CMOS Battery Dead? How to Replace the CMOS Battery

A Complementary Metal Oxide Semiconductor (CMOS) battery is a small, mostly button/coin cell battery that provides power to the small memory that stores information about the BIOS settings.

When the CMOS battery dies, BIOS resets its setting, including date, hard drive settings, boot order, etc., and in many situations, OS can't load properly.

When something like that happens, the only course of action is to replace the CMOS battery.

Updated: August 14, 2024.

What Kills a CMOS Battery?

On average, a CMOS battery lasts 3-10 years, sometimes even more.

The drain current of the CMOS battery is very low, but over time, it discharges the CMOS battery completely - the CMOS battery dies.

Also, the temperature inside computers is usually increased when compared with the temperature in the typical office or home - increased temperature also leads to an increased self-discharge rate.

So, constant current drain and increased temperature eventually kill the CMOS battery.

Note: when the computer is turned ON, the CMOS battery drain is practically zero - PSU (Power Supply Unit) provides power for the computer, BIOS memory included. So, if You use your computer very often, the CMOS battery load is decreased.

What Happens When CMOS Battery Dies?

When the CMOS battery dies, the BIOS memory is deleted, leading to the following:

• lost time/date,
• lost boot order,
• CMOS errors of various kinds will probably be displayed on the screen (CMOS Read Error, CMOS Checksum Error, CMOS Battery Failure, etc.)
• Operating System (OS) will not load,
• some computer peripherals may not work properly,
• BIOS password reset (if it was present), etc.

There are also other signs of a dead CMOS battery, but they all depend on the computer/server/laptop model.

CMOS Battery vs. RAID Disc Controller Battery

Before continuing further, there is also one thing that we must clarify.

Many servers and even PCs with advanced disc controllers feature lithium rechargeable RAID disc controller batteries.

The RAID disc controller battery provides power to the RAID disc controller memory chip in the case of power failure.

The reason is simple - in order to speed things up, when OS sends data to be written on the disc, the RAID controller stores that data in the RAID memory and signals to the OS that the data is written.

Since discs are much slower than memory, such activity speeds things up significantly.

However, if the power is lost, there is a danger of data inconsistency and corruption if the data is written into the memory, but not on the disc.

So, the RAID disc controller battery allows the RAID memory to store that data for a few days, usually 3-7 days.

And when the power is restored, data is first written from the memory to the disc, and only then the RAID disc controller gives the OS access to the data, preventing data inconsistency or corruption.

In the event of a weak RAID disc controller battery, it is possible to disable data caching on the disc controller until the new battery arrives - this slows down the disc subsystem, but that is better than data corruption.

Common CMOS Battery Models

The most common CMOS battery is a CR2032 lithium non-rechargeable 3.0V Lithium Manganese Dioxide (Li-MnO₂) battery, although other similar batteries are in use, including CR2025, CR2020, and CR2016.

Note that certain motherboards may use cylindrical lithium batteries or even rechargeable batteries based on NiCd, NiMH, or some of the lithium battery chemistries. But this is very rare, at least in recent years.

CR2032 and similar batteries feature a wide temperature range and very low self-discharge rate, allowing them to hold the charge for up to 10 years, sometimes even more.

But, if required, some devices support the use of Carbon-Monofluoride Lithium batteries, which labels start with 'B', for example, BR2032.

Carbon-Monofluoride Lithium batteries (for example, BR2032) feature a somewhat smaller capacity than Lithium Manganese Dioxide batteries (for example, CR2032), but have an even wider temperature operating range and ultra-low self-discharge rate, allowing them to easily operate 10 or more years.

Note: never change battery chemistry unless explicitly allowed by the device manufacturer.

How to Replace CMOS Battery

If You are not familiar with computers, the best course of action is to take your computer to the computer store/service and let them replace the CMOS battery for You - it will cost You more than the battery itself, but nonetheless, it is much cheaper than the new motherboard, memory modules, processor, etc.

Note: If the computer is within the warranty period, do NOT open it - take it to the computer dealership or store and let them replace the battery for You - opening the computer while under warranty renders the warranty void!

If your computer warns You about a weak CMOS battery, backup the BIOS to the USB memory stick.

If You can't back up the BIOS but You get warnings about the weak BIOS battery, it is (at least in theory) possible to replace the CMOS battery while the computer/server is turned ON, but this is not recommended - should You decide to do it, remember that whatever You do, it is your own responsibility.

Where is the CMOS Battery Located?

The location of the CMOS battery can vary depending on the type of device, but it is generally found directly on the motherboard of desktop computers, laptops, and some other electronic devices.

In desktop computers, the CMOS battery is usually easily visible and accessible once the side panel of the case is removed. It is typically positioned in a socket on the motherboard and resembles a large watch battery, most commonly a CR2032 lithium coin cell.

In laptops, accessing the CMOS battery can be a bit more challenging due to the compact nature of these devices. The CMOS battery might be located under the keyboard or beneath the back panel of the laptop.

It is often encased in a protective sleeve and connected to the motherboard via a small cable, making it slightly less straightforward to replace than in desktops. Some models may require the removal of several components before the CMOS battery becomes accessible, necessitating careful disassembly and reassembly.

For other devices, like all-in-one computers or specialized equipment, the CMOS battery might be deeply integrated within the system's architecture, requiring disassembly by a professional.

In these instances, consulting the device's manual or manufacturer's support website can provide specific guidance on the location and replacement procedure for the CMOS battery.

Replacing Computer CMOS Battery

In order to replace the CMOS battery, do the following:

• backup BIOS, if possible,
• turn Off the computer,
• unplug the power cord and press the On/Off button for 10-15 seconds,
• open the computer without touching the internal components,
• ground yourself using the anti-static wrist strap band,

Note: If You don't have an anti-static wrist strap band, touch the computer chassis in order to remove static electricity from your hands. Actually, it is a good practice to hold the computer chassis with one hand and to work with another hand.

• locate the battery and replace it with the new one,
• close the computer,
• plug in the computer and turn it On, go to the BIOS,
• either restore the BIOS from the backup or set it manually.

After the battery is replaced and the BIOS is set like it previously was, load the OS and check if everything is OK there.

And that would be all.

Dispose of the old battery as recommended/requested by local authorities - recycle it ...

CMOS Battery Frequently Asked Questions

Here are some of the most common CMOS battery Frequently Asked Questions (FAQ):

What is a CMOS battery?

A CMOS battery is a small, usually button/coin cell battery that provides power for the computer's motherboard memory while the computer or laptop is turned Off and even unplugged from the AC power.

How long does a CMOS battery last?

CMOS batteries last at least 3-10 years, with some models lasting even more.

What does CMOS battery failure mean? What happens if your CMOS battery dies?

That means that the motherboard memory is erased, and all the settings are gone, including date, password, and other settings required for normal computer operation.

Can a PC run without a CMOS battery?

Yes, it can, but probably not - when the CMOS battery dies, motherboard memory can be loaded from the ROM (default settings), but most computers don't operate using default settings.

Theoretically, you can set the BIOS after the computer starts and use your computer, but as soon as the computer is turned Off and unplugged, the motherboard BIOS memory is deleted ...

What kind of battery is the CMOS battery? Are CMOS batteries universal?

A typical CMOS battery is a CR2032 battery, but other batteries are used, such as CR2025, CR2020, etc.

Can you use any CR2032 battery for CMOS?

CR2032 battery is a standard 3.0V lithium non-rechargeable battery, so, in theory, "any" CR2032 battery should work.

However, it is highly recommended to go for high-quality CR2032 batteries from brands such as Panasonic, Energizer, Duracell, and similar.

How do I start my computer without a CMOS battery?

When the CMOS battery is dead or not present, the computer can be started the usual way - press the "On" button.

However, as soon as the computer starts to boot, it will present you with the screen requiring you to either load BIOS settings from the backup or to manually set the BIOS, which can be daunting task.

Note: some motherboards may refuse to go past the initial boot sequence when the BIOS battery is dead or not present.

BIOS Reset: How long to leave the CMOS battery out?

When trying to reset BIOS by taking the CMOS battery out, keep it out for at least 60-90 seconds, sometimes even more.

Reason: some motherboards are designed to allow the user to quickly replace the CMOS battery without losing the BIOS information and hence can withstand battery removal that lasts at least 20-30 seconds, sometimes even more.

Few Final Words

As one can see, replacing the old CMOS battery with the new one is rather easy.

But, if You plan to do it on your own, note that the static charge can destroy sensitive components easily, and if the computer is within the warranty period, it should be taken to the authorized service or dealership so that they replace the battery.

But, CR2032 and especially BR2032 and similar "CR" and "BR" batteries are intended to last for years - since most computers are used for 3-5 years, the possibility of having a weak or dead CMOS battery is low.

But, if that happens, the battery can be replaced relatively easily.