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 your screen, processor, and everything else. The CMOS battery is a small coin-shaped battery—usually the size of a watch battery—that sits on the motherboard and does one specific job: it keeps your laptop's internal clock and BIOS settings alive when the main battery is completely dead and the laptop is unplugged.
The CMOS battery is not something you charge. It sits permanently on the motherboard and slowly drains over years, not hours. When it dies, your laptop will still work perfectly fine—but you'll notice your system clock resets to a default date (often 2000 or 2001) every time you power off, and any custom BIOS settings you've made will vanish.
Most laptop owners never think about the CMOS battery because it lasts five to ten years and requires no maintenance. You only notice it's gone when it actually fails, which is rare in machines less than five years old.
Key Takeaways
- Your laptop has a CMOS battery separate from the main battery pack; it's a small coin cell on the motherboard that powers the internal clock and BIOS memory when the laptop is off.
- The CMOS battery does not charge and lasts five to ten years before it typically needs replacement.
- When a CMOS battery dies, your system clock resets to a default date every time you shut down, but your laptop continues to work normally otherwise.
- Replacing a CMOS battery requires opening the laptop case and usually costs between $30 and $100 in labor if you take it to a technician.
- You can test whether your CMOS battery is failing by checking whether your system date resets after you power off and unplug the laptop overnight.
What the CMOS battery actually does
CMOS stands for Complementary Metal-Oxide-Semiconductor. The battery powers a small chip on your motherboard that stores two things: your system's real-time clock and your BIOS settings. The BIOS is the firmware that runs before your operating system loads—it controls things like boot order, hardware detection, and security settings.
When your laptop is plugged in or running on the main battery, the motherboard draws power from those sources instead. But the moment you unplug the laptop and the main battery drains completely, the CMOS battery takes over. It keeps the clock ticking and the BIOS settings in memory so that when you plug the laptop back in and power it on, everything is where you left it.
Without a working CMOS battery, the clock stops. Every time you shut down and unplug, the system forgets what time it is. When you power on again, it defaults to a preset date—usually sometime in the early 2000s. Your operating system will then try to correct it if you're connected to the internet, but the BIOS itself has lost the information.
Signs your CMOS battery is failing
The most obvious sign is a system clock that resets to the wrong date every time you shut down and leave the laptop unplugged overnight. You'll power on the next morning and see the date is 2001 or 2002, even though you've set it correctly dozens of times before.
A second sign is that custom BIOS settings disappear. If you've changed boot order, disabled certain hardware, or adjusted power settings in the BIOS, and those changes don't stick after a full shutdown, the CMOS battery is likely dead. The settings revert to factory defaults.
Some laptops will also display a CMOS battery warning message during startup, though this is less common in consumer machines. You might see "CMOS battery low" or "CMOS checksum error" on the boot screen.
If your laptop is less than three years old and you're seeing these signs, the CMOS battery itself is probably not the issue—something else is draining it faster than normal, or there's a motherboard problem. If your laptop is seven years old or older, CMOS battery failure is the most likely cause.
How long a CMOS battery lasts
A typical CMOS battery lasts between five and ten years, depending on how often the laptop is used and how much the motherboard draws power. Laptops that sit in storage or are rarely used may see the battery last longer because it's not being drained. Laptops that are powered on and off frequently may see it fail sooner.
The battery doesn't wear out from charging and discharging like your main battery does. Instead, it slowly self-discharges over time. This is a chemical process that happens whether the laptop is on or off, so there's no way to slow it down or extend its life through your habits.
Most people replace their laptop before the CMOS battery dies, so you may never encounter this problem. But if you're using a laptop that's eight years old or older, a failing CMOS battery is worth checking for.
Replacing a CMOS battery yourself versus taking it to a technician
Replacing a CMOS battery is technically straightforward—you open the laptop case, locate the coin cell on the motherboard, pop it out, and insert a new one. The battery itself costs $5 to $15 online. However, opening a laptop requires tools, care, and knowledge of where the battery sits, which varies by model.
If you're comfortable with a screwdriver and following a disassembly guide specific to your laptop model, you can do it yourself. Search for "[your laptop model] CMOS battery replacement" on YouTube or your manufacturer's support site. Many manufacturers publish official disassembly guides.
If you're not comfortable opening the laptop, a repair technician can do it for you. Labor typically costs $30 to $100 depending on how difficult your specific model is to open. Some technicians may charge a flat fee; others charge hourly. Call ahead and ask for a quote.
One important note: before you open the laptop, unplug it and press the power button several times to discharge any remaining power. This prevents accidental damage to the motherboard.
What type of CMOS battery your laptop uses
Almost all laptops use a CR2032 coin cell battery. This is a standard battery you can buy at any pharmacy, electronics store, or online retailer for $3 to $8. It's the same battery used in watches, calculators, and car key fobs.
Some older or specialized laptops may use a CR2025 or CR2016, which are slightly thinner versions. The easiest way to know which one your laptop needs is to look at the battery that's currently in the motherboard—the model number is printed on it. You can also check your laptop's service manual or search for your model online.
When you buy a replacement, make sure you're getting a name-brand battery from a reputable seller. Cheap counterfeit coin cells can fail quickly or leak, damaging the motherboard. Panasonic, Energizer, and Duracell are reliable choices.
What happens if you ignore a dead CMOS battery
Nothing breaks. Your laptop will continue to work exactly as it did before. The only inconvenience is that your system clock resets every time you shut down, and any BIOS customizations you've made will disappear.
If you rely on an accurate system clock for work—for example, if you use timestamped files or security certificates—a dead CMOS battery can cause problems. Your operating system will usually correct the date if you're connected to the internet, but there may be a window of time where the clock is wrong.
For most users, a dead CMOS battery is more of an annoyance than a real problem. You can leave it alone indefinitely if the clock resetting doesn't bother you. But if you plan to keep the laptop for several more years, replacing it is inexpensive and takes an hour or less.
Frequently Asked Questions
Can I use my laptop if the CMOS battery is dead?
Yes, completely. A dead CMOS battery does not affect the laptop's ability to run. It only affects the internal clock and BIOS settings. Your laptop will work normally in every other way.
Will my laptop warn me when the CMOS battery is dying?
Some laptops display a warning message during startup, but most do not. The most reliable way to know is to watch for a system clock that resets to the wrong date after you shut down and unplug the laptop overnight.
Is a CMOS battery the same as the main battery I charge?
No. The main battery is large, rechargeable, and powers your laptop while it's on. The CMOS battery is a small coin cell that never charges and only powers the clock and BIOS memory when everything else is off.
How do I know if my laptop model uses a CR2032 or a different coin cell?
Check your laptop's service manual or search "[your laptop model] CMOS battery" online. You can also open the laptop and look at the battery itself—the model number is printed on the surface.
What should I do before I replace the CMOS battery myself?
Unplug the laptop and press the power button several times to discharge any remaining power on the motherboard. This prevents accidental electrical damage. Then follow a disassembly guide specific to your model.