Yes, your laptop has a CMOS battery, and it's separate from the main battery pack
Your laptop contains two different batteries. The large one you charge every day powers the screen, processor, and everything else you see and use. The CMOS battery is a small coin-shaped battery, usually the size of a watch battery, that sits on the motherboard inside your laptop. It has one job: keeping the system clock and BIOS settings alive when your laptop is completely powered off and unplugged.
The CMOS battery doesn't power your laptop. It only maintains the internal clock and a small amount of stored information about your hardware. When you shut down your laptop and unplug it, the main battery drains to zero, but the CMOS battery keeps working in the background. Without it, your laptop would forget the time and date every time you turned it off, and certain hardware settings would reset.
Most laptop owners never see or think about the CMOS battery because it lasts for years—typically three to ten years depending on the model and how often the laptop sits unplugged. When it finally dies, the symptoms are unmistakable and straightforward to fix.
Key Takeaways
- A CMOS battery is a small coin cell inside your laptop that maintains the system clock and BIOS settings when the laptop is off and unplugged.
- It is completely separate from your main rechargeable battery and does not power your laptop during normal use.
- CMOS batteries typically last three to ten years before they need replacement.
- A dead CMOS battery causes the clock to reset, BIOS settings to revert, or error messages about the date and time on startup.
- Replacing a CMOS battery requires opening the laptop case and costs between $20 and $100 depending on your model and whether you do it yourself.
What the CMOS battery actually powers
The CMOS battery powers only the CMOS chip—a tiny piece of memory on your motherboard that stores information. This chip holds your system clock, the date, and hardware settings like boot order, CPU speed, and RAM detection. It uses almost no power, which is why a small battery can keep it running for years.
When your laptop is on, the main battery powers everything, and the CMOS battery sits idle. When you shut down and unplug the laptop, the CMOS battery takes over and keeps the clock ticking and the settings stored. This is why your laptop knows what time it is the moment you turn it back on—the CMOS battery has been counting the seconds the whole time.
If the CMOS battery dies, the clock stops updating. The next time you start your laptop, it will show the wrong date and time, usually reverting to a default like January 1, 2000, or the date the laptop was manufactured. Some laptops also reset BIOS settings to factory defaults or show error messages about the date being incorrect.
Signs your CMOS battery is dying or dead
The most obvious sign is the system clock resetting. Every time you shut down and turn the laptop back on, the date and time are wrong—usually very far in the past. You may also see error messages during startup that mention the date, time, or CMOS battery itself.
Some laptops show a CMOS checksum error or a message like "CMOS battery low" or "System date and time not set." If you correct the date and time, shut down, and the problem happens again the next time you start up, the CMOS battery is almost certainly dead.
In rare cases, a failing CMOS battery can cause BIOS settings to reset unexpectedly, which might change your boot order or disable hardware features. This is less common than the clock issue but points to the same problem. A dead CMOS battery is not an emergency—your laptop will still work normally once you turn it on—but it is annoying and worth fixing.
How to replace a CMOS battery yourself
Replacing a CMOS battery is one of the simplest laptop repairs you can do yourself if you are comfortable opening the case. The process takes 10 to 20 minutes and requires only a screwdriver and a replacement battery. Most CMOS batteries are CR2032 cells, which cost $3 to $8 and are sold at any pharmacy, electronics store, or online retailer.
First, shut down your laptop completely and unplug it. Let it sit for a few minutes to discharge any remaining power. Open the bottom panel or access door—your laptop's manual will show you which screws to remove. Locate the CMOS battery on the motherboard; it is a round, silver coin-shaped cell, usually about the size of a dime, sitting in a small plastic holder.
Gently pry the battery out of its holder using a plastic spoon or a plastic pry tool—do not use metal, which can damage the motherboard. Note which side is facing up. Insert the new battery with the same side facing up, press it down until it clicks into place, and close the laptop. When you turn it back on, the clock should be correct, and the error messages should be gone.
If you are not comfortable opening your laptop, a repair shop can do this for you in 30 minutes to an hour. The labor cost is usually $30 to $75, plus the cost of the battery itself. Some manufacturers warranty this repair, so check your laptop's warranty before paying out of pocket.
Why your laptop needs a CMOS battery at all
Modern laptops use very little power when they are off, but the system clock and BIOS settings still need to be maintained. Without the CMOS battery, every time you unplugged your laptop, it would lose track of time and forget hardware settings. The next time you turned it on, it would have to ask you to set the date and time manually, and certain features might not work until you reconfigured them in the BIOS.
The CMOS battery is a legacy of desktop computers, where the same problem existed decades ago. Laptop designers kept the design because it is cheap, reliable, and solves the problem completely. The battery is so small and uses so little power that it lasts for years without maintenance.
Some newer laptops use a small supercapacitor instead of a battery to maintain CMOS settings. A supercapacitor charges when the laptop is on and slowly discharges when it is off, lasting weeks or months without power. However, most laptops still use a traditional CMOS battery because it is simpler and more reliable over the long term.
The difference between CMOS battery and main battery
Your laptop's main battery is large, rechargeable, and powers everything when the laptop is on. It stores enough energy to run your screen, processor, and all your programs for hours. When you plug in your laptop, the charger refills this battery. When you unplug it, this battery drains as you use the laptop, and eventually the laptop shuts down.
The CMOS battery is tiny, non-rechargeable, and powers only the system clock and BIOS settings when everything else is off. It never charges—it straightforward drains very slowly over years until it dies. You cannot replace the main battery by opening the case on most modern laptops; it is soldered to the motherboard or requires professional service. The CMOS battery, by contrast, is designed to be user-replaceable on most models.
A dead main battery means your laptop will not turn on or will shut down when ready when unplugged. A dead CMOS battery means your laptop works fine, but the clock resets and you see error messages. The two problems feel completely different because they are caused by different batteries doing different jobs.
When to replace the CMOS battery versus when to ignore it
If your laptop is showing the wrong date and time every time you start it up, and you have confirmed that the system clock is not just set wrong in Windows or macOS, then the CMOS battery is dead and worth replacing. The fix is cheap and takes 20 minutes if you do it yourself.
If your laptop is only a few years old and the clock is correct, do not worry about the CMOS battery. It will last several more years. If your laptop is very old—ten years or more—and you plan to keep using it, replacing the CMOS battery now is a good preventive step, even if it is not showing symptoms yet. A dead CMOS battery is not dangerous and will not damage your laptop, but it is annoying enough to fix while you have the case open for other maintenance.
If you are selling or giving away an old laptop, replacing the CMOS battery is a small gesture that shows the new owner the machine was well maintained. It costs almost nothing and takes minutes.
Frequently Asked Questions
Can I use my laptop if the CMOS battery is dead?
Yes, completely. A dead CMOS battery does not prevent your laptop from working. It only causes the clock to reset and BIOS settings to revert when the laptop is off. Once you turn it on, everything works normally. The only inconvenience is that the date and time will be wrong until you correct them manually or connect to the internet.
What type of CMOS battery do I need?
Most laptops use a CR2032 coin cell battery, which is the same type used in watches and calculators. Check your laptop's manual or search online for your specific model to confirm. Some older or specialized laptops may use a different type, but CR2032 is by far the most common.
Will replacing the CMOS battery erase my files or passwords?
No. The CMOS battery only stores the system clock and basic hardware settings. Your files, passwords, and operating system are stored on the hard drive or solid-state drive, which are powered by the main battery and do not lose data when the CMOS battery dies. Replacing the CMOS battery will not affect anything on your computer.
How do I know if it is the CMOS battery and not something else?
If the date and time reset to the same wrong value every time you shut down and turn the laptop back on, it is almost certainly the CMOS battery. If you see a specific error message mentioning "CMOS" or "battery," that confirms it. If the problem is something else—like the laptop not turning on at all or the clock being wrong only in the operating system—then it is a different issue.
Can I buy a CMOS battery at a regular store?
Yes. CR2032 batteries are sold at pharmacies, supermarkets, electronics stores, and online retailers. They cost $3 to $8 and are one of the most common battery types in the world. You do not need a special computer part—any CR2032 will work.