CMOS battery - small battery on motherboard that powers BIOS settings

What is a cmos battery and what does it do

  Reading time 6 minutes

Introduction to CMOS Battery

A CMOS battery, known as the power component behind the question “why is there a battery on my motherboard,” is a small, round cell found on the motherboard, which prompts users to ask, “where is the CMOS battery on a motherboard?” It ensures the computer’s BIOS settings are maintained and is essential for system start-up functionality. Think of it as the guardian of your system’s memory, holding onto important information while other components rest.

The Functions of a CMOS Battery

When it comes to functions, the CMOS battery plays a pivotal role in preserving BIOS settings and keeping the internal clock running. In essence:

  1. Preserving BIOS Settings: The CMOS battery is integral for upholding various BIOS configurations including hardware settings, system time and date, and boot sequences.
  2. Keeping the Internal Clock Running: Ever wondered, “why is there a battery on my motherboard?” The uninterrupted power supply for the real-time clock is the reason, ensuring that time and date settings are always correct.
 Replacing CMOS battery - how to change the battery on your computer's motherboard

Diving Deeper: How the CMOS Battery Works

CMOS Technology Explained

The CMOS battery — often questioned as “what does a CMOS battery look like” — is typically a coin-shaped lithium-ion cell seated on the motherboard, recognized for its endurance and reliability. It steadily powers the CMOS chip, which retains the BIOS settings. Here is a simple overview:

Lithium-ion (often)Retain BIOS settings3-5 YearsMotherboard
Power real-time clock

The Lifespan of a CMOS Battery

Addressing the question “how long does a CMOS battery last,” most can expect a robust lifespan ranging between:

  1. Average Lifespan: Usually between three to five years, this period can vary depending on several factors.
  2. Factors Affecting Durability: These factors include ambient temperature, battery quality, and the length of time before the battery was put to use in the system.

Symptoms of a Failing CMOS Battery

Common Indicators to Watch For

If your computer starts showing certain symptoms such as error messages or system time errors, it’s a sign to question the CMOS battery health. Identifiable problems may involve error messages like ‘CMOS Read Error’, or ‘System Battery Voltage is Low’, and system-related issues including loss of BIOS settings or issues with the hardware recognition on start-up.

Troubleshooting CMOS Battery Problems

To diagnose a failing CMOS battery, you should observe the BIOS settings after a full system reboot; if they have reset, it’s indicative of a battery problem. While user manuals provide specific diagnostic methods, seeking professional assistance is also advisable for those not confident in handling their device’s internals.

 Importance of CMOS battery - why a dead battery can cause issues with your computer

Replacing Your CMOS Battery

Step-by-Step Guide

Replacing the CMOS battery involves powering down the machine, preparing the necessary tools, and then accessing the battery on the motherboard for replacement. This procedure should only be attempted if proper static safety measures are in place:

  1. Power Down and Prepare: Make sure the computer is off and you’ve grounded yourself.
  2. Access and Replace: Locate the CMOS battery, which should be a simple case opening away, carefully remove the old battery and replace it with a new one.

Choosing the Right CMOS Battery for Your Computer

To choose the appropriate CMOS battery, always match the model number and voltage with the one previously installed. Opting for reputable brands can also ensure the longevity and functionality of your computer’s CMOS battery.

The Impact of CMOS Battery on Overall Computer Performance

A functioning CMOS battery is central to preserving personalized BIOS settings, ensuring stable and efficient operation. A good battery prevents data loss and configuration issues, thus proving its indirect yet vital effect on performance.

 Testing CMOS battery - how to check if your battery is still functioning properly

Conclusion: The Unseen Yet Essential CMOS Battery

An efficient CMOS battery may be small and inconspicuous but plays a critical role in ensuring your computer operates seamlessly. Regular monitoring of the battery can prevent abrupt system issues and maintain the reliability and efficiency of your computing experience.


  1. What happens if the CMOS battery dies?
    When the CMOS battery fails, the BIOS settings cannot be retained, leading to loss of system time and potential issues with BIOS configurations upon each startup.
  2. Can a dead CMOS battery cause a computer not to start?
    Typically, a computer will still start with a dead CMOS battery, but it might encounter BIOS-related issues that impact the boot process.
  3. Is it safe to replace the CMOS battery on my own?
    Replacing the CMOS battery is generally safe with the correct approach and safety precautions, such as grounding oneself to prevent damage from static electricity.
  4. How do I know what CMOS battery to buy?
    To find the correct CMOS battery, refer to the existing battery’s model number and voltage or consult the computer’s manual.
  5. Does the CMOS battery affect performance?
    A CMOS battery does not affect performance directly but maintaining BIOS settings can ensure the system operates according to user-defined configurations, contributing to overall performance efficiency.