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. Its only job is to power a tiny chip called the CMOS (Complementary Metal-Oxide-Semiconductor) that stores your computer's basic settings—things like the date, time, and boot order—even after you shut down and unplug the machine.

Without it, your computer would forget these settings every time you turned it off. You would see a message asking you to set the date and time again, or the machine might not start up the way you configured it. The CMOS battery is not powering your screen or your processor; it is only keeping that one small chip alive during downtime.

Most CMOS batteries last three to ten years, depending on how often your computer runs and how much the battery drains. When one dies, the fix is straightforward: remove the old battery and snap in a new one—a task most people can do in under five minutes with a screwdriver.

Key Takeaways

  • A CMOS battery powers only the settings chip inside your computer, not the whole machine, and keeps your date, time, and boot settings intact when the power is off.
  • Most CMOS batteries are coin-shaped (typically CR2032 size) and cost between $3 and $10 to replace.
  • A dead CMOS battery shows itself through a constantly wrong system clock, repeated requests to set the date and time, or boot errors mentioning CMOS.
  • Replacing a CMOS battery requires opening your computer case and unclipping or unsoldering the old battery, then inserting a new one in the same position.
  • If your computer is still under warranty or you are uncomfortable opening the case, a repair shop can replace it for $30 to $75 in labor.

How a CMOS battery works inside your computer

Your computer's motherboard contains a small chip that stores settings in what is called CMOS memory. This memory is volatile—it needs constant power to hold onto the data. When you shut down your computer, the main power supply cuts off, but the CMOS chip still needs to remember your settings. That is where the CMOS battery comes in: it provides a trickle of power to keep that chip awake.

The settings stored on CMOS include your system date and time, the order in which the computer checks drives to boot from (called the boot sequence), and hardware settings like whether certain ports are turned on or off. Some CMOS chips also store your BIOS password if you have set one. None of these settings require much power—just enough to maintain the electrical charge in the memory cells—which is why a tiny battery can do the job for years.

When you turn the computer back on, the CMOS chip hands off its stored settings to the BIOS (Basic Input/Output System), which uses them to start up the machine correctly. If the CMOS battery is dead, the chip loses power and forgets everything, so the BIOS has to use default settings instead.

Signs that your CMOS battery is dying or dead

The most obvious sign is a system clock that is always wrong. If you shut down your computer at 5 p.m., turn it back on the next morning, and the clock shows 1:00 a.m. on January 1, 2000 (or some other default date), your CMOS battery is dead or nearly dead. This happens because the chip lost power and reset to factory defaults.

You might also see error messages during startup that mention "CMOS checksum error" or "CMOS battery low." Some computers will ask you to enter the date and time every single time you boot, even though you set it correctly the last time. In laptops, a dying CMOS battery can sometimes cause the machine to forget which drive to boot from, leading to startup failures.

Another clue is if your BIOS settings keep reverting to defaults. You might have disabled a USB port or changed the boot order, but after a restart, those changes are gone. This usually means the CMOS chip could not hold onto the new settings because the battery was too weak.

What type of CMOS battery your computer uses

The vast majority of desktop computers use a CR2032 coin-cell battery—a flat, round battery about the size of a dime. You can find them at any pharmacy, grocery store, or hardware store for $3 to $10. Some older computers or specialized equipment might use a CR2025 (slightly thinner) or a CR2016, but CR2032 is by far the most common.

Laptops sometimes use a different design: instead of a removable coin cell, they may have a rechargeable CMOS battery soldered to the motherboard. When this type fails, you cannot straightforward swap in a new battery—the motherboard itself usually needs to be replaced, or a technician has to solder a new battery in place. Check your laptop's manual or contact the manufacturer to find out which type yours has.

When you buy a replacement, make sure it is the right chemistry. CR2032 batteries are lithium coin cells and are the standard. Do not confuse them with alkaline coin batteries (which have different part numbers) or button batteries meant for hearing aids.

How to replace a CMOS battery in a desktop computer

Before you start, shut down your computer completely and unplug the power cable from the wall. Wait a few seconds, then press the power button once to discharge any remaining electricity. This is important for your safety and to avoid damaging the motherboard.

Open your computer case. Most desktop cases have a side panel held on by two or three thumbscrews or a latch. Remove the panel and set it aside. Locate the CMOS battery on the motherboard—it is a shiny, coin-shaped object, usually silver or gold, about the size of a dime. It is often near the edge of the motherboard or close to the CPU.

The battery is held in place by a small metal clip or a plastic holder. Gently push the clip to the side or lift the holder, and the battery will pop out. Note which side is facing up (usually the positive side is marked with a + symbol). Insert the new battery the same way, with the positive side up, and let the clip snap back into place.

Close your case, plug in the power cable, and turn on the computer. You should see the BIOS boot screen. If the date and time are correct, the battery is working. If they are wrong, enter the correct date and time, save your settings, and restart. The CMOS battery should now hold those settings even after shutdown.

When to call a professional instead of replacing it yourself

If your computer is still under warranty, opening the case might void it. Check your warranty paperwork or contact the manufacturer before you proceed. Many manufacturers will replace a CMOS battery for free or a small fee if the machine is still covered.

If you are uncomfortable opening your computer or worried about static electricity damaging components, a local repair shop can do the job for you. Labor costs usually run $30 to $75, plus the cost of the battery itself. The whole job takes less than 15 minutes, so it is one of the cheaper repairs a shop can do.

For laptops with soldered CMOS batteries, you almost always need a professional. The battery is fused to the motherboard, and removing it requires a soldering iron and skill. A repair shop or the laptop manufacturer can handle this, though the cost may be higher because it involves more labor.

How long a CMOS battery lasts and when to replace it

A typical CMOS battery lasts between three and ten years. The exact lifespan depends on how often your computer is powered on and how much the CMOS chip is drawing power. A computer that runs 24/7 will drain the battery faster than one that is shut down every night. Older computers with less efficient power management also tend to drain CMOS batteries more quickly.

You do not need to replace a CMOS battery on a schedule. Replace it only when you see the signs—a wrong system clock, CMOS error messages, or settings that keep reverting to defaults. If your computer is working fine and keeping the correct time, the battery is still good.

If you replace the battery and the same problems come back within a few weeks, the issue might not be the battery itself. A failing motherboard, a loose power connection, or a BIOS setting that is not saving properly could be the real cause. In that case, take the computer to a repair shop for diagnosis.

Frequently Asked Questions

Can I use my computer without a CMOS battery?

Yes, but with limitations. The computer will boot and run, but it will forget the date, time, and any custom BIOS settings every time you shut it down. You will see error messages at startup and may have to reset the time repeatedly. It is not dangerous—just inconvenient.

Is a CMOS battery the same as the battery in my laptop?

No. Your laptop's main battery powers the entire machine. The CMOS battery is a separate, tiny coin cell inside the motherboard that only powers the settings chip. Laptops have both, though some newer models have rechargeable CMOS batteries instead of replaceable ones.

What happens if I install the CMOS battery backwards?

Nothing will break, but it will not work. The battery will not make electrical contact with the holder, so the CMOS chip will lose power and forget your settings. If your clock is still wrong after replacing the battery, check that the positive side (marked with a +) is facing up.

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

Not usually. Once you install the new battery, your BIOS settings should be restored from the chip's memory. If they are not, you may need to enter them again manually or look for a "Load Defaults" option in the BIOS menu. Your motherboard manual will show you where this option is.

Can a CMOS battery cause my computer not to start at all?

Rarely. A dead CMOS battery will cause error messages and a wrong clock, but it usually will not prevent the computer from booting entirely. If your computer will not start, the problem is probably something else—a failed power supply, a bad hard drive, or a motherboard issue. A CMOS battery problem shows itself through clock and settings errors, not a complete failure to boot.