Bad CMOS Battery

Discussion in 'PC Hardware' started by MrTsquare, Dec 20, 2014.

  1. MrTsquare

    MrTsquare Guest

    After buying a couple of 2032 cmos batteries last year to replace the
    one in my 5-year old build. I sprung for a new build including an ASUS
    Z97Pro series motherboard. Put that together in August 2014. recently,
    the new build began losing time, ie the time would continue to display
    the time that the system was last turned off. After resetting it a few
    times and playing with the choice of Internet Time servers displayed in
    Win7, I finally replaced the alledgedly 4 month old CMOS battery. Life
    is good and now time marches on. ONLY 4 MONTHS ON A CMOS BATTERY??

    MrTsquare, Dec 20, 2014
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  2. MrTsquare

    Al Drake Guest

    I have been having the same problem with the ones I use at work. I have
    them installed in electronic measuring devices and they just don't last
    as long as they use to.
    Al Drake, Dec 20, 2014
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  3. MrTsquare

    Paul Guest

    The important thing, is to keep track of how long the new ones last.
    Long term, you may decide the motherboard has a problem.


    The battery should last a little less than 3 years, if the PC is
    kept unplugged all the time (24 hours a day). If you run the PC 16
    hours a day, and unplug each day for 8 hours, then you'd expect
    9 years (or getting closer to the 10 year shelf life). If the
    PC has +5VSB (motherboard green LED lit all the time, whether
    sleep/hibernate/run), then there is no excuse for less than
    ten years from the battery. In other words, if you don't switch
    off at the back or unplug each day, the +5VSB provides the clock
    current and not the battery.

    The Asus motherboard box has a "Serial number" on the outside
    sticker. The first two characters of the serial number are
    the manufacturer date. My last purchase was "E4..........",
    which would be 2014 May or so. So I know my CMOS battery,
    even if the PC is left unplugged, the battery should last
    another two years minimum.

    These numbers are only approximate. They assume the Southbridge
    CMOS well draws maybe 10 microamps or so. The CMOS battery
    is only used, if the +5VSB is not running (the +5VSB runs that
    green LED on the Asus motherboard surface and is proof the
    supervisory circuits have power).

    If you place the "Clear CMOS" jumper in the wrong position,
    that could drain the battery. But you would figure that out,
    based on how that messed up the settings. If the motherboard
    has an electrical fault in that section of the design,
    and draws more than 10 microamps, that too could
    flatten the battery. There have been some defective motherboards,
    where they drained one battery after another, in short order.
    And it's some sort of motherboard problem.

    And that's why this is a "wait and see" situation. If each and
    every battery only lasts four months, then you'd suspect excessive
    current draw from that battery.

    You cannot draw more than 3000 microamps from the battery at
    any one time. There is a 1K ohm series resistor to the battery,
    which limits current flow. And that seems to be there for some
    sort of safety reason, is all I can figure. So at least you know
    the drain rate cannot exceed more than 300x the normal rate :)
    The battery should last at least one day, even with the worst of

    Paul, Dec 20, 2014
  4. MrTsquare

    MrTsquare Guest

    Interesting, Paul. Your figures on their lifetime do match my more
    limited experience as I am used to them lasting 5 years on a desktop. I
    don't believe I have the green led you refer to but then I have the
    whole thing turned off probably 20 hours a day, but then that's just
    like the like old ones that lasted 5 years. The original box for the MB
    is long gone and I'm not going to open the case to check, but the MB
    manual is dated April 2014. What occurs to me is that the ASUS stock of
    CMOS batteries may be old. Maybe they got a good deal on them 4 or 5
    years ago... ;<)

    MrTsquare, Dec 20, 2014
  5. MrTsquare

    Paul Guest

    Asus motherboards usually have a green LED. It's one of the few
    LEDs that they have consistently provided over the years. It's
    to prevent damage while working on the PC - if the green LED
    is lit, don't add or insert any RAM, PCI, PCI Express cards.

    Other brands don't connect a LED to the +5VSB, so you have no
    warning about when it is safe to work on the PC.


    My own informal records seem to get pretty close to the
    3 year figure. And that's for machines that sit in storage,

    There are datasheets for the CR2032 cells, and you can
    divide the milliamp-hour rating by 10 microamps to work out the
    expected "unplugged life" of the battery.

    What's quite curious, is why the figure is 10 microamps.
    A digital watch is rated 2 microamps, to perform the same
    function. Why do all the Southbridges cluster around the
    10 microamp number so closely ? You'd think some motherboards
    would do significantly better than others, and there would
    be a spread on battery life. My limited experience here,
    suggests the results cluster quite closely, for something
    that could have a random leakage component.

    Paul, Dec 20, 2014
  6. MrTsquare

    MrTsquare Guest

    Well at least its nice to have a "canary" like the time display to tell
    you its time to replace. Concerning the green light... After reading
    the right section of the book and powering down to verify, the Z97pro
    has a little "Power On" red light/push button right below the CMOS
    battery that performs the safety feature you discribed as well as the
    ability to "kitchen table" turn on test the MB.

    MrTsquare, Dec 20, 2014
  7. MrTsquare

    Paul Guest

    Interesting. So the ole Green LED got the boot :)
    It used to be a T-1 3/4 sized LED, which is a bit out of
    style now. The surface mount LEDs would be more popular with
    the staff on the soldering equipment (reflow machine).

    I downloaded the manual, but missed the illuminated switch detail.
    I was scanning for a "LED" feature.

    Paul, Dec 20, 2014
  8. MrTsquare

    bruce56 Guest

    I have built and bought dozens of PCs over the last 20 years.
    I have had at least 3 PCs with this problem, they were LGA775 or LGA1156
    boards, but different makes. The battery would go flat after a few months.
    So eventually it would boot up and say I have lost my settings,
    hit FX to load failsafe defaults and continue or something like that.
    I replaced with known new good batteries and they expired too.
    Yet these same boards had no other flaws at all.
    So I am puzzled what sort of fault causes this.
    bruce56, Dec 28, 2014
  9. MrTsquare

    Paul Guest

    Stick your multimeter across the 1K ohm current limiting
    resistor that comes from the battery.

    If the load is drawing 10 microamps, times 1K ohms, that would be
    10 millivolts (if normal). Any value higher than 10 millivolts,
    you'd have to figure out where the current was going, and that
    would not be easy.

    It could be excessive Southbridge leakage, but just as easily,
    a mistake in the motherboard design, causing the Southbridge to
    leak. Big chips like that, you have to be really careful about
    how the logic inputs are strapped when they're not being used.

    The CMOS/RTC circuit block sits in a "well", with transmission
    gates for interconnect to the rest of the chip. That scheme is
    intended to reduce leakage out of the well area, into other
    (unpowered) circuits. And keep the current draw under 10


    I learned about I/O leakage, on my first job. We had a
    setup like this.

    Power Power
    Supply Supply
    | |
    Circuit --- I/O ------- Circuit
    Board signals Board

    We turned off one of the supplies in the lab, expecting
    the circuit board to be safe to pull out of the system.
    Instead, the circuit board continued to run. Sufficient
    power flowed through the I/O signals, to keep the
    second circuit board running, including lighting up
    up the status LEDs on the faceplate. One side was powered
    by 5V. And the "phantom powered" circuit on the right,
    it was running at somewhere between 3.6V and 4V or so. The
    leakage doesn't charge up the other circuit to 5V or anything,
    but enough power is available, that it was still performing
    its logic function. This holds for "lightweight" circuits,
    like stuff using CD4000 series logic. Any logic family with
    a huge appetite for power, the leakage isn't enough to keep
    them running.

    The method Intel is supposed to use, is like this. The
    CMOS/RTC would be the circuit on the left. The circuit
    on the right would be the rest of the Southbridge. Killing
    the PSU output, causes the power to go off on the right hand
    side. The transmission gates are then set to "open circuit",
    so the current can't leak across. That helps the battery
    holding up the stuff on the left, to not end up powering
    the rest of the Southbridge on the right hand side.

    Power Power
    Supply Supply
    | |
    Circuit --- transmission---- Circuit
    Board gate Board

    Paul, Dec 28, 2014
  10. MrTsquare

    Flasherly Guest

    Had, once, a subbrand newer-name MB that would simply lose it's CMOS
    settings regardless the battery. Either of my Gigabytes AMD/Intel 775
    are both flawless or near so after years and years solid running. Then
    for a Gigabyte it would nothing short of scandalous to expect anything
    worse. Before Gigabyte I ran for a long time with Asus, now no longer
    a fair value consideration and among the most expensive. Before that,
    MSI, which interestingly has made something of a competitive comeback
    after nearly fading out.

    Stick to the best and there's no really no reason to have to deal with
    any of that nonsense.
    Flasherly, Dec 28, 2014
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