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 the CMOS chip — a tiny piece of memory that stores your computer's date, time, and hardware settings. When you shut down your machine, the CMOS battery takes over so that information does not disappear. Without it, your computer would forget the time every time you turned it off, and you would lose custom settings you made in the BIOS or UEFI.

Most computers use a CR2032 battery, which looks like a watch battery and costs between $3 and $8. It sits on the motherboard and lasts three to ten years depending on how often your computer runs and how old the battery gets. You will know it is failing when your computer keeps resetting the date and time to a default year, usually 2000 or 2005.

Key Takeaways

  • The CMOS battery powers the chip that stores your computer's clock, date, and BIOS settings when the machine is off.
  • A dead CMOS battery causes your computer to lose the correct date and time every time you restart, and may reset hardware settings to factory defaults.
  • Replacing a CMOS battery takes five to fifteen minutes and costs under $10, though the exact type varies by motherboard model.
  • You do not need to replace a CMOS battery unless it fails — most last between three and ten years.

How the CMOS battery powers your computer's memory

Your computer has two kinds of memory. RAM holds everything your operating system and programs are doing right now, but it needs constant power — the moment you turn off the computer, RAM empties completely. CMOS is different. It is a small chip that stores only a few pieces of information: the current date and time, your hard drive configuration, boot order, and any custom settings you made in BIOS or UEFI.

When your computer is running, the main power supply keeps CMOS updated. When you shut down, the CMOS battery takes over and supplies just enough power to keep that chip alive. It draws almost no current, which is why a single battery can last years. The moment you unplug the computer or the power fails, the battery is what stands between your settings and the void.

Signs your CMOS battery is dying or dead

The most obvious sign is that your computer keeps resetting the date and time. You set it to the correct date, shut down the machine, turn it back on, and it has reverted to an old date — often January 1, 2000, or whatever the motherboard's factory default is. This happens because the CMOS chip cannot hold the new time without power from the battery.

A failing CMOS battery can also cause your computer to forget custom BIOS or UEFI settings. You may notice that boot order changes, hard drive detection fails, or overclocking settings reset to default. Some computers will not start at all if the CMOS battery is completely dead, though this is less common. A few machines display an error message on startup — something like "CMOS checksum error" or "CMOS battery low" — which is a direct signal that replacement is needed.

How to replace a CMOS battery

Replacing a CMOS battery is straightforward if you are comfortable opening your computer case. First, shut down the machine completely and unplug it from the wall. Wait a few seconds, then open the case. Locate the motherboard — it is the large circuit board that everything plugs into. The CMOS battery is a round, silver coin-shaped object, usually about the size of a dime, sitting in a plastic holder on the board.

Press down gently on the clip or holder to release the battery, then slide it out. Note which way it was facing — the positive side usually faces up. Take the old battery to a computer shop or electronics store to confirm the model number (usually printed on the battery itself), or look up your motherboard model online to find the correct replacement. Insert the new battery the same way the old one was oriented, press it down until it clicks into place, close the case, plug in the power cable, and restart your computer. Set the date and time in your operating system, and you are done.

The whole process takes five to fifteen minutes. If you are not comfortable opening your computer, a repair shop can do it for $20 to $50 in labor, though the battery itself costs under $10.

CMOS batteries in laptops versus desktops

Desktop computers have an straightforward-to-access CMOS battery on the motherboard, usually held in place by a straightforward clip. Laptops vary widely. Some have a replaceable coin battery similar to a desktop, while others have the CMOS battery soldered directly to the motherboard, making replacement much harder or impossible without professional help. A few newer laptops use a rechargeable CMOS battery that charges whenever the main battery is plugged in, so it never truly dies.

If you own a laptop and suspect a CMOS battery problem, check your user manual or search for your specific model online before attempting to open it. Some laptops require removing the keyboard or other components just to reach the motherboard. If the battery is soldered in place, a repair shop is your only option, and the cost may be higher because of the labor involved.

How long a CMOS battery lasts

A typical CMOS battery lasts between three and ten years. The exact lifespan depends on how often your computer runs. A machine that stays on most of the time puts less load on the CMOS battery because the main power supply is handling most of the work. A computer that is shut down and restarted frequently, or one that sits unplugged for long periods, drains the CMOS battery faster.

Age also matters. Even if a CMOS battery is not actively powering anything, it loses charge over time. A battery that is five years old will not last as long as a brand-new one, even if the computer has been off the whole time. Once you replace it, mark the date somewhere so you have a rough idea when the next replacement might be needed.

What happens if you ignore a dead CMOS battery

A dead CMOS battery is annoying but not dangerous. Your computer will still start and run normally. The only real problems are that you will have to reset the date and time every time you restart, and you may lose any custom BIOS or UEFI settings you configured. Some programs that rely on accurate system time — like security software, backup tools, or online banking — may behave oddly or refuse to work if the date is wrong.

If your computer is part of a network or domain, a wrong system clock can prevent you from logging in or accessing shared resources. For most home users, the main inconvenience is resetting the time repeatedly. There is no harm in waiting to replace the battery, but doing so takes only a few minutes and costs almost nothing, so most people replace it as soon as they notice the problem.

Frequently Asked Questions

Can I use my computer without a CMOS battery?

Yes, your computer will run fine without one. The only issue is that it will forget the date, time, and any custom BIOS settings every time you shut it down. Some machines may not start at all if the CMOS battery is completely missing, but most will boot normally and straightforward reset to factory defaults.

Why does my computer keep saying the date is 2000?

That is the factory default date on most motherboards. When the CMOS battery dies, the chip loses power and reverts to its default state, including the default date. This is a clear sign the battery needs replacement.

Is a CMOS battery the same as a laptop battery?

No. A laptop battery powers the entire machine when unplugged. A CMOS battery is a tiny coin cell that powers only the CMOS chip. They are completely different and serve different purposes.

Do I need to replace my CMOS battery if the date keeps resetting?

Yes, that is the main reason to replace it. A functioning CMOS battery will hold the date and time indefinitely. If your computer keeps losing the correct date, the battery is failing and should be replaced.

What is the difference between CMOS and BIOS?

BIOS is the software that runs when your computer starts up. CMOS is the chip that stores the settings BIOS uses. The CMOS battery keeps those settings alive when the power is off; the BIOS itself does not need a battery.