The CMOS battery is a small coin-shaped cell inside your computer that keeps your system clock and BIOS settings running when the power is off

The CMOS battery sits on your motherboard — the main circuit board inside your desktop or laptop. On most desktop computers, it is a round, silver battery about the size of a watch battery, usually positioned near the edge of the motherboard where it is straightforward to access. On laptops, the CMOS battery is smaller and harder to reach, often soldered directly to the motherboard or tucked under layers of internal components.

When this battery dies, your computer will lose the time and date, forget your BIOS settings, and may refuse to start until you reset it. The battery typically lasts three to ten years depending on how often your computer is powered off and how much it sits unused.

Key Takeaways

  • The CMOS battery is a coin-shaped cell on your motherboard that keeps your system clock and BIOS settings alive when power is off.
  • On desktop computers, the CMOS battery is usually straightforward to spot and replace yourself; on laptops, you may need to disassemble the machine or take it to a technician.
  • A dead CMOS battery causes your computer to lose the date and time, forget BIOS changes, and sometimes fail to boot until reset.
  • Replacement batteries cost between $5 and $15 and are standard CR2032 or CR2025 cells available at any electronics or pharmacy store.

Finding the CMOS battery on a desktop computer

Open your desktop computer's case by removing the side panel. Look at the motherboard — the large green or brown board with circuits and components. The CMOS battery is a round, flat, silver disc, usually about the size of a dime or slightly larger. It sits in a small circular holder called a socket, most often near the edge of the board or close to the CPU (the large square chip in the center).

The battery is usually labeled with the model number — CR2032 or CR2025 are the most common. If you cannot spot it when ready, trace the power cables from your power supply; the CMOS battery is typically positioned away from the main power connectors, in a quieter corner of the board.

Once you locate it, note which way it faces — the flat side with the plus sign (+) should face up. This matters when you replace it.

Finding the CMOS battery on a laptop

Laptops are harder because the CMOS battery is buried deeper inside. You will need to remove the bottom panel or keyboard, which requires unscrewing several small fasteners and carefully disconnecting ribbon cables. The CMOS battery on a laptop is often smaller than a desktop version and may be soldered directly to the motherboard, making it impossible to replace without professional equipment.

Before you attempt this, check your laptop's service manual — search online for "[your laptop model] CMOS battery location" to see whether it is user-replaceable. If the manual shows it is soldered or requires removing the entire motherboard, take the laptop to a repair shop instead. The labor cost is usually $50 to $150, plus the battery itself.

How to replace the CMOS battery yourself

Power off your computer completely and unplug it from the wall. Wait at least 30 seconds to let any residual power drain. Open the case and locate the CMOS battery in its socket.

Press down gently on the battery or use a small flathead screwdriver to pry it out of the socket — most sockets have a small clip on one side that you can lift to release the battery. Do not force it; if it does not come out easily, you may be prying in the wrong spot.

Once the old battery is out, note the orientation. Insert the new battery with the plus side facing up, pressing down until you hear or feel a click. The battery should sit flush in the socket. Close your computer case, plug it back in, and power it on. Your BIOS settings will reset to factory defaults, so you may need to re-enter any custom settings you had changed.

What happens when the CMOS battery dies

A failing or dead CMOS battery causes several recognizable problems. Your computer will show the wrong date and time every time you restart it, even if you correct it before shutting down. You may see error messages during startup like "CMOS checksum error" or "CMOS battery low." Some computers will not boot at all until you reset the CMOS, which you can do by removing the battery for a few minutes or by moving a jumper on the motherboard (check your motherboard manual for the jumper location).

Your BIOS settings — things like boot order, RAM speed, or security settings — will revert to defaults. This is usually harmless, but if you had customized your BIOS for performance or compatibility, you will need to re-enter those settings after replacing the battery.

Choosing and buying a replacement CMOS battery

The most common CMOS batteries are CR2032 and CR2025, both coin-cell lithium batteries. Check your old battery or your motherboard manual to confirm which one you need. These are standard batteries sold at pharmacies, grocery stores, electronics retailers, and online — they cost between $5 and $15 for a single cell, or less if you buy a pack of several.

Do not buy the cheapest option you find; a quality brand like Energizer, Duracell, or Panasonic will last longer and is worth the small extra cost. Avoid batteries from unknown sellers on marketplace sites, as counterfeit cells are common and may fail quickly or damage your motherboard.

When to take your computer to a technician

If your laptop requires disassembly beyond removing a bottom panel, or if the CMOS battery is soldered to the motherboard, have a repair shop handle it. If you open your desktop computer and cannot locate the battery after a careful search, take a photo of your motherboard and post it to a computer forum or bring it to a technician — they can identify the battery location in seconds.

If replacing the battery does not solve your startup problems or error messages, the issue may be elsewhere on the motherboard, and a technician can run diagnostics to confirm.

Frequently Asked Questions

Can I run my computer without a CMOS battery?

No. Without a CMOS battery, your computer will lose the date, time, and BIOS settings every time you power off. Some computers will not boot at all without it. The battery is essential.

How long does a CMOS battery last?

Most CMOS batteries last three to ten years. Computers that are powered off frequently or sit unused for long periods may drain the battery faster. If your computer is on 24/7, the battery lasts longer because it is constantly being trickle-charged by the system power supply.

Is it safe to replace the CMOS battery myself?

Yes, on desktop computers. The battery is low-voltage and cannot shock you. Unplug the computer first, wait 30 seconds, and follow the steps above. On laptops, only attempt it if your manual says the battery is user-replaceable and not soldered.

What if I put the battery in backwards?

The battery will not make contact and your computer will not recognize it. Your BIOS will show an error or the clock will be wrong. straightforward remove it and flip it over so the plus side faces up.

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

Your BIOS will reset to factory defaults automatically when you replace the battery. You do not need to do anything extra. If you had custom settings, you will need to re-enter them after the replacement.