CMOS batteries typically last three to ten years, depending on how often your computer runs and the ambient temperature where it sits

A CMOS battery is a small coin-cell battery (usually CR2032) that keeps your motherboard's real-time clock and BIOS settings alive when the computer is powered off. It does not power your computer during use—the main power supply does that. The CMOS battery only maintains a tiny amount of data in the background, so it drains very slowly.

Most CMOS batteries last between three and ten years. A computer that runs eight hours a day in a cool room may get ten years from one battery. A computer that runs 24/7 in a warm server room may need a replacement in three to four years. Room temperature matters more than you might expect: heat accelerates chemical breakdown inside the battery, so a machine in a 75°F office will outlast one in an 85°F closet.

Key Takeaways

  • CMOS batteries drain slowly because they only power the clock and BIOS memory when the computer is off, not during normal operation.
  • A battery typically lasts three to ten years, with longer life in cooler rooms and computers that spend time powered down.
  • The most common sign of a failing CMOS battery is the system date and time resetting to a default value every time you restart.
  • Replacing a CMOS battery takes five to fifteen minutes and costs between five and twenty dollars for the battery itself.
  • A dead CMOS battery does not damage your computer or data—it only prevents the clock from keeping time when powered off.

Signs Your CMOS Battery Is Dying

The clearest sign is that your system date and time reset to a default value—often January 1, 2000, or the date the motherboard was manufactured—every time you restart the computer. You set the correct date and time, shut down, power back on, and it has reverted. This happens because the CMOS battery can no longer hold the clock data.

You may also see error messages during startup that mention "CMOS checksum error" or "CMOS battery low." Some computers display "System date and time not set" or ask you to press a key to continue. These messages mean the motherboard detected that the backup power is failing.

In older machines, a dying CMOS battery can cause the BIOS settings to reset to factory defaults. This might mean your hard drive is no longer recognized, your boot order changes, or your CPU speed drops. Newer computers are less affected by this because they store more settings in non-volatile memory that does not depend on the CMOS battery.

How to Check Your CMOS Battery

The easiest check is to observe whether your system time drifts. Set the correct date and time, shut down the computer completely (not sleep mode), wait a few hours or overnight, and power it back on. If the time has reset or drifted significantly, the battery is likely weak or dead.

If you want to inspect the battery itself, you will need to open the computer case and locate the motherboard. The CMOS battery is a round, silver coin-cell battery about the size of a dime, usually held in a clip or socket on the motherboard. Look for a label that says "CR2032" or similar. You do not need to remove it to check it—just note the model number so you can buy a replacement if needed.

Do not attempt to test the battery with a multimeter unless you are comfortable with electronics. A voltage reading below 2.8 volts suggests the battery is near the end of its life, but the test itself is not necessary for a home user. If your clock keeps resetting, the battery needs replacement regardless of what a meter says.

Factors That Shorten Battery Life

Heat is the primary culprit. CMOS batteries are chemical devices, and heat accelerates the chemical reactions that deplete them. A computer in a warm room, a laptop that runs hot, or a machine in direct sunlight will drain its CMOS battery faster than one in a cool, climate-controlled space. If your computer runs in an environment above 80°F regularly, expect the battery to last toward the shorter end of the range.

Continuous operation also matters. A computer that runs 24/7 drains the CMOS battery faster than one that is powered down most of the time. The battery is always supplying current to the real-time clock circuit, even if the amount is tiny. Over months and years, that constant draw adds up.

The age of the computer itself can play a role. Older motherboards sometimes have less efficient power management in the CMOS circuit, which means the battery drains faster. Very old machines from the 1990s and early 2000s may see CMOS battery failure in five years or less, while modern computers often stretch to eight or ten years.

How to Replace a CMOS Battery

Replacing a CMOS battery is straightforward and takes five to fifteen minutes. First, power off the computer completely and unplug it from the wall outlet. Wait a few seconds to may support all power is discharged. If your computer has a removable power cable, remove it. If it has a built-in power supply, straightforward unplugging is enough.

Open the computer case according to your machine's manual. Most desktop cases have a side panel held by two or three screws or thumb screws. Locate the motherboard and find the CMOS battery—it is a round, silver coin-cell, usually held in a small clip or socket. Gently push the clip to the side or lift the battery out of its socket. Note which side faces up (the positive side usually faces toward you).

Take the old battery to a store or check the label to confirm the model number (usually CR2032). Buy an identical replacement—they cost between five and fifteen dollars. Insert the new battery into the socket with the positive side facing the same direction as the old one was. You should hear or feel a small click as it seats. Close the case, plug in the power cable, and power on the computer.

When you restart, the BIOS may ask you to confirm the date and time or press a key to continue. Set the correct date and time, save the settings, and restart again. The system should now keep the correct time even after being powered off.

What Happens If You Do Not Replace It

A dead CMOS battery does not damage your computer or erase your data. Your files, programs, and operating system remain intact. The only consequence is that the system clock resets every time you power off, which can cause minor inconveniences.

If you do not notice or care about the time being wrong, you can leave the battery dead indefinitely. However, some programs rely on accurate system time—email clients may not sync properly, security certificates may appear invalid, and scheduled tasks may not run at the right moment. For most home users, a wrong system clock is annoying but not critical.

In a business or server environment, a dead CMOS battery is more serious because accurate timekeeping affects logging, backups, and security protocols. In those settings, replacement should happen as soon as the battery fails.

Frequently Asked Questions

Can I use a different battery model if I cannot find the exact one?

No. CMOS batteries come in specific sizes and voltages. CR2032 is the most common, but some older or specialized motherboards use CR2025, CR2016, or other models. Using the wrong size may not fit the socket, and using the wrong voltage can damage the motherboard. Always match the model number exactly.

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

Usually not. When you replace the battery, the BIOS settings may revert to factory defaults, but you can straightforward set them again—date, time, and any custom settings you had. If you had overclocking settings or custom boot options, you will need to re-enter those. Most users only need to set the date and time.

How do I know if my computer has a CMOS battery or uses something else?

Nearly all desktop and laptop computers built in the last thirty years use a CMOS battery. If your computer is from the 1990s or later, it almost certainly has one. The only exceptions are very new machines with built-in supercapacitors instead of batteries, but these are rare in consumer computers. If you open the case and do not see a coin-cell battery, check your motherboard manual.

Is it safe to replace the CMOS battery myself?

Yes, it is very safe. The CMOS battery supplies only a few milliamps of current—far too little to cause injury. The main precaution is to power off and unplug the computer first, and to avoid touching other components unnecessarily. If you are uncomfortable opening your computer case, a repair shop can do it for ten to thirty dollars in labor.

Will replacing the CMOS battery fix other computer problems?

Only if those problems are caused by an incorrect system clock or corrupted BIOS settings. A dead CMOS battery can cause boot failures, unrecognized hard drives, or error messages during startup. If your computer has other issues—slow performance, crashes, or software errors—a new CMOS battery will not fix them.