A failing CMOS battery can cause a number of issues with a computer system. The CMOS battery, also known as the RTC (Real Time Clock) battery, provides power to the CMOS chip on the motherboard which stores the system’s BIOS settings. When this battery starts to lose its charge, it can lead to symptoms like:
Date and Time Resetting
One of the most common symptoms of a failing CMOS battery is the system’s date and time resetting every time you turn on or reboot the computer. This happens because the CMOS chip loses power when the computer is off and can no longer keep track of the date and time. At startup, the BIOS has to guess at the date and time or reset it to a default value, leading to it being incorrect whenever you power on the system.
BIOS Settings Reverting to Defaults
Along with the date and time, the CMOS chip also stores all the system’s BIOS settings configured in the BIOS setup utility. These include settings like the boot order, CPU parameters, RAM timings and voltages, onboard device settings, and more. A failed CMOS battery causes loss of power to the CMOS chip, leading to all BIOS settings resetting back to their factory defaults. This can lead to issues like:
- Boot device changing, preventing the system from booting up properly
- Overclocking or undervolting settings returning to default, impacting performance
- Customized fan profiles reverting to default, affecting system cooling and acoustics
Having to re-enter BIOS settings every time you boot the system can be annoying and time consuming.
Error Messages at Boot
The sudden loss of CMOS settings due to battery failure can sometimes lead to errors during the Power-On Self Test (POST) sequence. You may encounter errors like:
- “CMOS Checksum Error”
- “Default configuration used”
- “CMOS Date/Time Not Set”
“Configuration Change Warning”
These errors indicate the BIOS detected its CMOS settings have been corrupted or lost, and is attempting to recover by reverting to failsafe defaults. While not dangerous, error messages at boot are a telltale sign of a CMOS battery that needs replacing.
Difficulty Getting Into BIOS Setup
Some motherboards require you to enter the BIOS setup utility in order to make changes to the BIOS configuration. However, a failed CMOS battery can sometimes prevent you from accessing the BIOS setup at all. When you attempt to invoke the BIOS setup key during boot (typically Delete, F1, F2 etc.), it fails to come up.
This happens because the CMOS has lost its memory contents, causing critical BIOS configuration issues that prevent the BIOS from initializing properly. Replacing the CMOS battery should allow normal access to BIOS setup again.
System Instability or Unpredictable Behavior
An unstable CMOS memory contents and continually fluctuating BIOS settings caused by a dying CMOS battery can also translate into erratic or unpredictable behavior from the computer itself. Issues like:
- Spontaneous crashes or lockups
- Freezes during bootup
- Peripheral devices intermittently disconnecting and reconnecting
- Unexplained intermittent blue screens or system halts
While such symptoms could arise due to other faulty hardware as well, they point to a CMOS battery failure if they occur in conjunction with losing time/settings or error messages at boot.
No Power or Fans Not Spinning but Motherboard Lights On
In rare cases, a completely dead CMOS battery can cause other serious startup issues like the system powering on but no fans spinning, no video output, yet motherboard lights coming on. This can happen because some motherboards require a valid CMOS configuration to initiate the power on sequence and allow voltage to reach the other components. When the CMOS battery is fully dead, an invalid CMOS state may prevent power from properly reaching the fans, memory, video card etc. Replacing the CMOS battery should restore normal power on functionality.
Conclusion
In summary, the most likely symptoms caused by a failing CMOS battery in order of commonness are:
- Date and time resetting every boot
- BIOS settings reverting to defaults
- Error messages during boot
- Difficulty accessing BIOS setup utility
- System instability or erratic behavior
- Partial or no power with fans not spinning
Any of these symptoms are a clear sign that the CMOS battery has failed or is on its way out, and should be promptly replaced. Pay attention especially if the date/time is constantly wrong or BIOS settings need fixing every boot. Catching a CMOS battery failure early can save you headaches down the line.
Frequently Asked Questions
Why does the CMOS battery need replacing?
The CMOS battery is a standard coin-cell battery that provides constant power to maintain the CMOS chip’s memory contents even when the system is powered off. Like any other battery, it eventually runs out of charge through normal use over 2-5 years, leading to the failure symptoms described.
How do I replace the CMOS battery?
Locate the CMOS battery on your motherboard – it looks like a large watch or calculator battery. Gently remove it from its holder and press in the new battery aligned with positive/negative markings. Make sure the battery clip clicks back into place holding it down. Load BIOS defaults after to clear any corrupted settings.
Is it safe to replace the CMOS battery yourself?
Yes, replacing the CMOS battery is a safe, simple DIY fix. Just be sure to first turn the system off and disconnect AC power before prying out the old battery. Handle it by the edges and don’t short the positive/negative terminals. Dispose of lithium batteries properly.
Do I need to reset the BIOS after replacing the battery?
After putting in a new CMOS battery, it’s recommended to load BIOS defaults or optimized settings. This clears out any corrupted BIOS values leftover from the failed battery and starts fresh with the new battery.
How do I prevent CMOS battery failure in the future?
There is no surefire way to prevent eventual CMOS battery death, but avoiding extended periods of disuse can prolong battery lifespan. If the computer will be in storage for a long time, remove the CMOS battery until ready to use again. Store it at moderate temps – high heat also accelerates discharge.
Troubleshooting Steps
If your system is exhibiting signs of a failing CMOS battery, follow these troubleshooting steps:
- Check the date/time in BIOS setup or OS – if consistently incorrect on every boot, likely CMOS battery issue.
- Enter BIOS and load optimized defaults – if settings keep reverting, CMOS battery is likely bad.
- Visually inspect the CMOS battery on the motherboard – if leaking or corroded, needs replacing.
- Boot into BIOS setup with AC power off – if BIOS resets instantly, battery may be dead.
- Replace CMOS battery with an equivalent new one – match the voltage and size.
- Load BIOS defaults again after battery swap and ensure date/time is now correct.
If symptoms persist even after battery replacement, there may be an issue with the CMOS chip itself rarely. Contact your motherboard or system manufacturer for further troubleshooting.
Preventative Measures
Here are some tips to help prevent CMOS battery failure and extend the battery lifespan:
- Avoid exposing the computer to high temperatures for long durations as heat accelerates battery discharge.
- Ensure the battery clip or holder makes good contact to avoid corrosion over time.
- When putting a system in storage, remove the CMOS battery and store it separately.
- Periodically check the battery voltage in BIOS or using a multimeter.
- Replace the CMOS battery as a precaution every 5 years or so.
- Choose reputable brand name batteries from reliable suppliers.
- If board has battery-low warning, enable it in BIOS to get early failure indication.
While CMOS battery failure cannot always be prevented, following these tips can help delay the inevitable for as long as possible.
Additional Information
CMOS Battery Background
The CMOS battery powers the CMOS (Complementary Metal Oxide Semiconductor) chip present on PC motherboards. This chip requires power at all times to maintain the computer’s BIOS/UEFI firmware settings configured in the BIOS setup utility.
Unlike volatile RAM that loses data once power is removed, the CMOS chip uses EEPROM or flash memory to store configuration data. The small lithium coin cell battery provides just enough backup power to preserve CMOS memory contents when the system is powered down.
Typical CMOS Battery Specifications
- Voltage: 3V
- Chemistry: Lithium-ion or lithium-manganese dioxide
- Diameter: 16mm to 20mm
- Thickness: 2mm to 5mm
- Capacity: 220mAh to 300mAh
- Service life: 3 to 10 years
Common replacement CMOS batteries include the CR2032 and CR2025 lithium cells, among other size variations. Match the original battery’s specifications for best results.
CMOS Battery Location
The CMOS battery can usually be found near the BIOS chip or southbridge chip on the motherboard. It is often close to the onboard clock generator or real-time clock chip. Consult the motherboard manual for exact CMOS battery placement if uncertain.
On laptops, the CMOS battery may be located under the keyboard, inside the palm rest compartment, or beneath the bottom cover. Search the laptop’s service manual for the CMOS battery if needed when replacing.
Resetting the CMOS
As well as replacing the battery itself, resetting the CMOS can sometimes help resolve odd system issues arising from CMOS failure. Motherboards have a CMOS reset jumper or button to clear the CMOS memory contents.
After replacing the battery, resetting the CMOS forces the system to rebuild all BIOS settings from scratch. However, this also requires re-entering custom configurations like overclocks and boot order.