Motherboards have a small CR2032 battery to power a chip called the CMOS. This chip saves your computer's settings and keeps the clock running when the power is off. Without this battery, your computer would forget its settings and lose track of time every time you unplug it.

Picture this: you turn on your computer and the clock shows the wrong year. Your custom settings are gone, and a strange error message appears on screen. These problems happen when the tiny coin battery on your motherboard dies. Every desktop and laptop motherboard uses one of these batteries to remember important information.

The battery powers a small memory chip that holds your BIOS or UEFI settings. It also runs the Real-Time Clock (RTC) that tracks the date and time. This setup has been standard in personal computers since IBM introduced it in the early 1980s.

What Does the CMOS Battery Actually Do?

The CMOS battery has two main jobs. First, it keeps your BIOS settings saved when the computer is off. Second, it runs the clock that tracks time and date. Both jobs need constant power, which is why your motherboard needs a battery.

Saving Your BIOS Settings

Your computer's BIOS stores many important settings in a special type of memory called SRAM. This memory needs constant power to hold onto data. When your computer is on, the power supply provides this power. When it's off, the CMOS battery takes over.

The BIOS settings your computer saves include:

  • Boot Order: Which drive your computer checks first when starting up
  • Hardware Settings: CPU speed, memory timing, and fan controls
  • Security Options: Passwords and Secure Boot settings
  • Power Settings: What happens when you press the power button and sleep options
  • Device Controls: Which USB ports work and how storage drives connect

Running the System Clock

The Real-Time Clock chip tracks the current time and date. This clock runs all the time, even when your computer is completely off. It uses very little power, about 1 to 10 microamps, which is why a small battery can run it for years.

Why Accurate Time Matters: Your computer uses the system clock for more than just showing you the time. It checks if security certificates are valid, runs scheduled tasks, names files with the right dates, and syncs with other computers on your network. Wrong system time can block you from secure websites and cause software to stop working.

Technical Specs of the CMOS Battery

Almost every motherboard made today uses the same type of battery. The CR2032 lithium coin cell has become the standard because it provides the right voltage in a small package that lasts for years.

  • Battery Type: CR2032 lithium coin cell (used in about 95% of motherboards)
  • Voltage: 3.0 volts when new, works until it drops below 2.0 volts
  • Capacity: About 220 to 240 milliamp hours (mAh)
  • Size: 0.79 inches (20mm) wide and 0.13 inches (3.2mm) thick
  • Power Draw: The CMOS chip uses only 1 to 10 microamps when the computer is off
  • Location: Usually near the CPU socket, memory slots, or expansion card area

How to Tell if Your CMOS Battery is Dying

When your CMOS battery starts to fail, your computer shows clear warning signs. These symptoms get worse as the battery voltage drops below its working level. Catching these signs early lets you replace the battery before you lose all your settings.

Common Warning Signs

  • Clock Resets: Your computer shows the wrong time or date after being turned off, often resetting to a date in the past
  • Lost Settings: Your BIOS settings go back to defaults every time you restart
  • Boot Errors: Messages like "CMOS checksum error" or "CMOS battery failure" appear when starting up
  • Wrong Boot Device: Your computer tries to start from the wrong drive or asks you to press F1 to continue
  • Hardware Not Found: Hard drives, USB ports, or other devices suddenly stop being detected
  • Slower Performance: Your CPU runs at base speed instead of its normal faster speeds
Early Warning: If your clock occasionally shows the wrong time or you sometimes get random BIOS errors that go away after restart, your battery is getting weak. Plan to replace it within the next month or two before it fails completely.

How Long Do CMOS Batteries Last?

CMOS batteries typically last between 5 and 10 years. The exact lifespan depends on how often you use your computer and where you keep it. Regular computer use actually helps the battery last longer because the motherboard draws power from the wall outlet when turned on.

  • Daily Use: Computers used every day typically get 8 to 10 years from their battery
  • Weekly Use: Systems used once or twice a week usually last 5 to 7 years
  • Storage: Computers sitting unplugged for months drain their batteries faster, often lasting only 3 to 5 years
  • Hot Environments: Temperatures above 85°F (29°C) can shorten battery life by 20% to 30%
  • Battery Brand: Name brands like Panasonic, Energizer, and Duracell typically last 2 to 3 years longer than generic batteries

How to Replace a CMOS Battery

Replacing a CMOS battery is one of the easiest computer repairs you can do. It takes about 5 to 10 minutes and costs only $3 to $5 for a new CR2032 battery. You can buy these batteries at most grocery stores, pharmacies, or electronics shops.

Step by Step Instructions

  1. Shut Down Completely: Turn off your computer and unplug the power cord from the wall
  2. Remove Static: Touch the metal case to discharge any static electricity from your body
  3. Open the Case: Remove the side panel to see the motherboard inside
  4. Find the Battery: Look for a silver coin shaped battery in a small plastic holder
  5. Note Your Settings: Write down your current BIOS settings or take photos before removing the battery
  6. Remove the Old Battery: Push the small metal clip aside and lift the battery straight up
  7. Install the New Battery: Put the new CR2032 in with the plus sign (+) facing up, then press until it clicks
  8. Close Everything Up: Put the side panel back on and plug in the power cord
  9. Reset Your Settings: Turn on the computer and set the correct time, date, and any other settings you noted
After Replacing: All your BIOS settings will reset to factory defaults when you remove the old battery. You'll need to set the time and date, choose your boot order, and restore any custom settings. This is why writing down your settings before you start is so helpful.

Do Modern Motherboards Still Need CMOS Batteries?

Yes, even the newest motherboards still need CMOS batteries. While modern UEFI firmware can store many settings in flash memory that doesn't need power, the Real-Time Clock still requires constant power to keep running.

Modern motherboards use the CMOS battery for:

  • Timekeeping: The RTC chip still needs power to track time when the computer is off
  • Security Features: Some security settings are stored in battery backed memory on purpose so they can be reset if needed
  • Fast Boot Data: Hardware information saved for quick startup lives in battery powered memory
  • Wake Features: Wake-on-LAN and scheduled wake times need the clock to know when to turn on
  • Emergency Recovery: Backup boot settings stored in battery powered memory help recover from problems

What Happens if You Remove the CMOS Battery?

Removing the CMOS battery is actually a common troubleshooting step. When you take out the battery for about 30 seconds to a minute, all the BIOS settings reset to factory defaults. This can fix problems caused by wrong settings or corrupted data.

Some people remove the CMOS battery on purpose to clear a forgotten BIOS password. Others do it to reset overclocking settings that made their computer unstable. Just remember that removing the battery erases all custom settings, so you'll need to set everything up again.

The Bottom Line

Every motherboard needs a CMOS battery to save settings and keep time when the power is off. This small CR2032 coin cell costs just a few dollars and lasts 5 to 10 years with normal use. When it dies, you'll notice the clock resetting and BIOS settings disappearing.

Replacing the battery is simple and takes just a few minutes. Look for signs of a failing battery like wrong time displays or startup errors. When you see these signs, pick up a new CR2032 battery and swap it out before the old one dies completely.