While how long a CMOS battery lasts can be years, the right time to change it is when your computer starts showing specific symptoms, not when a timer runs out.
What is a CMOS battery?
A CMOS battery is a small, coin-shaped battery on a computer’s motherboard that powers the real-time clock and preserves BIOS/UEFI settings when the device is unplugged or turned off. It ensures your system remembers the date, time, and hardware configuration even after a complete power loss. This battery is separate from the main laptop or desktop power supply and is typically a lithium cell mounted in a holder for easy replacement.
Factors affecting the lifespan of a CMOS battery
Several factors influence how long a CMOS battery lasts, and none of them are documented as a fixed number by manufacturers. The following seven elements play the largest role:
- Battery quality: Higher-quality batteries use better materials and tend to last longer.
- Usage frequency: Frequent power cycling and repeated access to BIOS settings can increase wear.
- Operating temperature: High temperatures accelerate battery deterioration; low temperatures can cause faster drain.
- Power draw: Systems with high-performance components or extensive hardware configurations place a heavier load on the battery.
- Voltage: Operating at a voltage higher than recommended can shorten battery cell life.
- Environmental conditions: Humidity, dust, and contaminants can cause corrosion or damage.
- Manufacturer reputation: Trusted manufacturers typically produce more reliable batteries.
Signs of a dying CMOS battery
When a CMOS battery begins to fail, it shows clear symptoms. Watch for these seven indicators:
- Incorrect date and time: The system displays the wrong time or date after a reboot or power loss because the battery can no longer maintain the hardware clock.
- CMOS checksum errors on startup: Error messages about CMOS settings or checksum problems appear during boot.
- Boot failures or delays: The system takes longer to boot or fails to boot entirely.
- BIOS settings resetting to default: Configuration changes revert after a restart or power loss.
- Random system instability: Unexpected freezes, crashes, or restarts occur due to inconsistent power from the failing battery.
- Inability to save BIOS changes: Modifications to BIOS settings are not retained after saving.
- Low battery warnings: Some motherboards display a low battery alert.
These symptoms can also stem from other hardware or software issues, but if you see multiple signs, replacing the battery is a good first step.
What to do if your phone shows the wrong time
Phones do not use a user-replaceable CMOS battery, so an incorrect time on a smartphone is not a battery problem. Instead, check the device’s date and time settings using the steps below. If the options are dimmed or unavailable, follow the troubleshooting guidance.
iPhone
- Go to Settings > General > Date & Time.
- Turn Set Automatically on to have the iPhone get the correct time over the network and update for the current time zone.
- If you want to change the time manually, turn Set Automatically off, then change the date and time displayed.
- To change the time zone manually, turn off Set Automatically, tap Time Zone, enter a location in the search field, and tap the result.
- If Set Time Automatically is dimmed, open Settings > Privacy & Security > Location Services > System Services and turn on Setting Time Zone.
Apple also documents 24-Hour Time in the same settings screen, but it is not available in all countries or regions. Apple says some networks do not support network time, so in some countries or regions iPhone may not automatically determine the local time. Apple's troubleshooting page says to make sure the device is connected to the internet, install the latest software updates, then turn on Set Time Automatically. Apple's retrieved iPhone pages apply to iPhone, with one page explicitly shown for iOS 14.0 and another for iOS 13.0, plus a current iOS guide. Apple's troubleshooting page covers iPhone, iPad, and Apple Vision Pro for time and time zone issues.
Android
- Open Settings > System > Date & time.
- Turn Automatic date and time on to update the time automatically.
- Turn Automatic time zone on to set the time zone automatically.
- Alternatively, open the Clock app, tap More > Settings > Change date and time.
- To set the time zone manually, go to Change date & time > Time zone and, if present, turn off Set automatically.
Google's Clock help also documents Open your device's Clock app > More > Settings > Change date and time. The AOSP documentation says automatic time detection and automatic time zone detection can be controlled in System > Date and Time in the Settings app. Google's retrieved help pages cover Android devices generally and do not give a narrower model list.
Frequently Asked Questions
What is a CMOS battery?
It is separate from the main laptop or desktop battery.
How long does a CMOS battery last?
No official numeric lifespan is documented by manufacturers. Longevity depends on factors like battery quality, usage frequency, temperature, and power draw. Replacement should be triggered by signs of failure rather than a fixed interval.
What are the signs of a failing CMOS battery?
The most common signs include incorrect date and time, CMOS checksum errors on startup, boot failures or delays, BIOS settings resetting to default, random system instability, inability to save BIOS changes, and low battery warnings from the motherboard.
Can I replace the CMOS battery myself?
Yes, replacing a CMOS battery is generally a simple process that can be done by the user. Follow your device’s manual for specific instructions, and be cautious when handling motherboard components. If you are unsure, consult a professional.
What happens if I ignore a failing CMOS battery?
Ignoring a failing battery can lead to incorrect date and time settings, which may cause problems with software applications, potential data loss, and difficulty connecting to the internet. In some cases, failure to replace the battery may result in permanent damage to the CMOS chip or other motherboard components, requiring more extensive repairs.
For the wider topic, see replacing watch and device batteries.
