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Thread: CMOS Battery help

  1. #1
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    Default CMOS Battery help

    I've been trying to fit an external battery to my 386DX-33
    (Here's the motherboard diagram. Mine in a slightly newer version, but seems to be almost identical.)
    The original NiMH battery has, sensibly, been previously removed. I've found something that seemed like it should work as an external battery. It's supposed to be for a cordless phone, but it's also a NiMH with 3.6 volts and even what seems to be the right connector.

    However, it doesn't seem to be working.

    The diagram above shows that J10 is supposed to be for an external battery. With my multimeter I've found which pin is positive and which is negative/ground.
    In the attached image, the pin on the far left is positive, and the one next to it is negative.
    I can't find what, if anything, the other two connect to though...

    So I hooked up my new battery, but CMOS settings still aren't stored for more than a couple of hours, and I always get a "battery low" error.

    I've tried leaving the compy on for ages to charge the battery in case it was simply flat when I got it, and that doesn't work either.

    I would love some help please! Pictures attached.
    Attached Images Attached Images

  2. #2
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    That's because the external battery jumper isn't intended for rechargeable batteries, but for primary batteries. Just get a 3x or 4x AA or AAA battery holder and some alkaline cells. You'll be good for at least a decade. Be sure to situate the batteries so that if they leak,no harm will be done.
    Last edited by Chuck(G); July 19th, 2019 at 08:33 AM.

  3. #3
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    What Chuck said. I like to use Tadiran lithium thionyl chloride 1/2AA cells in a 1/2AA holder, since they're very unlikely to leak unless mechanically damaged. Like this:


  4. #4

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    As Chuck said, the external battery connector does not charge the battery. There are usually a couple of diodes between the + pin and the RTC/CMOS to prevent charging current from reaching the external battery from the power supply. The resistance of these diodes in the flow direction does cause a voltage drop between the battery and the RTC, so a 3.6V battery may not be sufficient as an external battery - 3 or 4 alkalines in series as Chuck suggests (or equivalent) is the ticket.

    The 4-pin external battery connectors on motherboards I have seen seem to have +BATT at pin 1 and -BATT at pin 4. Sometimes they want pins 2-3 jumpered when using an on-board rechargeable battery, other times they are not connected to anything, or to ground.

  5. #5
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    Well, looks like I might have to find someone who needs a battery for their cordless phone haha!

    Thanks for the help


    [edit]
    Just a thought, would it be worth trying to jumper pins 3-4? I know it's typically 2-3, but it really seems to me that on this particular board pin 2 is negative/ground. My multimeter clearly shows continuity with the legs of the BIOS chip. (Likewise pin 1 seems to be positive). 3 and 4 are the only ones with no clear purpose, so perhaps they need to be jumpered to tell the board there's an external battery connected?
    [/edit]
    Last edited by Bworp; July 19th, 2019 at 07:24 PM.

  6. #6
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    The use of a NiMH or NiCd battery in this application has always confused me. The CMOS RTC draws but 50 microamps or so in quiescent mode. On the other hand, an NiMH or NiCd battery self-discharges at a faster rate, so what's the point in using a rechargeable supply? An alkaline battery will last its shelf life.

  7. #7

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    It's too bad that mercury batteries are taboo.

  8. #8
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    Well, I think that silver-oxide ones are still made, but really, plain old alkalines do just fine. I'm not sure that lithiums would give you any additional life.

  9. #9

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    Elon must have the answer....

  10. #10
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    Quote Originally Posted by Chuck(G) View Post
    Well, I think that silver-oxide ones are still made, but really, plain old alkalines do just fine. I'm not sure that lithiums would give you any additional life.
    The hermetically sealed lithium cells should give longer shelf life. Apparently a big reason why potted RTC/NVRAM modules, with the battery in the encapsulation, last so long is that the potting slows the evaporation of electrolyte past imperfect cell seals. There are industry and scholarly articles on the matter.

    Quote Originally Posted by KC9UDX View Post
    Elon must have the answer....
    Yeah, throw it out and buy a new one

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