A CMOS battery typically lasts three to ten years

The CMOS battery is a small coin-cell battery inside your computer that keeps the system clock and BIOS settings alive when the machine is powered off. Most CMOS batteries last between three and ten years, though the actual lifespan depends on how often you use your computer, the ambient temperature where it sits, and the specific battery model your motherboard uses.

When a CMOS battery dies, your computer will still turn on and run normally — but it will lose the date and time, reset BIOS settings to factory defaults, and sometimes forget boot order or hardware configurations. You will notice the clock is wrong the moment you power up. The battery does not affect performance while your computer is running; it only matters when the machine is off.

Replacing a CMOS battery is straightforward and costs between five and twenty dollars for the battery itself, plus a few minutes of your time. You do not need to reinstall Windows or lose your files.

Key Takeaways

  • A CMOS battery typically lasts three to ten years depending on usage patterns and motherboard design.
  • When the battery dies, your computer will show the wrong date and time and reset BIOS settings to defaults, but will otherwise run normally.
  • Replacement batteries cost five to twenty dollars and take less than five minutes to swap out.
  • Frequent power cycling and high ambient temperatures can shorten battery life.
  • You can test whether your CMOS battery is failing by checking if the system clock resets every time you shut down.

What shortens CMOS battery life

CMOS batteries drain faster in certain conditions. Computers that sit in warm environments — above 80 degrees Fahrenheit — experience accelerated battery drain. If your desktop or laptop is near a heater, in direct sunlight, or in an unventilated cabinet, the battery will wear out sooner than the three-to-ten-year average.

Frequent power cycling also matters. A computer that powers on and off multiple times per day draws more from the CMOS battery than one that stays on most of the time or remains off for long stretches. Laptops that are regularly shut down and restarted tend to drain the battery faster than always-on servers or desktops that run continuously.

Some older motherboards and budget models use lower-quality CMOS batteries that fail closer to the three-year mark. High-end or newer boards often include better batteries that reach eight to ten years. The motherboard manual or manufacturer specifications will sometimes list the expected lifespan, though most do not.

Signs your CMOS battery is failing

The clearest sign is that your system clock resets to a default date every time you shut down the computer. When you power it back on, the time will be wrong — often showing a date like January 1, 2000, or the date the motherboard was manufactured. If you manually set the clock and it holds while the computer is on, but resets when you turn it off, the CMOS battery is almost certainly dead or dying.

A second warning sign is that BIOS settings revert to defaults after shutdown. If you have customized boot order, disabled certain hardware, or changed power settings in BIOS, and those changes disappear the next time you start up, the battery is losing its charge. Some computers will also display a message during startup like "CMOS checksum error" or "CMOS battery low," though not all motherboards show this warning.

A failing battery does not cause crashes, freezes, or performance problems while the computer is running. If your machine is slow or unstable, the CMOS battery is not the cause. The battery only affects what happens when the power is off.

How to replace a CMOS battery

Before you start, power off the computer completely and unplug it from the wall. Wait at least thirty seconds. This ensures no residual power remains in the system. If you are working on a laptop, also remove the battery if it is user-removable.

Open the computer case or access panel. On desktops, this usually means removing the side panel. On laptops, you may need to remove the bottom panel or keyboard, depending on the model. Locate the CMOS battery on the motherboard — it is a round, silver coin-cell battery about the size of a dime, typically labeled CR2032 or similar. Take a photo of its position before you remove it.

Gently pry the battery out of its holder using a plastic spoon, plastic lever, or your fingernail. Do not use metal tools, as they can short the motherboard. Note which side faces up — the positive side usually faces up. Insert the new battery in the same orientation, pressing down until it clicks into place. Close the case, plug in the computer, and power it on.

The BIOS will likely show a checksum error on first boot. Enter BIOS setup (usually by pressing Delete, F2, or F12 during startup — check your motherboard manual) and let it auto-detect hardware, or manually restore your custom settings. Set the date and time, save, and exit. Your computer should now retain these settings when powered off.

CMOS battery types and where to buy them

The most common CMOS battery is the CR2032, a 3-volt lithium coin cell. Some older computers use CR2025 or CR2016 batteries, which are slightly thinner. A few specialized boards use AA or AAA batteries, though this is rare in modern machines. Check your motherboard manual or look at the battery itself to see the model number before you buy a replacement.

You can purchase CMOS batteries at electronics retailers like Micro Center or Best Buy, at pharmacies and supermarkets in the watch battery section, or online from Amazon or eBay. A single CR2032 battery costs between two and five dollars. Some retailers sell multipacks of five or ten for eight to fifteen dollars, which is useful if you maintain multiple computers. Do not buy the cheapest option — a reputable brand like Panasonic, Energizer, or Duracell will last longer than an off-brand battery.

Preventing premature CMOS battery failure

Keep your computer in a cool environment. If you have a choice, place your desktop in a room that stays below 75 degrees Fahrenheit. may support the case has adequate airflow and is not blocked by dust or objects. A cooler motherboard means a longer-lasting CMOS battery.

If you own a laptop or desktop that you use infrequently, consider leaving it plugged in rather than powering it off completely. A computer that stays powered on or in sleep mode does not drain the CMOS battery as quickly as one that is shut down and restarted regularly. This is a minor factor compared to temperature, but it does add up over years.

There is no way to extend a CMOS battery's life once it begins to fail. Replacement is the only solution. However, knowing the signs means you can replace it before it causes problems like missed scheduled tasks or lost BIOS configurations.

Frequently Asked Questions

Can a computer work without a CMOS battery?

Yes, the computer will run normally while powered on. However, it will lose the date, time, and BIOS settings every time you shut it down. Some programs that rely on accurate system time may behave oddly, and you may see error messages during startup. Replacing the battery is inexpensive and takes minutes, so it is worth doing rather than living without one.

Will replacing the CMOS battery erase my files or Windows?

No. The CMOS battery stores only the system clock and BIOS settings, not your operating system or personal files. Replacing it will not affect Windows, your programs, or your data. You may need to reset a few BIOS options after replacement, but nothing on your hard drive changes.

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

No. The main battery in a laptop powers the entire computer while it is unplugged. The CMOS battery is a separate, tiny coin cell inside the motherboard that only keeps the clock and BIOS settings alive when the main battery is removed or the laptop is off. Replacing one does not affect the other.

What happens if I install the CMOS battery backwards?

Installing it backwards will not damage the motherboard, but it will not work. The battery straightforward will not make contact with the holder, and the computer will behave as if no battery is present — losing time and settings on shutdown. Remove it, flip it over so the positive side faces up, and try again.

Do all computers have a CMOS battery?

Most desktop and laptop computers built in the last twenty years have one. Very old computers may use a different method to store BIOS settings. Some specialized devices like tablets or phones do not have a user-replaceable CMOS battery. If you are unsure whether your machine has one, check the motherboard manual or open the case and look for a round coin-cell battery.