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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.
    IMG_2347.jpg

    IMG_2348.jpg

  2. #2
    Join Date
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    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

    Default

    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
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    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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