If your computer isn't booting up properly, or it shows the incorrect date and time every time you log in, chances are that your CMOS battery is depleted. Even though it sounds complicated, replacing your CMOS battery is a simple affair. But before that, it's important to understand what exactly is the CMOS battery, and what role it plays.


What Is the BIOS?

It's essential to understand the meaning of a computer's Basic Input/Output System (BIOS) before moving on to the CMOS. The BIOS is a piece of firmware that is integrated into your computer. It performs simple, but important functions on your computer, especially while booting.

It initializes the input devices on your computer, such as the keyboard and mouse. Additionally, it performs a power-on self test (POST) that checks the computer hardware before proceeding to boot into the OS. It also sets up the CMOS which forms the BIOS's non-volatile memory, i.e. it retains information even when the computer is powered off.

This is just rudimentary information about the BIOS: if you want to learn more, there are numerous other concepts related to the BIOS.


What Is a CMOS Battery?

Old PC motherboard.

CMOS stands for 'complementary metal-oxide semiconductor' and, in a nutshell, it stores information such as the time, date, and the hardware configuration of the system.

The CMOS battery is responsible for keeping the CMOS and BIOS powered on even when the computer is unplugged. This is because the CMOS keeps track of the date and time in real-time. Additionally, the hardware configuration and system settings need to be accessed as soon as the computer boots. If the CMOS battery runs out, your computer loses all this information and the BIOS reverts to its default settings.

Related: Why Does My Motherboard Have a Battery?

Most CMOS batteries last for about 2-10 years. That is a wide range because there are some factors that influence this. The CMOS battery is never used whenever your computer is plugged in. In laptops, the CMOS battery may also charge itself whenever it is plugged in.


This doesn't mean that your CMOS battery will discharge in a few weeks if you don't plug in your computer. Most CMOS batteries will have a shelf life of at least a couple of years, even if you don't plug in your computer at all.

When to Replace the CMOS Battery

Your computer will start acting abnormally when the CMOS battery is discharged. Here are a few of the tell-tale signs:

  1. Incorrect date or time: The most common sign of a discharged CMOS battery is incorrect date and time on your computer. Usually, the date and time will be reset to a time way back in the past.
  2. Input devices begin malfunctioning: Since the BIOS is responsible for managing devices such as the keyboard and mouse while booting, they may malfunction if the CMOS battery has been discharged. The mouse cursor could become inaccurate, or there could be a delay in the movement. The keyboard and mouse could also stop working altogether, since the BIOS acts as an intermediary.
  3. Missing driver: It's also possible that recently installed drivers may disappear on account of a failing CMOS battery. These drivers will most likely be related to the input/output devices connected to your computer.
  4. Power on/off issues: While there could be plenty of reasons why your computer isn't booting at all, one of them could be a failing CMOS battery. Alternatively, your computer can also stop responding to the power off button.
  5. A beeping sound/error messages: A constant beeping sound and error messages associated with the CMOS could also indicate a failing CMOS battery. The error messages include a CMOS checksum error, CMOS read error, CMOS battery failure, and system battery voltage is low.

How to Remove the CMOS Battery

Removing the CMOS battery is a relatively simple process, even if you haven't handled computer components before. Simply follow the steps listed below.

Location of the CMOS Battery on a motherboard.

Before you proceed to handle PC components, get rid of any static discharge you may have in your body. You can do this by simply touching the metal part of your computer's cabinet.

How to Remove the CMOS Battery on a Desktop Computer

  1. Remove the PC cabinet's cover to reveal the parts housed inside. This differs across different cases, so refer to the manual. Usually, you'll have to unscrew a couple of bolts and slide off the cover.
  2. Ensure there is enough light to see the motherboard clearly. Remove any dust that may have accumulated.
  3. Now, look for a round battery on the motherboard. It should be about the size of a coin.
  4. Carefully remove the battery from its slot. Remember to note down the positive (+ve) and negative (-ve) sides of the battery. Usually, the positive side will face upwards.
  5. If you're replacing the CMOS battery, insert the new battery now, while being careful it's in the same orientation as the previous one.
  6. Put back the cover on your computer cabinet.

Related: Common Mistakes That'll Damage or Ruin Your Motherboard


How to Remove the CMOS Battery on a Laptop

  1. On a laptop, the process of removing the CMOS battery is a bit more complicated due to its compact form factor and extra screws. Usually, it's better to take the help of a professional.
  2. Flip the laptop over, unscrew the bolts, and remove the casing to reveal the inner components. Make sure the laptop is shut down, and the charging cable is disconnected.
  3. First, remove the laptop's battery and then look for the CMOS battery on the motherboard. It should look exactly like the one on a desktop computer.
  4. Exercising caution, remove the CMOS battery.
  5. Remove the new battery in the same orientation as the previous one.
  6. Re-insert the laptop's battery and put the laptop's casing back on.
  7. Tighten all the bolts on the casing, and power on your laptop to ensure it is working correctly.

CMOS Demystified

The CMOS is not difficult to understand and, as is evident from the sections above, replacing the CMOS battery is also a breeze. Additionally, knowledge about other important PC components can also prove to be useful.

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