A CMOS battery keeps your computer's 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 a chip called the CMOS (Complementary Metal-Oxide-Semiconductor). This chip stores your computer's basic settings: the system date and time, boot order, hard drive detection, and hardware configuration. When you shut down your computer, the CMOS battery takes over and keeps those settings in memory. Without it, your computer would forget everything the moment you unplugged it.

You will never interact with the CMOS battery during normal computer use. It sits on the motherboard—the main circuit board inside your machine—and typically lasts three to ten years depending on the model and how often your computer runs. Most people only learn about it when their computer starts showing signs the battery is dying: the clock resets to 2000 or 2005 every time you restart, or the computer forgets which drive to boot from.

Key Takeaways

  • The CMOS battery powers a memory chip that stores your computer's date, time, and hardware settings when the machine is off.
  • A failing CMOS battery shows up as a clock that resets to an old date after every restart, or boot errors about missing drives.
  • Replacing a CMOS battery takes 10 to 20 minutes on a desktop and involves removing the side panel and locating a coin-shaped battery on the motherboard.
  • CMOS batteries cost between $5 and $15 and are sold at electronics retailers under the name CR2032 or similar model numbers.
  • Laptop CMOS batteries are soldered to the motherboard in most modern models, so replacement requires professional service or motherboard replacement.

How the CMOS battery powers your computer's memory

Your computer has two types of memory. RAM (random-access memory) holds everything your programs are doing right now, but it needs constant power—the moment you turn off the computer, RAM empties completely. The CMOS chip is different: it uses almost no power and holds only a small amount of data, so a tiny battery can keep it running for years.

When you change a setting in your BIOS or UEFI (the firmware menu you see before Windows or Mac loads), you are writing to the CMOS chip. The CMOS battery ensures that setting stays there even after a power loss. Without the battery, every restart would wipe those settings and force your computer to guess at its own configuration—which usually fails.

Signs your CMOS battery is failing

The most obvious sign is a clock that resets. If you shut down your computer and come back the next day to find the date and time have jumped back to 2005 or 2000, your CMOS battery is nearly dead. This happens because the battery no longer has enough charge to keep the clock running overnight.

Other warning signs include boot errors mentioning "CMOS checksum error" or "CMOS battery low," the computer forgetting which hard drive to boot from, or losing custom BIOS settings you know you saved. Some computers will also beep repeatedly on startup or refuse to recognize newly installed hardware. On laptops, you might see a message asking you to set the date and time every time you power on.

These problems do not mean your computer is broken—they mean the CMOS battery needs replacement. The computer itself will work fine once you swap it out.

How to replace a CMOS battery in a desktop computer

Desktop CMOS batteries are user-replaceable and the job takes about 10 to 20 minutes. First, shut down your computer completely and unplug the power cable. Press the power button once to discharge any remaining electricity. Open the side panel of your case—usually held by two thumbscrews or a latch on the back.

Locate the CMOS battery on the motherboard. It looks like a large coin or watch battery, typically silver-colored, and is usually near the edge of the board or close to the CPU socket. Note which way it sits in the holder—the positive side (marked with a + symbol) should face up. Gently press down on the clip holding the battery until it springs out, or slide the battery out of its holder depending on the design.

Take the old battery to an electronics store or pharmacy to find the replacement. Most CMOS batteries are CR2032 cells, but your motherboard manual or a quick search for your motherboard model will confirm the exact type. Insert the new battery with the positive side facing up, press it down until the clip holds it firmly, and close your case. Plug in the power cable and restart your computer.

Your computer may ask you to set the date and time on the first boot. You may also need to re-enter any custom BIOS settings you had before, though many computers restore these automatically.

Why CMOS batteries in laptops are harder to replace

Laptop CMOS batteries are usually soldered directly to the motherboard rather than sitting in a removable holder. This design saves space but means you cannot swap the battery yourself without specialized equipment. If your laptop's CMOS battery fails, you have two options: take it to a repair shop where a technician can desolder and replace the battery (usually $50 to $150 in labor), or replace the entire motherboard if the repair cost exceeds the laptop's value.

Some newer laptops have moved away from CMOS batteries altogether, using a small capacitor or a rechargeable battery that charges whenever the laptop is plugged in. Check your laptop's manual or contact the manufacturer to find out which system yours uses.

Where to buy a replacement CMOS battery

CMOS batteries are inexpensive and widely available. A CR2032 battery costs $5 to $15 depending on the brand and where you shop. You can find them at electronics retailers like Best Buy, online at Amazon, or at pharmacies and grocery stores in the watch battery section. Some computer repair shops will sell you one and install it for a small fee if you do not want to open your computer yourself.

When you buy a replacement, check the part number on your old battery or look up your motherboard model to confirm you are getting the right type. Most CMOS batteries are CR2032, but some older or specialized boards use CR2025 or other variants. The wrong size will not fit or will not hold a charge properly.

Frequently Asked Questions

Can I use my computer while the CMOS battery is dead?

Yes, your computer will work fine. You will just see error messages on startup and the clock will reset every time you restart. The CMOS battery only powers the settings chip, not the computer itself. Replace it when you have time—there is no rush.

Will replacing the CMOS battery erase my files?

No. Your files live on your hard drive or SSD, which has its own power supply. The CMOS battery only stores hardware settings and the system clock. Replacing it will not touch your documents, photos, or programs.

How long does a CMOS battery last?

Most CMOS batteries last three to ten years. Older computers or those that sit unplugged for long periods may need replacement sooner. There is no way to predict exactly when yours will fail, but you will see clear warning signs like a reset clock before it stops working entirely.

What happens if I install the CMOS battery backwards?

The battery will not make contact with the holder and your computer will behave as if the battery is missing—the clock will reset and settings will not save. straightforward remove it, flip it over so the positive side faces up, and reinsert it. No damage occurs from installing it backwards.

Do I need to reset anything in BIOS after replacing the CMOS battery?

Your computer may ask you to set the date and time on the first boot. Some BIOS settings may reset to factory defaults, so if you had custom settings (like boot order or overclocking), you may need to re-enter them. Check your BIOS menu after the replacement to see what needs adjustment.