What is CMOS battery and RAM CMOS on motherboard?

This post is lesson 18 of 42 in the subject Computer Hardware

CMOS battery and RAM CMOS are two concepts that are often confused. Let’s take a closer look at these 2 concepts.


The full English name of CMOS is Complementary Metal-Oxide-Semiconductor. It is a term that refers to a type of technology used to make integrated circuits. This technology allows the device to consume very little energy.

RAM CMOS is a very small memory chip integrated in the Southbridge chipset, powered by a 3V CMOS battery. So the data in RAM CMOS is not lost when computer shutdown.

RAM CMOS is used to store BIOS configurations, provided to the CPU during boot. When starting the computer, the POST (Power-On Self Test) process starts, the CPU will read and follow the instructions in the RAM CMOS.

If RAM CMOS loses data due to out of CMOS battery, CPU will read the default CMOS version written on ROM.

2. CMOS battery on mainboard                  

CMOS battery is a battery with a voltage of 3V, to power the RAM CMOS, to help maintain the parameters set in the BIOS/CMOS Setup Utility such as date and time, password protection, etc.


The phenomenon indicates that the CMOS battery is exhausted

The date and time are displayed incorrectly when restarting the computer. In many cases, this is leading to be unable to access the internet.

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Posted by Vinh Le

Composer at ITLearningCorner.

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