A CMOS battery keeps your computer's internal clock and settings running 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 completely, the CMOS battery takes over so that information does not disappear. Without it, your computer would forget the time and revert to factory settings every time you powered it back on.

CMOS stands for Complementary Metal-Oxide-Semiconductor. The chip itself uses almost no power, which is why a single small battery can run it for three to ten years depending on the computer model and how often the machine sits powered off. Most CMOS batteries are CR2032 cells — the same type used in watches and car key fobs — and cost between $5 and $15 to replace.

Key Takeaways

  • A CMOS battery preserves your computer's date, time, and BIOS settings when the machine is completely powered down.
  • Most CMOS batteries last three to ten years and are inexpensive coin-cell batteries like the CR2032.
  • A dead CMOS battery causes your computer to lose the correct time, reset BIOS settings to defaults, and sometimes fail to recognize hard drives or other hardware.
  • Replacing a CMOS battery requires opening the computer case and removing a small battery from the motherboard — a task most people can do in under ten minutes with basic tools.

Signs your CMOS battery is failing or dead

The most obvious sign is that your computer's clock resets to an incorrect date and time every time you restart it. You may also see error messages during startup mentioning CMOS, checksum errors, or BIOS settings. Some computers display a message like "CMOS battery low" or "System date and time not set."

A failing CMOS battery can also cause your computer to forget which hard drives or storage devices are installed, or to lose custom BIOS settings you configured for performance or boot order. In rare cases, the computer may refuse to start at all, though this is more common in older machines. If you see these signs, the battery is likely near the end of its life or already dead.

How to locate and replace a CMOS battery

Replacing a CMOS battery requires opening your computer case. On a desktop, this usually means removing the side panel — typically held by two or three screws or thumb screws. On a laptop, you may need to remove the bottom panel or keyboard, which varies widely by model. Check your computer's manual or search for your specific model number plus "CMOS battery replacement" to see the exact steps.

Once inside, locate the motherboard — the large circuit board where all components connect. The CMOS battery is a round, flat coin-cell battery about the size of a dime, usually sitting in a small plastic holder. Press the clip or lever holding it in place, and the battery will pop out. Note which side faces up (usually marked with a + or −), then insert the new battery the same way. The whole process takes five to ten minutes.

You do not need special tools — a plastic spoon or coin can release the battery clip if your fingers do not fit. Wear an anti-static wrist strap if you have one, though most people replace the battery without incident. After reinstalling the battery, close the case, power on the computer, and check that the date and time are correct in your system settings.

Where to buy a replacement CMOS battery

CR2032 batteries are sold at most electronics retailers, pharmacies, and supermarkets. You can find them at Best Buy, Micro Center, Walmart, Target, and Amazon, usually in the watch battery or electronics section. Prices range from $5 to $15 for a single battery, though multi-packs cost less per unit if you want spares.

Some computer repair shops will replace the battery for you if you do not want to open the case yourself. Labor typically costs $20 to $50 depending on the shop and how difficult your specific computer is to open. For most people, buying the battery and doing it yourself is faster and cheaper.

CMOS battery lifespan and when replacement becomes necessary

A CMOS battery typically lasts between three and ten years, depending on how often your computer is powered off and how much the CMOS chip is being used. Computers that run continuously or are rarely shut down put less demand on the battery. Older computers and laptops that spend long periods in storage or powered down will drain the battery faster.

You do not need to replace the battery on a schedule — only when you see the signs of failure listed above. Some people replace it during routine maintenance or when opening the case for other reasons, which is reasonable since the battery is inexpensive. If your computer is more than seven or eight years old and you have never replaced the CMOS battery, it is probably worth doing preemptively to avoid unexpected clock resets or BIOS errors.

What happens if you ignore a dead CMOS battery

A dead CMOS battery is not an emergency. Your computer will continue to work, but it will lose the correct date and time every time you restart it. This can cause problems with file timestamps, scheduled tasks, and security certificates that depend on the system clock being accurate. Some software may refuse to run if the date is too far off.

More importantly, your BIOS settings will reset to factory defaults, which can affect boot order, hardware detection, and performance settings you configured. If you have custom overclocking settings or specific boot preferences, those will be lost. The computer will still start and run, but you will need to reconfigure those settings each time you restart until you replace the battery.

CMOS battery versus the main system battery

Do not confuse the CMOS battery with your computer's main power supply or, in a laptop, the rechargeable battery that powers the entire machine. The CMOS battery is separate and much smaller — it only powers the CMOS chip, not the processor, memory, or display. A dead CMOS battery will not prevent your computer from starting or running; it will only cause the clock and settings to reset.

If your laptop will not hold a charge or your desktop loses power when ready when unplugged, that is a problem with the main battery or power supply, not the CMOS battery. Those are different issues requiring different solutions.

Frequently Asked Questions

Can I use a different battery instead of a CR2032?

Not safely. The CR2032 is the standard CMOS battery, and using a different size or type can damage the motherboard or fail to fit the holder. Always replace it with another CR2032 or the exact battery type specified in your computer's manual.

Do I need to turn off the computer before replacing the CMOS battery?

Yes. Shut down the computer completely, unplug the power cable, and wait a few seconds before opening the case. This prevents electrical damage to the motherboard and ensures your safety while working inside the machine.

Will replacing the CMOS battery erase my files or programs?

No. The CMOS battery only stores the date, time, and BIOS settings — not your files, programs, or operating system. Everything on your hard drive remains untouched. Your BIOS settings will reset to defaults, but your data is safe.

What if my computer does not have a visible CMOS battery?

Some newer computers use a rechargeable CMOS capacitor instead of a replaceable battery. If you cannot find a coin-cell battery on the motherboard, check your computer's manual or contact the manufacturer. You may need professional service to replace a soldered capacitor.

How do I know which side of the battery goes up?

The positive side (marked with a +) should face up toward you when you look at the battery in its holder. The old battery will show you the correct orientation — match it when you insert the new one. If you are unsure, check your motherboard manual or search for your specific model online.