Computer Draining CMOS Battery?
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Thread: Computer Draining CMOS Battery?

  1. #1
    Join Date
    Jul 2003
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    Computer Draining CMOS Battery?

    When I had to replace my computer's CMOS battery for the first time, I didn't even know what such a thing was. The battery had worked for years. Then I changed it, but the new battery didn't last very long. I figured it happens, it was a bad buy, so I just got another one. That one didn't last long either, however, and I had to replace it quite quickly. And just now I again get a CMOS checksum error. That's the third one then, in a row, that doesn't live very long. So I'm wondering: was I merely lucky with my first battery, and is the lifetime of a CMOS battery just not very long? Or have I been extremely unlucky in my purchases (all random brands, by the way)? Or is it possible that there is something wrong with my computer, that it is somehow destroying these batteries very quickly? I grabbed that last option out of thin air, I have no idea if something like that is possible, but I am just wondering.

  2. #2
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    The battery pulls a tiny amount of power from the supply even when the computer is off to keep the charge. The battery may run down if one unplugs the PC or disconnects it from a power source, say turning of the switch on a multi outlet strip, for long periods of time. Then the battery may run down because it does not have enough time to recharge.
    Also, are you sure you replaced the battery with the correct one? Check with the motherboard manual if you have one or can find it on the web.
    J.
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  3. #3
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    Also, nothing says the new battery has not been sitting around for about 10 years and is about dead itself.

    Seen that quite a few times.
    SMILE
    and post back. Let us know if it worked.
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  4. #4
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    It sounds to me like a mobo component has failed, and is causing the battery to drain rapidly. Normally a mobo battery will last for years, and you'd have to be very unlucky to get three bad ones in a row.

  5. #5
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    Thanks for your replies, guys.

    Just wondering, how much time does the battery need to recharge? Because my computer is on an extension cord, a multi-outlet strip, as jseidel said, with a power switch and I keep that switch off during the day when I'm not at home. It is usually switched on for at least a few hours every day because my laptop draws power from the same source and between my desktop and my laptop, at least one is usually on when I am here. I have had this setup ever since I had the computer, so the first battery was under the same conditions.

    I never checked if I bought the right battery for my specific motherboard, I assumed all motherboards used the same type of battery, really. The battery is the right size, at least. Are there more types of batteries that have the same size?

    If a component on my motherboard has failed and is thus causing this, what components might that be? And how would I go about finding out whether or not this is the case?

  6. #6
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    The battery doesn't recharge, it is a straightforward Lithium coin cell.

  7. #7
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    I see, I was under the impression that jseidel was saying that the battery recharged, but perhaps I am misunderstanding.

  8. #8
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    In all likelihood, jseidel was a bit off with that one, it's now very rare for motherboards to use rechargeable CMOS backup batteries. If yours was one of the rare ones and you fitted the wrong battery, that could cause a problem, but normally rechargeables weren't user replaceable.

    Assuming yours was a lithium button cell, such as a CR2032, you could check the current drain using a digital multimeter if you have access to one. By sliding a thin piece of plastic between the contact finger and the battery case, you could insulate it, and complete the circuit with the meter on a current range instead. The Real Time Clock and BIOS data memory area that the battery is maintaining should only draw a few tens of micro-amps. If the meter indicated a larger current, say some milli-amps, then there is a fault on the motherboard that's draining the battery.

  9. #9
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    I know I'm a noob here, but I've seen this problem before. Sadly in all likelyhood this maybe a component on the motherboard. The battery circuit is controlled by diodes, I can't recall if they are zener or regular 2n4901s (little orange beads). Eventually these devices will fail especially if the system to a spike of some kind.

    When the computer is on, the battery is disconnected by the diodes from the charging circuit, and the power is supplied to the CMOS from the power supply. When the power is off, the battery feeds the CMOS through the diode protection circuit because diodes act as valves and only allow current to flow in one direction. If the battery keeps draining, this means that one of the valves had failed, and is drainng the voltage right out of the battery.

    In the past, I used to repair these circuits on some motherboards in video terminals and early PCs. These boards used Tadrian Lithium 3.5V batteries. (Pink cans). Eventually the battery would die, and had to be replaced, or sometimes if the system took an electrical spike, the circuit would fail and we'd have to change the diodes and perhaps some other components. (Thinking back over 20 years can get tough sometimes ).

    Anyway, given that many of the components have shrunk down to the size of ant footprints, and the motherboards are multilayer now, it's quite difficult to replace any hard-wired components. Sadly the motherboard should be replaced as this is a sign that there maybe other things that could be failing as well.

    Good luck,

    John

  10. #10
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    I'd say you're probably right in your suspicion John. These days the most likely component doing that duty would be a common cathode dual shottky diode, an SMT component.

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