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Thread: CMOS battery issue

  1. #1

    Default CMOS battery issue

    I recently got a 486 motherboard that appears to have had its barrel battery replaced with a CR2032 coin style. The issue is that I am still getting a checksum error after around 15 minutes of it being turned off. I replaced the battery to be sure and checked it with a multi meter, but it still loses its memory. I'm trying to figure out what other problem areas I should be looking at. I don't see any visibly bad traces. Unfortunately, I can't seem to find a lot of info on this board. The model is: AT AV-8540/8541 if that helps anyone. The only other thing I was thinking was running a battery externally, but I'm not sure about where it would connect to on this board. Any help would be appreciated, thanks!

    Picture of the battery jumper on the board.


  2. #2
    Join Date
    Jan 2007
    Pacific Northwest, USA
    Blog Entries


    Can you identify your board on Th99 or at least one that resembles it closely? The point is that I'm not sure that a CR2032 non-rechargeable battery is a good replacement for a NiCd barrel battery, unless a blocking diode has been installed.

  3. #3


    Thanks that link was really helpful! I got an external one plugged in and working. Yeah charging a non rechargeable battery had concerned me a bit, but I just got this off ebay. Now that it is no longer getting a CMOS error I will go ahead and add the diode now. With the voltage drop of the diode would I need to add a second battery to compensate?

  4. #4
    Join Date
    Apr 2015
    Austin, Texas


    The forward voltage drop of a generic silicon diode (like a 1n400x) is generally 0.7-0.8v. This would really only work if you had something like 3x AA cells, a CR2032 would drop too low.

    You could try a schottky barrier diode like a 1n5817, which has a forward voltage drop of 0.15-0.2v.


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