Inside your computer hides a small coin cell battery most people have never heard of until something goes wrong with it. It is called the CMOS battery and it has an important job. Let us explain what it does, what happens when it dies and why a computer sometimes “forgets” what time it is.

What the CMOS battery is

The CMOS battery is a small coin cell battery on the motherboard (most often the CR2032 type). It powers a small memory in which the computer keeps the basic BIOS settings and especially the current time and date, even when the computer is off and unplugged.

Thanks to it, the computer knows what time it is the moment you turn it on, and remembers the settings you made in the BIOS.

What it is for

  • Keeps the time and date, even when the computer is off.
  • Stores the BIOS settings (boot order, hardware settings).
  • Works in the background for years, commonly lasting several years.

What happens when it dies

The CMOS battery drains over time, it is a normal wear component. When it does, it shows up quite typically:

  • The computer forgets the time and date. After every shutdown the clock returns to the past (often to some old date).
  • The BIOS settings reset to the factory state.
  • A message may appear at startup like “CMOS checksum error” or “CMOS battery low”.
  • Problems with websites and login. A wrong time causes certificate errors, this relates to the SSL certificate.

The good news is that the computer keeps working, it just does not remember the time and settings. The solution is simply to replace the battery with a new one, it is a cheap and quick fix.

What happens when you remove and reinsert it

This is good to know: if you remove the CMOS battery and put it back, the computer’s BIOS settings are erased and return to the factory state. This is exactly what is used as a BIOS reset, for example when a BIOS password is forgotten or when a bad setting causes the computer not to start.

After reinserting the battery (or inserting a new one), you will therefore have to set the time and possibly the boot order and other things you adjusted in the BIOS again. It does not harm the computer or the data, only the settings reset.

How to tell it needs replacing

  • The computer loses the time and date after shutdown.
  • It reports a CMOS or battery error at startup.
  • The BIOS settings reset without your intervention.

If this is happening, it is time for a new battery. On older computers it is a common maintenance matter that belongs to computer maintenance.

Conclusion

The CMOS battery is a tiny but important component that keeps the time and BIOS settings, even when the computer is off. When it dies, the computer starts forgetting the time and resetting settings. Replacement is cheap and simple, and removing the battery can also be used to reset the BIOS, for example with a forgotten password.

Does your computer forget the time or report a CMOS error? Get in touch, we will replace the battery and set the settings properly.