CMOS Battery
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CMOS Battery

Yes, there is a battery on your system. Usually it is a CR2032 watch style battery (on desktop systems).  Any number of things can happen when your battery voltage drops below 3.0 volts (Normal is 3.3 volts) or 10% voltage drop. When testing a CMOS battery, set your voltmeter to BATTERY and NOT to VDC.  The reason is that on the VDC setting the battery will show 3.3 volts when in reality the battery is almost dead (like the real voltage is .1 volts!). 

Problems caused by low CMOS batteries:

  •  Hard drives not being detected.

  •  Boot problems like devices not loading or having to reinstall device drives upon every boot up

  •  Devices like modems and network card disappear

  •  Constantly getting errors downloading files from the internet (This can also be a problem with your NIC card)

  •  Clock is not up to date, changes or gains or loses time after shutdown.

  •  System Freezes with no apparent cause. (Windows HANGAPP does not respond at all)

  •  Actually very bizarre problems that make no sense.

  •  Random problems

  •  Working hardware from another system or brand new hardware either doesn't work, or works improperly.

  •  Very erratic system behavior.

  •  Changes to BIOS revert back to previous settings after power up.

This is by no means a complete list of possible failures or issues that are CMOS battery related, but before replacing a motherboard or power supply, CHECK THE BATTERY!

After changing the CMOS battery or after a BIOS flash, ALWAYS clear your NVRAM (check with motherboard or system manufacturer for procedure)

Should you replace your battery and still have the problems, then the issue is your power supply or RAM.

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Last modified: 03/11/09