CMOS stands for “Complementary Metal Oxide Semiconductor.” The battery powers the BIOS firmware in your laptop.

BIOS needs to remain operational even when your computer isn’t plugged into a power source. That’s where the battery comes in. When your computer gets unplugged, BIOS relies on the battery for power.

You’ll find these batteries in both laptops and desktop PCs, but it’s used more frequently in a laptop. That’s because laptops are usually unplugged for a longer amount of time than desktop PCs. Most desktop PCs are unplugged from their power source very infrequently.

The CMOS battery gets charged whenever your laptop is plugged in. It’s only when your laptop is unplugged that the battery loses charge. Most batteries will last 2 to 10 years from the date they’re manufactured. The more you leave your laptop plugged in, the longer your battery will last.

The Motorola 146818 chip was the first RTC and CMOS RAM chip to be used in early IBM computers, capable of storing a total of 64 bytes of data. Since the system clock used 14 bytes of RAM, this left an additional 50 bytes for storing system settings. Today, most computers have moved the settings from CMOS and integrated them into the southbridge or Super I/O chips.

How long does the CMOS battery last?
The standard lifetime of a the battery is around 10 Years. However, this can vary depending on the use and environment where the computer resides.

Which devices use this type of battery?

  • Digital logic circuits
  • SRAM (Static RAM)
  • Microprocessors
  • Microcontrollers

How would I know if my CMOS battery is failing?
If the CMOS battery is failing, the computer cannot maintain the correct time or date on the computer after it’s turned off. For example, after turning your computer on, you may notice the time as been set to 12:00 P.M. and the date is reset to January 1, 1990. This error indicates the  battery has failed.


Another indication the  battery is failing is if any of the following messages displayed when the computer is booting.

  • CMOS Read Error
  • CMOS Checksum Error
  • CMOS Battery Failure
  • System battery voltage is low