Yes, laptops have a CMOS battery separate from the main battery pack
Your laptop actually has two batteries. The large one you charge every day powers the screen, processor, and everything else you use. The CMOS battery is a small coin-shaped battery, usually the size of a watch battery, that sits on the motherboard. It powers only the BIOS—the firmware that runs before Windows or macOS even loads.
The CMOS battery is not rechargeable. It lasts between three and ten years depending on how often your laptop is plugged in. When it dies, your laptop will still work perfectly when powered on, but it will forget the date, time, and some hardware settings the moment you unplug it. You will not lose files or damage the machine.
Key Takeaways
- The CMOS battery is a small coin cell on the motherboard that keeps the BIOS clock and settings alive when the laptop is off and unplugged.
- A dead CMOS battery causes the date and time to reset every time you power on, but does not affect files or damage the laptop.
- CMOS batteries last three to ten years and are not rechargeable; they must be physically replaced.
- Replacing a CMOS battery requires opening the laptop case and usually costs $50 to $150 in labor if you take it to a technician.
What the CMOS battery actually powers
CMOS stands for Complementary Metal-Oxide-Semiconductor. The battery powers a small amount of memory on the motherboard that stores BIOS settings. The BIOS is the layer of code that runs when you first press the power button, before the operating system loads. It tells the laptop what hardware is installed, what the current date and time are, and how to boot up.
When the CMOS battery is healthy, it keeps this information alive even when the laptop is completely powered off and unplugged. The moment the battery dies, the BIOS loses power and forgets everything except the factory defaults. The next time you turn on the laptop, it will show the wrong date and time—usually something like January 1, 2000 or the date the laptop was manufactured.
The CMOS battery does not power your screen, keyboard, or any of the components you interact with. It is purely a housekeeping battery for the firmware layer underneath the operating system.
Signs your CMOS battery is dying or dead
The clearest sign is that your date and time reset to an incorrect value every time you shut down and restart. You might notice the clock is wrong when ready after booting, or you might see it only after the laptop has been off for a few hours or days. If you correct the date and time, it will stick while the laptop is on, but reset again the next time you power cycle.
Some laptops will also show a CMOS battery warning in the BIOS setup screen, which you can access by restarting and pressing a key like F2, F10, or Delete during startup (the exact key varies by manufacturer). You may also see a message like "CMOS battery low" or "System battery voltage is low" on the boot screen.
In rare cases, a dead CMOS battery can prevent the laptop from booting at all, but this is uncommon. Most of the time, the laptop works normally—it just forgets the time.
How long CMOS batteries last
A CMOS battery typically lasts between three and ten years. The variation depends mainly on how often your laptop is plugged in. If you leave the laptop plugged in most of the time, the main battery pack is doing the work, and the CMOS battery drains very slowly. If you unplug the laptop frequently and let it sit unused for long periods, the CMOS battery drains faster because it is the only power source keeping the BIOS memory alive.
Laptop age matters too. A five-year-old laptop is more likely to need a CMOS battery replacement than a one-year-old machine, straightforward because the battery has been in use longer. There is no way to extend the life of a CMOS battery—it is not something you can maintain or recalibrate. When it dies, it must be replaced.
How to replace a CMOS battery yourself
Replacing a CMOS battery is a straightforward repair if you are comfortable opening your laptop. The battery is a small coin cell, usually a CR2032 or similar type, that sits in a plastic holder on the motherboard. You can buy a replacement battery at any pharmacy, electronics store, or online retailer for $3 to $10.
To replace it, you will need to power off the laptop completely, unplug it, and remove the main battery if it is user-removable. Then you will need to open the case—usually by removing screws from the bottom panel. Once inside, locate the CMOS battery (it looks like a large watch battery), gently pop it out of its holder, and insert the new one with the positive side facing up. Close the case, reinstall the main battery, and power on.
The exact steps vary by laptop model. Before you start, search for your specific model number plus "CMOS battery replacement" to find a guide with photos. If you are not comfortable opening the laptop, a technician can do this repair in 30 minutes to an hour, usually for $50 to $150 in labor.
What happens if you ignore a dead CMOS battery
Ignoring a dead CMOS battery will not damage your laptop or cause data loss. The machine will continue to work normally while it is powered on. The only inconvenience is that the date and time will be wrong every time you restart, which can cause minor issues like incorrect file timestamps or problems with time-sensitive software.
Some security certificates and authentication systems rely on the system clock being correct. If your CMOS battery is dead and the date is set to 2000, you might have trouble connecting to find websites or logging into accounts that use time-based verification. This is rare, but it is worth fixing if you encounter it.
There is no urgency to replace a CMOS battery—it is not a safety issue or a sign of hardware failure. You can replace it whenever it is convenient, or you can leave it alone if the wrong date does not bother you.
CMOS battery versus the main laptop battery
The two batteries serve completely different purposes. The main battery pack is large, rechargeable, and powers everything you see and use—the screen, processor, keyboard, and storage. It holds a charge for hours and is what you plug in to recharge. The CMOS battery is tiny, non-rechargeable, and powers only the BIOS firmware. It lasts years without recharging because it draws almost no power.
When the main battery dies, the laptop shuts down when ready. When the CMOS battery dies, the laptop still works fine—it just forgets the time. They are independent systems, and the failure of one does not affect the other.
Frequently Asked Questions
Can I replace the CMOS battery myself without voiding the warranty?
Most manufacturers allow you to replace the CMOS battery yourself without voiding the warranty, since it is a standard maintenance part. However, opening the laptop case may void the warranty depending on your specific model and manufacturer. Check your warranty documentation or contact the manufacturer before you open the case. If you are unsure, have a technician do it instead.
What type of CMOS battery does my laptop use?
Most laptops use a CR2032 coin cell battery, but some use CR2025 or other types. The easiest way to find out is to search your laptop model number plus "CMOS battery type" online, or open the case and look at the battery itself—the type is printed on it. You can also call a repair shop with your model number and they will tell you when ready.
Will replacing the CMOS battery erase my files or settings?
No. Replacing the CMOS battery will reset the date and time to the factory default, and it may reset some BIOS settings like boot order. Your files, passwords, and operating system settings will not be affected. After you replace the battery, straightforward set the correct date and time, and everything will work normally.
Is a CMOS battery the same as the main laptop battery?
No. The main battery is large, rechargeable, and powers the entire laptop. The CMOS battery is a tiny coin cell that powers only the BIOS firmware. They are separate systems with different purposes and lifespans.
Do all laptops have a CMOS battery?
Almost all laptops with a removable or replaceable battery have a CMOS battery on the motherboard. Some very thin or sealed laptops may have the CMOS function built into the main battery system, but this is rare. If you are unsure whether your laptop has one, search your model number plus "CMOS battery" online.