile memory meaning all of these settings would be lost as soon as power was taken away. Since many people don’t leave their computers on 24/7, manufacturers put a coin battery on the motherboard to ensure the settings wouldn’t lost in case of power loss.

Fast forward to modern times, it turns out that motherboard manufacturers don’t use CMOS for BIOS settings anymore partly because those old CMOS chips only held 128 bytes. Nowadays systems need to store lots more BIOS data.

Instead they choose a chip with non-volatile RAM or NVRAM that not only has a higher capacity but can store data persistently even if there’s no power going to the board. So, what’s the point of using those CMOS chips that require a battery to keep the settings from getting lost?

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Here is the reason!

The reason there’s still a CMOS battery is because there’s still a CMOS chip but instead of handling the BIOS nowadays it’s only responsible for time keeping, just like a regular old wrist watch. Your computer needs constant power  to keep an accurate time.

So, that battery ensures your computer’s clock will tick away properly even if you’ve left it unplugged for an extended period. Interestingly many servers can keep an accurate time without a CMOS battery even if the power goes out. 

Instead of using a CMOS chip to keep time locally, servers often connect to an NTP server at boot time. NTP stands for network time protocol which is used to connect to a different server over the internet to fetch the time accurately. Though this isn’t commonly used on home PCs.

So, that little battery on your motherboard is still important. In fact if you remove it you’ll probably find that your BIOS settings will still reset just like they did in the old days. 

People were so used to removing the battery as a means of resetting the BIOS that most PC manufacturers simply kept this functionality in place as an easy way to clear settings. Of course there are a good number of motherboards out there that feature a clear CMOS button either on the board itself or on the rear I/O to reset your BIOS settings.

The CMOS battery isn’t something you have to worry about very often. They typically last for years before they might need to be replaced. But you might want to have a copy of your BIOS settings saved somewhere especially if you’re the type of person who spend hours getting thier RAM timings just right.