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How to check for a CMOS Battery Failure?

What is a CMOS Battery?

The CMOS battery is an important part of the computer, because it helps the computer remember its settings and about itself. It’s important that this battery is in good health. There are some symptoms which you can see for signs of a weak or dead CMOS battery.

The CMOS battery makes the CMOS memory remember settings, including date and time settings. If the battery is weak or dying, the clock will get reset to 12:00 and the date to Jan 1, every time you start the computer. The computer may also shutdown, unexpectedly. It may also ask where it should boot from, when you power it on. These are some signs of a weak, dying or dead CMOS battery.

Many computers will give a warning message about the CMOS battery, when you boot them up. Nowadays, CMOS batteries are not replaceable and are built inside the motherboards, when you see symptoms of a dying CMOS battery, you probably have to replace the motherboard. Check with your motherboard manufacturer, if you have a new motherboard, since the manufacturers consider this a defect. You may be eligible for a replacement motherboard.


This battery is connected to the south bridge chip, to power both CMOS memory and RTC circuits. You can see the CMOS battery within its motherboard socket in the above picture.


The above is an example of a south bridge chip. The CMOS memory and the Real Time Clock (RTC) are both embedded in this chip, nowadays.

CMOS_Battery4    CMOS_Battery3


When your computer starts up, the BIOS may give a warning message like "CMOS checksum error, defaults loaded". It will also tell you to press a key to enter the BIOS or just continue. Go into the BIOS and change the date/time and other settings that need to be changed. Then, save and quit. After restarting, if you still get the above message, you should replace the CMOS battery.

If you don’t get any such warning, check that the time and date on your system is correct. If the time resets to 12:00 and date resets to Jan 1, this is an indication of a dying CMOS battery and you should replace it. You should first confirm this by setting the time and date in your BIOS. If they reset again after restarting the computer, replace the battery.

If the computer suddenly shuts down or you get hardware errors like not detecting the hard drive, CD drive and RAM, this may cause the computer to shutdown. First check that all the cables are properly plugged in. Then, remove and re-insert the RAM. If the system still shuts down, you should try to replace the CMOS battery first, since it’s cheaper than trying to replace other parts.

Check the CMOS battery voltage, if you have a multimeter. The voltage across a good battery should be about 3 volts. When it’s below 2.5 volts, it may start giving problems. If the battery voltage is below 1.8 volts, the battery is dead.

Replacing the CMOS battery

When you remove the CMOS battery, all the hardware settings stored in the CMOS memory are lost. So, you should record or note down the CMOS settings before removing the battery. Go into the BIOS by pressing the required key (mostly F1, F2, Del or F10). Note down any customized settings like hard drive configuration, CPU speed adjustments or port activation/deactivation etc. Then, shutdown the system and unplug.

When you have replaced the battery, enter the BIOS once again and select "Load BIOS Setup Defaults". (Depending on your BIOS Manufacturer, you may have "Load Standard Defaults", "Load Fail-Safe Defaults" or just "Load Defaults"). Then, reconfigure the customized settings that you noted earlier. Save the settings and exit the BIOS.


Replacement CMOS batteries can be purchased at any electronics stores or online. Compare the engraved serial numbers before purchasing the battery.

Remove power and discharge static electricity before touching delicate electronics.

Below are links to some videos on how to remove and replace a CMOS battery:

How to replace your PC’s battery

CMOS battery replacement

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