CMOS Battery
CMOS Battery
What Is CMOS?
CMOS stands for Complementary Metal-Oxide Semiconductor.
In computers, CMOS refers to a small amount of memory on the motherboard that stores important system settings, such as:
System date and time
Boot order (which device starts first)
CPU settings
Memory (RAM) settings
Hardware configuration
BIOS/UEFI settings
These settings must be saved even when the computer is turned off.
What Is the CMOS Battery?
The CMOS battery is a small coin-shaped battery on the motherboard that provides power to the CMOS memory when the computer is unplugged or turned off.
The most common CMOS battery is:
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CR2032
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It typically looks like a silver coin.
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_______
/ \
| CR2032 |
\_______/
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What Does the CMOS Battery Do?
The battery keeps:
Date and time accurate
BIOS/UEFI settings saved
Hardware configuration retained
Without the battery, these settings may be lost when power is removed.
CMOS and BIOS Relationship
BIOS/UEFI
The firmware that starts the computer and checks hardware during startup.
CMOS
Stores the configuration settings used by the BIOS/UEFI.
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CMOS Battery
↓
CMOS Memory
↓
BIOS/UEFI Reads Settings
↓
Computer Starts
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Signs of a Failing CMOS Battery
Incorrect Date and Time
You may notice the clock resets every time the computer is powered off.
BIOS Settings Reset
Boot order and other settings return to default values.
CMOS Checksum Error
You may see messages like:
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CMOS Checksum Error
CMOS Battery Failure
Time and Date Not Set
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during startup.
How Long Does a CMOS Battery Last?
Typically:
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3 to 10 years
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depending on:
Motherboard
Usage
Battery quality
Replacing a CMOS Battery
Basic steps:
Shut down the computer.
Unplug power.
Open the case.
Locate the coin-cell battery.
Remove and replace it with the same type.
Reconfigure date and time if necessary.
Clearing CMOS
Sometimes technicians remove the CMOS battery or use a motherboard jumper to:
Reset BIOS passwords
Fix boot issues
Restore default BIOS settings
This process is called clearing the CMOS.
A+ Certification Exam Tip
Remember:
Component FunctionCMOS Stores BIOS/UEFI settings
CMOS Battery Powers CMOS memory when computer is off
BIOS/UEFI Firmware that starts the computer
CR2032 Most common CMOS battery type
Easy Memory Trick
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CMOS = Stores Settings
CMOS Battery = Saves Settings
BIOS = Uses Settings
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Simple definition: CMOS is the memory that stores your computer's hardware configuration and BIOS settings, while the CMOS battery keeps those settings and the system clock functioning when the computer is powered off.
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