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 that powers the CMOS chip — a tiny piece of memory that stores your system's date, time, and hardware settings. When you shut down your computer, the CMOS battery takes over so those settings don't vanish. Without it, your machine would forget what time it is every time you powered it down, and you'd lose custom settings like boot order or fan speeds.

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 years. Most CMOS batteries are CR2032 cells — the same size as a watch battery — and they typically last between three and ten years depending on how often your computer runs and how old the battery was when it left the factory.

Key Takeaways

  • A CMOS battery stores your computer's date, time, and hardware settings when the machine is powered off.
  • The most common CMOS battery is a CR2032 coin cell, which costs between two and eight dollars to replace.
  • Signs your CMOS battery is failing include a clock that resets to 2000 or 2005, lost BIOS settings, or boot errors mentioning the date.
  • Replacing a CMOS battery takes ten to thirty minutes and requires only a screwdriver and steady hands; no technical knowledge is needed.

What happens when a CMOS battery dies

When the CMOS battery runs out, your computer will still turn on and run normally — but it will forget its settings the moment you power it down. The most obvious sign is a clock that resets to a fixed date, usually sometime between 2000 and 2005, depending on when the motherboard was made. You'll see this date wrong in Windows or macOS every time you boot up.

Beyond the clock, you may lose custom BIOS settings: boot order, overclocking profiles, fan curves, or security features you configured. Some machines will show error messages during startup mentioning CMOS checksum failure or asking you to press a key to continue. On older systems, a dead CMOS battery can prevent the machine from booting at all, though this is less common in modern computers.

How to tell if your CMOS battery needs replacing

The clearest sign is a system clock that's stuck in the past. If you shut down your computer, wait a day, and the clock shows a date from years ago when you power it back on, the CMOS battery is almost certainly dead. This happens even if your computer is connected to the internet and would normally sync the time automatically — the battery failure prevents the system from even starting the sync process.

A second indicator is repeated BIOS error messages or a request to reset BIOS settings every time you boot. If you've set a custom boot order or changed any hardware settings and they revert to defaults after shutdown, the CMOS battery is losing its charge. On some machines, you may also see a beeping sound during startup, which can signal a CMOS problem, though beeping codes vary by manufacturer.

How to replace a CMOS battery yourself

Replacing a CMOS battery is one of the simplest computer repairs. First, power off your machine completely and unplug it from the wall. Wait a few seconds, then open the case — on a desktop, this usually means removing a side panel held by two or four screws. On a laptop, you may need to remove the bottom panel, which typically has six to twelve screws.

Locate the CMOS battery on the motherboard. It's a round, flat, silver coin about the size of a dime, sitting in a small plastic holder. Gently pry it out — some holders have a small tab you can push to eject the battery, while others require light pressure with a plastic tool or your fingernail. Note which side faces up, then insert the new CR2032 battery the same way. Close the case, plug in the power, and boot up. Your BIOS will likely ask you to confirm the date and time on first startup.

The entire process takes ten to thirty minutes for most people. You need only a screwdriver (usually Phillips head) and steady hands. If you're uncomfortable opening your machine, a computer repair shop can do it for fifteen to fifty dollars, though the battery itself costs two to eight dollars.

Where to buy a replacement CMOS battery

CR2032 batteries are sold everywhere: drugstores, supermarkets, electronics retailers, and online. They're the same batteries used in watches, calculators, and car key fobs, so you won't have trouble finding one. A single battery costs between two and eight dollars depending on the brand and where you shop. Name brands like Energizer and Duracell are slightly more expensive but tend to last longer than generic versions.

If you're not certain which battery your machine uses, check your motherboard manual or search your computer model online — most systems use CR2032, but some older or specialized machines may use CR2025 or CR2016. The number refers to the diameter and thickness; putting in the wrong size won't damage anything, but it won't fit properly in the holder.

How long a new CMOS battery lasts

A fresh CMOS battery typically lasts between three and ten years. The exact lifespan depends on how often your computer runs, how much power the CMOS chip draws (which varies by motherboard age and design), and the battery's manufacturing date. Newer batteries tend to last longer than old stock that's been sitting on a shelf.

If you replace your CMOS battery and it dies again within a year or two, the problem may not be the battery itself. A failing motherboard can drain a CMOS battery faster than normal, or a short circuit on the board can prevent the battery from holding a charge. If this happens, the motherboard may need repair or replacement, which is more expensive than a battery swap.

CMOS battery versus the main system battery in laptops

Laptops have two separate batteries: the main rechargeable battery you charge daily, and a small CMOS battery on the motherboard. The main battery powers the computer when it's unplugged. The CMOS battery, usually a CR2032 or similar coin cell, keeps the clock and settings alive even when the main battery is completely drained and the laptop is off.

If your laptop's clock resets when powered off, the CMOS battery is the problem, not the main battery. Replacing the CMOS battery in a laptop is slightly more involved than in a desktop because you have to remove the bottom panel and navigate around other components, but the process is the same: locate the battery, pry it out gently, and insert a new one.

Frequently Asked Questions

Can I use any CR2032 battery, or does it have to be a specific brand?

Any CR2032 will work. Brand doesn't matter for function, though name brands like Energizer and Duracell tend to last slightly longer. Generic batteries are cheaper and work just as well if you're replacing a dead battery soon anyway.

What if I replace the CMOS battery and the clock still resets?

Make sure the new battery is inserted correctly — the positive side (marked with a +) should face up. If it's correct and the problem persists, the motherboard may have a hardware fault draining the battery faster than normal, or a short circuit preventing proper charging. A technician can diagnose this.

Is it safe to open my computer to replace the CMOS battery myself?

Yes, if you unplug the power cable first and wait a few seconds. Touch a metal part of the case to discharge any static electricity before handling components. The CMOS battery itself is safe to handle — it's not toxic and won't shock you.

Will my computer work without a CMOS battery?

Yes, it will run normally while powered on. However, it will forget the date, time, and any custom BIOS settings every time you shut it down. Some older systems may refuse to boot without a CMOS battery, but most modern computers will start up fine.

How do I know if my laptop has a replaceable CMOS battery?

Most laptops do, but some newer models have the CMOS battery soldered to the motherboard, making it non-replaceable without professional service. Check your laptop's manual or search the model number online. If it's replaceable, the manual will show where it's located.