CMOS Battery

What is CMOS Battery?

A CMOS or Complementary Metal Oxide Semiconductor battery provides the necessary power to the CMOS chip to run even when the computer system is turned off. It is just like a watch type battery that powers the BIOS firmware.

In simple words, the CMOS battery is designed to maintain the date, time, BIOS configuration and other information that are necessary for the computer system during startup.

Understanding CMOS Battery

What is CMOS Battery

In order to understand CMOS battery you should know first that the acronym CMOS here is sort of an out of date misnomer.

This term in fact comes from the computers in which a primary battery with a long life was used to provide the backup power to the low power but volatile CMOS memory.

This specific term in point of fact signifies a low power technology used to manufacture a chip and has got nothing to do with batteries.

It is due to this technology that enables these batteries to last for a long time though.

Even if you do not plug in your system at all or often, the CMOS battery will last for several years as well.

Typically, the CMOS needs very little power and therefore the battery can last for several years more than others.

Most CMOS batteries may last anywhere between 2 and 10 years depending on the usage and several other external factors influencing the environment and duration of the power-off stage.

Moreover, the CMOS battery will not be used when the computer system is running. In laptop computers, on the other hand, the CMOS battery can charge itself as well when the device is plugged in.

Everything that CMOS memory and the BIOS contain such as the boot up configuration and the Real Time Clock is maintained by the power backup provided by the battery when the computer system is unplugged. Hence it is called the CMOS battery.

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If the CMOS battery dies, the computer will lose all boot-up information and the BIOS will go back to its default settings.

However, all computer systems do not need such a battery to operate such as:

This is because these specific types of computers typically do not require any RTC or CMOS. All the necessary firmware contents are usually stored in a Serial Peripheral Interface or SPI NOR Flash chip.

This chip can retain the data stored in it even without power.

The CMOS batteries used today are usually CR2025 or the same as a 3V lithium coin cell.

Functions of CMOS Battery

The main function of the CMOS battery is to help the CMOS memory to run properly by supplying the power required to it apart from a few other functions performed along with it.

Ideally, the CMOS refers to the chip on the motherboard that holds the necessary information for boot up which is lost when the system loses power for any reason or is turned off.

It is the CMOS battery that provides the power to it when it is turned off.

The battery therefore indirectly helps in the local timekeeping function of the computer so that the system remembers when it is booted again after being shut down.

The CMOS battery also provides the necessary backup to the static volatile Random Access Memory that contains the BIOS or Basic Input Output System of the computer.

The CMOS battery keeps on running when the processor, motherboard, hard drive and RAM stop working when a computer is shut down. That way, it keeps the RTC ticking so that the correct time and date is displayed during the next boot up.

However, there is a catch. The CMOS battery does not provide power to the computer system when it is powered on. It only supplies a small amount of power when, and only when the system is turned off.

Typically, the functions of the CMOS battery can be summarized in the following points:

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The bottom line is that the CMOS battery helps the computer to maintain the time and other settings when the system is turned off.

Uses of CMOS Battery

The CMOS batteries are primarily used to provide the backup power to the Complementary Metal Oxide Semiconductor of the computer to maintain the values of the primary parameters.

These parameters, such as the Real Time Clock or RTC are used by the computer when it boots again.

Several computers also hold the current date and time in the CMOS memory which needs a power backup which is supplied by this battery so that the memory can increment these values so that it is updated correctly even when the computer is switched off.

The CMOS battery is also used to help the memory to maintain the configuration parameters that will decide which particular disk to boot from.

Types of CMOS Battery

There are different types of battery used for CMOS and over time, these types changed from large battery packs to small cylindrical lithium batteries to the modern CR2032 coin shaped battery type.

Typically, the types of CMOS batteries vary according to the model.

As for the desktop computers from the 1990s and modern ones, almost all of them use the CR2032 coin cell batteries. These are typically placed on a coin cell holder.

However, the older desktop computers mostly used coin cells with tabs that were soldered to the main board directly.

On the other hand the laptop computers use a variety of batteries right from the CR2032 batteries in some like in the desktop variants to the smaller coin cell batteries.

There are even some laptop models that use two such small coin cells wrapped in heat shrink.

These have a wire at one end and a connector on the other to allow easy replacement of those that are removable. Yes, there are a few that may be soldered in place with the main board.

The computers of the mid-90s and earlier typically used larger batteries which were prone to leaks and caused damages to the motherboard.

A few models even used several coin cell batteries stacked in a plastic wrap that were soldered to the main board while others used a single large cylindrical cell that were soldered as well.

Earlier than that, computers had a battery integrated into the clock chip and therefore these could not be replaced without replacing the entire chip. A few were even soldered in place, making matters even worse.

Some computers of the 1980s used smaller cylindrical lithium CMOS batteries while others used large packs of batteries mounted inside the case and connected to the main board by a wire.

What Happens If the CMOS Battery Dies?

The time and date will have to be reset every time the system is turned on when the CMOS battery dies.

There may also be some other notable issues in the computer system in such situations such as the operating system being prevented from loading because the hardware settings will be lost when the system is turned off.

Sometimes you may even see specific error messages such as ‘CMOS read error’ or ‘CMOS checksum error’ during boot when the battery dies or is failing.

Questions & Answers:

Does a Computer Need a CMOS Battery?

Yes, a computer needs a CMOS battery to maintain the date and time and also to find all the necessary settings in the BIOS for a proper startup.

Well, if you do not need the date and time in the system to be displayed correctly then there is no need for a CMOS battery.

Where is the CMOS Battery Located in the Desktop Computer?

Typically, the coin shaped CMOS battery is located inside the computer case in the node canister attached to the motherboard. Sometimes, this battery may also be located in a socket on the main board of the computer.

How Long Does a CMOS Battery Last?

Typically, the CMOS battery will last for about three years if the switch in the Power Supply Unit is turned off. Under extreme usage it may be able to supply the necessary power for about a year after which it should be replaced.

‘Replaced’ is the keyword here because the CMOS battery is not rechargeable like the Lithium-ion batteries. Trying to recharge this battery may cause an explosion.

Can a Computer Run without a CMOS Battery?

Yes, of course your computer can run just fine even without the CMOS battery because the system and the motherboard will boot up just fine in most cases.

However, the time and date will be incorrect. Also, the settings in the BIOS or Basic Input Output System will be reset and you will not be able to save the changes.


After reading this article now you know that the CMOS battery may not be a component in your computer that may affect its performance directly.

However, it may cause some issues during the day-to-day use of the system that will be unable to remember the hardware settings during startup.