A CMOS battery keeps your computer's internal clock and settings alive when the power is off

The CMOS battery is a small coin-shaped battery inside your desktop computer or laptop that powers the CMOS chip — a tiny piece of memory that stores your system date, time, and hardware settings. When you shut down your computer, the CMOS battery takes over so those settings don't vanish. Without it, your machine would forget the time every time you powered it down and lose custom configurations you've made in the BIOS or UEFI menu.

Most CMOS batteries last three to ten years depending on how often your computer runs and how old the battery was when installed. You'll know it's failing when your system date resets to a default year (often 2000 or 2001), your BIOS settings revert to factory defaults, or you see error messages about the real-time clock on startup.

Key Takeaways

  • A CMOS battery is a small coin cell (usually CR2032) that keeps your computer's clock and BIOS settings alive when power is off.
  • You need to replace it when your system date keeps resetting, your BIOS settings won't stay saved, or you see clock-related error messages on boot.
  • Replacement takes 10 to 20 minutes on a desktop and involves opening the case, locating the battery on the motherboard, and swapping it out.
  • A new CMOS battery costs between $5 and $15 and is the same type you'd find in a watch or calculator.

How to tell your CMOS battery is dying

The most obvious sign is a system date that resets every time you restart. You'll turn on your computer and see the date is January 1, 2000, or some other default year, even though you've set it correctly before. This happens because the CMOS chip has lost power and forgotten what year it is.

A second warning sign is BIOS settings that won't stick. You change something in the BIOS menu — like boot order, fan speed, or RAM timing — save it, and restart. When you go back into BIOS, the setting has reverted to the factory default. This means the CMOS chip couldn't hold onto the change because the battery is too weak.

You may also see error messages on startup like "CMOS checksum error," "CMOS battery low," or "Real-time clock error." These are the motherboard's way of telling you the CMOS battery voltage has dropped below the minimum needed to keep data safe.

Where the CMOS battery sits and what it looks like

On a desktop computer, the CMOS battery is mounted directly on the motherboard, usually near the edge or in a corner. It's a round, flat silver disc about the size of a dime — most commonly a CR2032 battery, though some older boards use CR2025 or other coin-cell types. You can see the battery number printed on the surface.

On a laptop, the CMOS battery may be soldered to the motherboard or mounted in a small plastic holder. Some newer laptops don't have a replaceable CMOS battery at all; instead, they use a supercapacitor that charges while the machine is plugged in. If your laptop has one, you'll need to check your service manual or contact the manufacturer to know for sure.

The battery sits in a small metal clip or socket that holds it in place. The clip usually has a tab or lever you can gently pry to release the battery.

How to replace a CMOS battery on a desktop

Start by shutting down your computer completely and unplugging the power cable from the wall. Wait 30 seconds to let any residual power drain from the system. This is a safety step — it prevents accidental damage to the motherboard while you're working inside.

Open your computer case. Most desktop cases have two side panels held by thumbscrews or a latch. Remove the panel that gives you access to the motherboard. You may need to remove a few components like a graphics card or RAM stick to reach the CMOS battery, depending on your case layout.

Locate the CMOS battery on the motherboard. It's the small round silver disc. Gently pry the metal clip holding it in place using a plastic spoon, a plastic pry tool, or a flathead screwdriver. The clip will lift up slightly, and the battery will pop out. Note which side was facing up — the positive side usually faces outward.

Insert the new battery with the same orientation as the old one. The clip will snap back down and hold it in place. Close your case, plug in the power cable, and turn on your computer. Go into the BIOS menu (usually by pressing Delete, F2, or F12 during startup) and reset your date and time, then save and exit.

What type of battery to buy and where to find it

The vast majority of desktop motherboards use a CR2032 lithium coin-cell battery. You can buy one at any pharmacy, supermarket, electronics store, or online retailer for $5 to $15. They're the same batteries used in watches, calculators, and car key fobs, so they're widely stocked.

Before you buy, check your current battery to confirm the model number printed on it. If it says CR2032, you're set. If it says CR2025, CR2016, or something else, buy that exact type instead. Using the wrong size can damage the clip or fail to make proper contact.

Avoid buying the cheapest no-name brands if you can. A name-brand battery from Energizer, Duracell, Panasonic, or Maxell will last longer and is unlikely to leak. The price difference is usually only $1 or $2.

What happens if you ignore a dead CMOS battery

Your computer will still work, but it will be annoying to use. Every time you start it up, the date will be wrong, which can cause problems with file timestamps, software licensing, and security certificates. Some programs won't run if the system date is too far off.

If you've customized your BIOS settings — like overclocking, RAM timing, or boot order — those changes will disappear every time you restart. You'll have to reset them manually each time, or they'll revert to defaults and your machine may run slower or behave differently than you expect.

The CMOS battery itself won't cause hardware failure or data loss. Your hard drive and files are safe. But the user experience degrades, and the longer you wait, the more annoying it becomes. Replacing it is cheap and takes less than half an hour.

CMOS battery replacement on a laptop

Laptop CMOS batteries are harder to access than desktop ones because you have to disassemble more of the machine. On some models, you can remove the bottom panel and reach the battery in 10 minutes. On others, you have to remove the keyboard, trackpad, or even the motherboard itself.

Check your laptop's service manual or search for your specific model number plus "CMOS battery replacement" to find step-by-step instructions. If the battery is soldered to the motherboard, you cannot replace it yourself — you'll need to contact the manufacturer or a repair shop.

Many newer laptops use a supercapacitor instead of a battery. This component charges whenever the laptop is plugged in and doesn't need replacement. If your laptop is less than five years old, check the manual to see whether it has a replaceable battery or a supercapacitor.

Frequently Asked Questions

Can a CMOS battery cause my computer not to start?

No. A dead CMOS battery will not prevent your computer from booting. You'll see error messages and your date will reset, but the machine will still power on and run normally. If your computer won't start at all, the problem is something else — likely the power supply, motherboard, or RAM.

How long does a CMOS battery last?

Most CMOS batteries last between three and ten years. Batteries in computers that run constantly tend to last longer than those in machines that are shut down frequently. The age of the battery when installed also matters — if your computer is ten years old, the original battery is probably near the end of its life even if it hasn't failed yet.

Is it safe to replace the CMOS battery myself?

Yes, it's safe on a desktop computer. The battery holds very little power and poses no shock risk. Always unplug your computer first and wait 30 seconds before opening the case. On a laptop, the process is riskier because you may damage other components during disassembly — check your manual or consider having a repair shop do it.

What's the difference between CMOS and the main battery in a laptop?

The main battery in a laptop powers the entire machine when it's unplugged. The CMOS battery (if present) is a separate, tiny battery that only powers the clock and BIOS settings when the laptop is off and unplugged. Replacing the main battery is different from replacing the CMOS battery.

Will replacing the CMOS battery erase my files or passwords?

No. The CMOS battery only stores the system date, time, and BIOS settings. Your files, passwords, and operating system are stored on your hard drive or SSD and are not affected by the CMOS battery. After replacement, you may need to reset your BIOS settings if you had customized them.