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what is cmos battery

 Every time a computer starts it needs a CMOS battery to save certain information about the hardware installed in it which usually includes the number of hard drives installed, the amount of memory installed, the date-time, and more. In previous days this information was provided by the user whenever the computer was turned on. 

However, with advances in computer science, this important information is stored in the CMOS chip installed on the motherboard. This chip helps save important information and provides it at startup.

A CMOS chip (Complementary metal–oxide–semiconductor) requires very little power to operate and achieves this from a battery. This battery can usually last for several years provided the computer is in regular use. 

You will need to replace the CMOS battery if your computer does not keep track of the date and time or if you receive the following error messages when you set up your computer.


Alternatively known as RTC (real-time clock), NVRAM (non-volatile RAM) or CMOS RAM CMOS for short is a complementary semiconductor to metal oxide. 

CMOS is a battery-powered semiconductor chip inside computers that stores information. This information ranges from system time and date to system hardware settings for your computer.

History of CMOS

The Motorola 146818 chip was the first RTC and CMOS RAM chip used in early IBM computers capable of storing a total of 64 bytes of data. 

Because the system clock used 14 bytes of RAM this left an additional 50 bytes for storing system settings. Today most computers have switched settings from CMOS and integrated them with Southbridge or Super I / O chips.

How long does a CMOS battery last?

The standard lifespan of a battery is about 10 years. However, this can vary depending on the use and environment in which the computer is located.

How do I know if my CMOS battery is failing?

If the CMOS battery fails the computer cannot save the correct time or date on the computer after it shuts down. 

For example, after turning on the computer you may notice a time set to 12:00. And the date is reset to January 1, 1990. This error indicates that the CMOS has failed.

Another indication that a CMOS battery is failing is if one of the following messages is displayed when the computer boots.

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