What are the CMOS battery and RAM CMOS on the motherboard?

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

The CMOS battery and RAM CMOS are two distinct concepts that are sometimes mistaken for each other. Let’s examine each one more closely.


CMOS is short for Complementary Metal-Oxide-Semiconductor, which is a type of technology used to create integrated circuits. The technology enables devices to consume very little energy.

RAM CMOS is a small memory chip that is integrated into the Southbridge chipset and powered by a 3V CMOS battery. This means that data stored in RAM CMOS is not lost when the computer shuts down.

RAM CMOS is used to store BIOS configurations that are provided to the CPU during boot. When the computer starts up, the POST (Power-On Self Test) process begins, and the CPU reads and follows the instructions in the RAM CMOS.

If the CMOS battery runs out, RAM CMOS may lose data. In this case, the CPU will read the default CMOS version that is written on ROM.

2. CMOS battery on the mainboard

This battery supplies 3V of power to the RAM CMOS. This helps maintain the parameters set in the BIOS/CMOS Setup Utility, such as the date and time, password protection, and other settings.


The phenomenon suggests that this battery needs to be replaced. When you restart the computer, the date and time are displayed incorrectly, which often leads to an inability to access the internet.

It is important to note that different types of mainboards may have different CMOS battery configurations – some may require the use of a specific kind of battery or have different installation procedures. Refer to your computer’s user manual or manufacturer’s website for instructions on replacing your mainboard’s battery.

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