What is a CMOS Battery? How It Works & How to Replace One [2024]
Reviewed by Huzaifa Haroon- The CMOS chip handles crucial BIOS settings and keeps the Real Time Clock running, while the CMOS battery, a small lithium coin cell, powers this chip to ensure settings and time are maintained even when your PC is off.
- The CMOS battery is important because it saves BIOS/UEFI settings, keeps the Real Time Clock accurate, and powers the CMOS memory. Typically, it lasts around 3 years when the PC is unplugged.
- If you're seeing wrong date/time, BIOS passwords resetting, messed up boot sequences, beeping sounds at startup, unresponsive peripherals, and checksum errors, your CMOS battery is dying and needs to be replaced.
CMOS stands for Complementary Metal Oxide Semiconductor and despite its size, it does much more than you think. Let’s clarify a few things first. A CMOS chip is vastly different than the CMOS battery on your motherboard.
Table of Contents
Introduction: What is “CMOS”?
CMOS is a MOSFET-type technology used for logical operations. In simple terms, it is used to make chips that have low static power consumption. This chip quite efficiently stores your important BIOS configurations, powers the RTC (Real Time Clock), and holds many other important settings.
For the last decade or so, the CMOS chip has been integrated as a part of the ICH / South Bridge chip on your motherboard.
The coin-shaped object on the contrary is the CMOS battery, a Lithium coin cell. This is what powers the aforementioned chip/memory. While storing BIOS/UEFI preferences can be offloaded to say your Hard Disk, since it is non-volatile, a battery is required to keep the time and date in check (Real Time Clock).
The terms CMOS battery and CMOS chip/memory are often used interchangeably, but it is best to know the difference between both.
READ MORE: What is Power-On Self-Test in Computers? POST Explained ➜
How Exactly Does a PC Function?
Let’s start from the basics. Your CPU is the brain of your computer, it does all the complex calculations and determines your PC’s performance. The CPU is attached to a motherboard, on which all the I/O is connected. You use a keyboard and mouse for input and in most cases a monitor displays the output. The BIOS establishes the relationship between all these hardware devices.
When you boot up your PC, your BIOS identifies and configures the hardware connected. If you have 2 SSDs, one SATA and one M.2, say you want the SATA to be your primary for whatever reason, the BIOS does all the handling. The BIOS or UEFI is the first software that loads up, even without an Operating System.
But let’s say you want to save these preferences, I mean who wants to set a manual overclock or change the boot device priority repeatedly every single time their PC boots up? This is where the CMOS memory/chip comes into play. Just like how you require a Hard Drive to store your games, your BIOS requires the CMOS memory to store all the settings and configurations.
Let’s talk more about the importance of CMOS in how your PC works.
↪ The Role of a CMOS Battery
If you’ve assembled your computer for the first time and boot up, the CMOS is empty. The BIOS starts collecting information regarding your PC, like your CPU frequency (Overclocks, if any), memory speeds (XMP), memory latency, which settings you prefer to keep on/off, and so on. It saves this data in the CMOS memory.
The next time you boot up your PC, all this data is tallied against the stored information in the CMOS memory. If you replace any hardware part or change a setting, that information is also immediately saved to the CMOS memory.
Most importantly, a CMOS battery is crucial to maintain the RTC (Real Time Clock) on your computer. This allows proper tracking of time even when your PC is powered off. While most PCs nowadays are connected to the Internet, it is still convenient to have a one-stop solution for all systems. Let’s recap what we have discussed thus far, that is the CMOS battery:
- Saves your BIOS/UEFI preferences and settings.
- Maintains the Real Time Clock.
- Provides power to the CMOS memory.
Since all batteries have a limited lifespan, you may need to replace your CMOS battery eventually. Typically, CMOS batteries last for around 3 years when unplugged from the PSU. But once they start getting weak, you may notice all sorts of problems.
READ MORE: 2 ways to change ‘Critical Battery Percentage Levels on Windows’ ➜
Issues with the CMOS Battery
A few issues that may arise due to a weakened or dying CMOS battery are as follows:
1. Incorrect Date and Time
As the CMOS battery is primarily responsible for powering the RTC, it may lead to incorrect date and time on your PC. While this can be overcome by synchronizing your PC’s date/time with the Internet, it is a crystal-clear symptom of a failing CMOS battery.
Likewise, certain websites will display an error message stating “Your clock is ahead/behind. This indicates that the date and time are not synchronized properly and is most likely caused by your CMOS battery.
READ MORE: How to Fix Real Time Clock Error ➜
2. BIOS Passwords May Get Reset
BIOS Passwords are different from Windows user passwords since they prevent unauthorized users from accessing your system BIOS. As the BIOS data is dependent on the CMOS memory and battery, the BIOS may frequently get reset due to a malfunctioning battery. This in turn also resets all BIOS-level passwords on your system.
If you manage an organization or a school, this can lead to many vulnerability and security issues. As a side note, you can use this to your advantage as well. If you somehow forget your BIOS password, just remove your CMOS battery, wait for a bit (10–15 minutes), reinsert it and your password will be reset.
3. Disturbed Boot Device Sequence
If your PC out of nowhere boots off another Hard Disk, or your primary boot device suddenly becomes the secondary or even undetected, it could be a telltale sign of a failing CMOS battery.
As we explained above, all this boot sequence data is saved in the CMOS memory. If the CMOS battery, which supplies power to the CMOS memory gets weak, this data may get corrupted or even lost.
4. Constant Beeping Sound
If you hear a beeping sound when your PC starts, it might be a beep code indicating a failing CMOS battery. Such beeps occur when your PC undergoes the procedure of POST (Power On Self Test). While each code has a separate meaning, specific to each OEM, this in tandem with the aforementioned problems hints towards an issue with the CMOS battery.
For example, 10 beeps on an AMI BIOS indicate a CMOS shutdown register read/write issue.
5. Irresponsive Peripherals
Another sign of a dying CMOS battery is if your keyboard and mouse stop responding or show weird behavior. If you use a custom keyboard layout, it may get reset. This may be a result of improperly configured drivers, courtesy of the CMOS battery.
6. Checksum Error
When you boot your computer or laptop with a failing CMOS battery, you may experience a CMOS checksum error. This basically implies that your BIOS has not been able to verify the details stored regarding your hardware in the CMOS memory, as we explained above.
The CMOS memory holds all the information stored in the BIOS for the 1st boot. When you boot for the 2nd time, the new information is matched against the data stored from the 1st boot.
READ MORE: How to Fix the CMOS Checksum Error on Windows? ➜
How to Reset/Replace the CMOS Battery
All electronic appliances have a limited lifespan. After a number of years, your CMOS battery might not supply enough voltage to the CMOS memory, resulting in the aforementioned problems.
If your CMOS battery needs a replacement, or if you just want to reset your BIOS, the hardest part is to actually locate the battery iteslf. The rest is just pulling it out and plugging it back in. Matter of fact, CMOS batteries are quite cheap and easy to replace. Follow these steps to do so:
- Take necessary ESD protections.
- Open your PC’s case and locate the motherboard on which your CPU cooler, GPU and RAM have been installed. (Reference image from GIGABYTE)
- Locate the CMOS battery, typically shaped like a silver coin. In the above image, it is located just below the PCIe Slot. You may need to disconnect cables and remove drives or even your GPU to gain access to the CMOS battery.
- If you are on a laptop or a server motherboard, kindly check the manufacturer’s guide for more details. Some laptops have a casing that gives access to all components beneath. Other laptops may have several layers, each packing different components. In the worst-case scenario, you’d have to unscrew all the casings and explore a bit.
- Next, use your fingers to hold the edge of the battery and gently pull it out. Some manufacturers will opt to use a clip that you must pull up to remove the battery.
- Once the battery is out of the socket, gently reinsert the replacement in the socket and you’ve successfully replaced your CMOS battery. As a reminder, this process is easier than it sounds.
READ MORE: How to Run A Computer Performance (Benchmark) Test on Windows ➜
Conclusion
The CMOS battery is responsible for storing your BIOS settings and maintaining the date and time on your PC. As is the case with all cells, the CMOS battery may degrade over time if left unpowered for a long while. You will start to notice some serious hiccups if your battery starts to fail.
It is highly recommended you swap out your faulty CMOS battery for a new one. CR2032 Lithum cells cost as low as $1 and replacing your existing CMOS battery for a new one is quite straightforward.
FAQs
No. CMOS is a technology that is used to make chips, mainly the CMOS memory. The CMOS battery powers the CMOS memory. The BIOS on the contrary initializes the hardware and stores related data in the CMOS memory for future use.
No! Modern PCs use an RTC (Real Time Counter) which keeps track of the date and time on your PC. It requires an active power source to do so. Furthermore, many other BIOS-related settings are dependent on the CMOS battery.
It depends. While most CMOS batteries can last for around 5-10 years, you may find a few that die within just 3 years. Luckily enough, replacing them isn’t a hassle since most of them are inexpensive CR2032 Lithium cells.
Reviewed by Huzaifa Haroon
