The CMOS battery is alternatively referred to as an RTC (real-time clock), NVRAM (non-volatile RAM) or CMOS RAM, CMOS is short for complementary metal-oxide-semiconductor. CMOS is an onboard, battery-powered semiconductor chip inside computers that stores information. This information ranges from the system time and date to system hardware settings for your computer. The picture shows an example of the most common CMOS coin cell battery (Panasonic CR 2032 3V) used to power the CMOS memory.
History of CMOS:
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 CMOS battery is around 10 Years. However, this can vary depending on the use and environment where the computer resides.
Which devices use CMOS?
Digital logic circuits
SRAM (Static RAM)
Microprocessors
Microcontrollers
Need of CMOS battery in Computers:
Did you ever notice the computer keeping the accurate time records with utmost precision, and that too without any external interaction, when we turn the system off? And when you do turn it on back, it shows the same time.
There are quite many systems that are present in computers, laptops, mobiles, etc. that keep on running on. Their power consumption is very less, even when we turn off the system. But the Clock system is one of those systems that will consume significant power. The iteration for the clock system will happen when the system (computer, laptop, mobile, etc.) is switched off.
The reason for this is CMOS, which is an abbreviation for Complementary Metal-Oxide-Semiconductor. It is also many times referred to as CMOS RAM, NVRAM (known as Non-Volatile RAM) and RTC (Real-Time Clock). It is a specialized semiconductor chip, which is battery-powered, used to store various information and is situated inside the system. In layman terms, it can be referred to as a special kind of battery that is present inside the computer. The information that is stored can vary from the simple time and date of the system to the hardware settings for the system. The most commonly used CMOS coin-cell battery that is used to get the CMOS memory powered up is Panasonic CR 2032 3V.
Usually, these CMOS batteries have an average life expectancy of around five years, although it highly depends on its usage and in what environment the computer resides. The depletion rate of the CMOS battery charge is linearly dependent on the power-off stage duration of the system. When the computer is shut down, the RAM, HDD, motherboard, processor, etc. are entirely turned off. It is this CMOS battery that keeps on running. So until this CMOS battery dies down, the computer will show the correct time, even when it is shut down for months and when it is without any internet access. But in practical use, the CMOS battery stores much more information than just date or time. Everything that the BIOS needs, is practically stored by the CMOS battery.
But if the CMOS battery fails, the computer would not be able to keep up with the correct date and date on the computer after it is shut down. For example, if you turn off the computer when the CMOS battery has died down, the date will be automatically set to January 1, 1990, and the time will be set to noon.
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 has been set to noon and the date is reset to January 1, 1990. This error indicates the CMOS battery has failed.
Another indication the CMOS battery is failing is if any of the following messages are displayed when the computer is booting.
CMOS Read Error
CMOS Checksum Error
CMOS Battery Failure
System battery voltage is low
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