What is a CMOS Battery? How It Works & How to Replace One
Reviewed by Huzaifa HaroonCMOS stands for Complementary Metal Oxide Semiconductor. Even though it’s a small chip on your motherboard, it does a lot more than you might expect. First, let’s clear up a common misunderstanding: the CMOS chip and the CMOS battery are two different things.
CMOS is a MOSFET-based technology used inside the motherboard, mainly because it allows for logical operations while using very little static power. This chip is where important BIOS settings are stored, it powers the RTC (Real Time Clock), and it keeps other key configuration details safe as well.
For over a decade, the CMOS chip has been integrated into the ICH (I/O Controller Hub) or South Bridge area of your motherboard.
The coin-shaped part on your motherboard is the CMOS battery, a Lithium coin cell. This battery provides the small amount of power the CMOS chip needs to keep its settings when your PC is turned off. While most modern systems can save some BIOS or UEFI settings to a hard drive or other non-volatile storage, the battery is still required to keep the time and date running accurately, as the RTC can’t keep time without power.
The terms CMOS battery and CMOS chip/memory are often mixed up, but it’s useful to know that the battery powers the chip—the two are not the same.
How Exactly Does a PC Function?
Let’s start at the beginning. Your CPU is the heart of your computer—it handles most of the calculations and is crucial for performance. The CPU connects to the motherboard, and all your components—like keyboard, mouse, and monitor—get plugged in there too. The BIOS helps everything work together by managing communication between these hardware devices.
When you turn on your PC, the BIOS checks and configures the attached hardware. For example, if you have two SSDs—one is SATA and the other is M.2—and you want to boot from the SATA drive, the BIOS stores this preference. BIOS (or UEFI) is the very first software your PC runs, even before any operating system loads.
Nobody wants to re-enter all their BIOS settings—like manual overclocks or boot device priorities—every time they turn on their PC. That’s why CMOS memory exists. Just like your games are stored on a hard drive, your BIOS settings are saved in the CMOS memory.
Now, let’s explore why the CMOS is so important for keeping your PC running smoothly.
↪ The Role of a CMOS Battery
When you power on your computer for the first time, the CMOS starts empty. The BIOS collects information about your PC—like CPU settings (including any overclocks), memory speeds (XMP profiles), and which features you have turned on or off—and saves that data to CMOS memory.
The next time your PC starts, it checks the stored CMOS data to make sure nothing changed. If you swap hardware or make changes in BIOS, those settings are updated in CMOS memory right away.
Most importantly, the CMOS battery keeps your RTC (Real Time Clock) running when your PC is off, so your system always knows the correct time and date. While modern PCs often sync the clock over the internet, it’s still handy to have the battery in place. So to summarize, the CMOS battery:
- Stores your BIOS/UEFI settings and preferences.
- Keeps the Real Time Clock accurate.
- Provides power for the CMOS memory when your PC is unplugged.
Since every battery wears out eventually, you will likely need to replace your CMOS battery at some point. Typically, these batteries last around 3 years if the computer is unplugged. Weak batteries can cause a wide range of problems.
Issues with the CMOS Battery
Here are some of the most common problems that may happen if your CMOS battery is getting weak or has died:
1. Incorrect Date and Time
Because the CMOS battery powers the Real Time Clock, if the battery is failing you may keep seeing the wrong date and time on your PC. While you can fix this temporarily by syncing your computer’s time with the internet, it’s a very clear sign that your CMOS battery is dying.
Also, some websites will display errors like “Your clock is ahead/behind.” If this happens, it’s usually because your system clock isn’t in sync, often due to a weak or dead CMOS battery.
2. BIOS Passwords May Get Reset
BIOS passwords are different from your regular Windows passwords—they protect BIOS settings from unauthorized users. Since these are saved in CMOS memory, a bad battery can reset them unexpectedly. If the CMOS battery is failing, your BIOS passwords can be erased without warning.
If you’re managing computers for an organization or school, this can cause security issues. On the other hand, if you ever forget your BIOS password, you can reset it by removing the CMOS battery for about 10-15 minutes and then putting it back in.
3. Disturbed Boot Device Sequence
If your computer suddenly tries to boot from another hard drive, or you notice your previously set boot order is lost, it’s likely due to a failing CMOS battery.
As discussed above, your boot sequence is saved in CMOS memory. If the battery runs low, this important data can be lost or corrupted.
4. Constant Beeping Sound
If you hear a beeping sound when your PC starts up, it could be a beep code related to a failing CMOS battery. These beeps are generated when your computer runs POST (Power On Self Test). Each beep pattern means something specific (depending on the manufacturer), but if you notice beeps along with other issues listed here, the CMOS battery could be the culprit.
For example, 10 beeps on AMI BIOS systems usually indicate a CMOS shutdown register problem.
5. Irresponsive Peripherals
If your keyboard, mouse, or other devices stop working or behave unpredictably after starting your PC, a dying CMOS battery could be the reason. If you use a custom keyboard layout, the settings might reset due to corrupted or missing CMOS data.
6. Checksum Error
If you turn on your PC and see a CMOS checksum error message, this means your BIOS can’t verify the data in CMOS memory, most likely because of a weak battery.
CMOS memory stores all the settings from your BIOS for the first boot. When you restart, your PC tries to match new information with what’s stored—if there’s a mismatch because the battery failed, you see errors like this.
How to Reset/Replace the CMOS Battery
Every electronic part wears out eventually, and the CMOS battery is no exception. After a few years, it may not hold enough voltage, and you’ll see the earlier problems.
If your CMOS battery is dead or you just want to reset your BIOS, the hardest step is usually just finding the battery! The rest is simple—just pull it out and pop a new one in. CMOS batteries are cheap and easy to replace. Here’s how to do it:
- Take the necessary ESD (Electrostatic Discharge) precautions to protect your components.
- Open your PC’s case and find the motherboard, where your CPU cooler, GPU, and RAM are installed. (Here’s a visual guide from GIGABYTE):
- Look for the CMOS battery—it’s usually a shiny coin-shaped cell. In most motherboards, it’s under or near the PCIe Slot. You may have to unplug cables, move drives, or take out your GPU to reach it.
- If you’re on a laptop or a server motherboard, check your manufacturer’s manual for details. Some laptops allow easy access, others might require removing several covers or the entire bottom panel.
- With your fingers, gently pull the edge of the battery up. Some systems use a small metal clip you have to push aside before the battery comes out.
- Once removed, gently insert the new battery in the same spot. That’s it—you’ve replaced your CMOS battery! Don’t worry, it’s easier than it sounds.
Reviewed by Huzaifa Haroon
