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OT: Can CMOS battery on PC motherboard be hot-swapped?

Discussion in 'Electronic Design' started by Joerg, Feb 24, 2013.

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  1. Joerg

    Joerg Guest


    Got a Dell Vostro 200 mini tower with XP on there that seems to be a bit
    off in the realtime clock lately. Around five years old so needs a new
    3V coin cell on the mobo.

    In order not to lose all the setup stuff, can those CR2032 coin cells be
    hot-swapped while the PC is running?

    Of course using ESD straps, being careful and all that.
  2. SoothSayer

    SoothSayer Guest

    the BIOS flash update utility has a backup feature which keeps settings
    as well.
    you back it up, swap the battery, and then re-apply the backed up flash

    And yes, you CAN apply an external 3V source to the MOBO. Just
    remember that you will be fighting the very low internal resistance of a
    dead battery for the few seconds you are under power as you remove it,
    and for the few seconds you re under external power as you install the
    new one.

    Be very careful about polarity, and do not exceed 3 volts so you do not
    begin "feeding" the battery.

    As a side note, many modern motherboards have enough capacitance at
    that sub-circuit location to retain things long enough for you to swap
    the battery. I do not know if yours is such a design.

    another side note is that RTCs in PCs are not very accurate with the
    original Pc deign and crystal. Newwer designs may use more accurate
    clock sources, but most do not, and your most accurate clock possible is
    your cell phone, or iPad or other cell system connected or GPS enabled
    device. Always bet to keep your PC's clock referenced to such a more
    accurate source.

    Internet updates are prone to latency errors , so do not expect better
    than plus or minus about 1.5 seconds in that realm.

    The phone or GPS device is the clear winner, and a good modern wrist
    watch usually tracks those pretty well once set. Usually within mere
    seconds per year. The phones are ALWAYS accurate as they are ALWAYS
    constantly updated.
  3. whit3rd

    whit3rd Guest

    I've done it. The backup function of the battery means the circuitry
    only draws battery power while power is OFF (and I do mean OFF,
    'standby' of ATX power supplies is an ON state, even though
    the computer seems powered down).

    There's also a hold capacitor, usually a few seconds of power is assured while
    swapping the coin cell.
  4. Joerg

    Joerg Guest

    Thanks. Now I've got to find that sort of utility, somewhere.

    If I had a flash update utlity :)

    So I assume even with a running PC one would still have to hook up a 3V
    supply in order not to lose data?

    No clue, I think it's a Foxconn G33M mobo in there.

    Actually, without web updates the RTC in this PC was remarkably accurate
    for all those years. Well, until yesterday. I rarely had to set it.
  5. Joerg

    Joerg Guest

    So maybe I should just hot-swap then.

    That's odd. Because the PC here is always sent to hibernate and the
    power is turned off. But it remains connected to 120VAC. Still, this
    morning the RTC was off again by more than five minutes from yesterday.

    And then the coin cell slips and our dog carries in to the kitchen,
    "Look, ma, I found bling-bling!" :)
  6. Joerg

    Joerg Guest

    Done, new battery is in. Did a power-down into hibernate, came back up
    like usual. The old battery still measures 3.272V but when loading it
    with as light a load as 10k it collapses to 2.5V. With 1k it goes to
    1.6V. So I guess the old battery was truly exhausted. It was made by
    "Newsun" in Japan, never hear of that brand.

    While I had the PC open I saw that I have quite a build-up of dog hair
    in there, got to take it apart and clean it. Our big lab likes to curl
    up down there when I am doing SPICE runs and warm air comes out. It's
    amazing how tight of a space they can snuggle themselves into.

    Supposedly there is a Dell tool but I haven't found it yet.

    Thanks for the help, Jeff.
  7. Joerg

    Joerg Guest

    It ain't the same. In his opinion :)

    Unfortunately on the Dells the intake is on the left side and that's
    exactly where the Labrador snuggles up. Raising the PC would not help
    but make it more prone to keeling over. For example when he hears the
    Fedex truck and tries to travel from underneath the PC table to the
    front door in milliseconds.


    What makes it sweat is when you are almost done with a switcher and then
    have to enter real xfmr coupling of 0.97 and such. Important to gauge
    the dissipation in snubber parts. That really taxes a PC.

    I guess then the digital camera is the only option. Or paper and pen :)
  8. Baron

    Baron Guest

    Joerg Inscribed thus:

    Don't mess about ! Do it live. Use an anti static strap. Take care not
    to damage the cell holder while removing the old battery and don't let
    it flip out onto the circuit board. Make sure that you put the new
    battery in the right way up, positive uppermost.

    It should not take more than a few seconds to do the swap.

    PS. A wood or plastic toothpic might help depending on the holder type.

  9. Baron

    Baron Guest

    Joerg Inscribed thus:
    A tip that might be usefull... A yellow led with the leads sprung across
    the battery will give an instant indication of the battery state.
  10. Joerg

    Joerg Guest

    Well, it's already done. So not I am wondering about the laptops.
    They've got to have such batteries as well. Maybe time to check those out.
  11. Jasen Betts

    Jasen Betts Guest

    yeah. but be careful if you drop it it could short something out,
  12. Jasen Betts

    Jasen Betts Guest

    The BIOS flash is not the config CMOS NVRAM
    dead battery is about 50 ohms and 2.7 volts
    NTP is much better than that.
  13. JW

    JW Guest

    That's $10. I use Time synchronizer from Softnik Technologies. Better yet,
    it's free.
  14. Joerg

    Joerg Guest

    Why does the change have to be quick? Isn't the circuit powered as long
    as the laptop is? Because then one could solder in a new battery and
    re-use the connector instead of shelling out lots of dough for a
    specialty battery plus shipping charges.

    Hey, it's nice, this morning the PC showed the correct time again :)

    What I really don't understand why in this day and age they don't write
    the settings into flash. I mean, we even successfully do that on totally
    cheapo uC design.
  15. Joerg

    Joerg Guest

    That was not a very smart decision by the industry. Serviceablity has
    suffered as a consequence.

    And we know how that does. Battery is out ... cell phone rings ... oh,
    got to take this one real quick ... :)

    My iron can be operated sans ground. With a laptop it's not really that
    tough anyhow because one can unplug the charger (many of those aren't
    grounded anyhow). Just watch for ESD.

    Whoops :)

    Not really. Almost everything beyond 0Fh gets written once and then left
    alone the next five years of so. Ok, maybe someone buys a new drive and
    enters that. This would cause two write cycles to that location in five
  16. Guest

    If you're using Windows (XP and up for sure, don't remember about 2000),
    it has a built-in time sync functionality, *as long as the machine is
    not part of a domain*. Just go to the time and date settings in Control
    Panel and you should have an option for "Internet Time". It will
    default to the server (which probably round-robins to
    several machines at Microsoft) but you can add other servers if you

    If your Windows machine is part of a domain, its time is synchronized
    from the domain controller as part of the networking protocol. If the
    domain controller is listening to an Internet time server, then it will
    propagate the correct time to its clients.

    If you have old Windows or otherwise want some kind of independent
    time-setting software, nistime-32bit.exe is free of charge from . It says: "The program
    can be configured to query the server periodically and run in the
    background." I have used it before on Windows 98 and it seems to work
    OK; the minimum query interval is one hour. That NIST page also has a
    link to a list of other time-setting software for various OSes.
    (Trivia: it's called -32bit.exe, not to distinguish it from the 64-bit
    version, but from the 16-bit version!)

    If you have Linux, you can run ntpd. Most recent distributions do this
    and provide some kind of happy clicky control panel. The real skinny is
    in /etc/ntp.conf . ntpd can get its reference time over the Internet,
    or from a local GPS receiver.

    I don't know what OS X runs under the hood but I wouldn't be surprised
    if it's some flavor of ntpd. The knobs are apparently at Applications >
    System Preferences > Date & Time .

    Matt Roberds
  17. Joerg

    Joerg Guest

    Same here, I think we somehow must have the same forefathers :)

    Although my dad was a pro when it comes to programming, and so is my
    sister, so maybe it's just me who didn't get the programmer's gene or
    where it mutated into a circuit design gene.

    Wouldn't it be a zero-cost piece of cake to at least write those to hard
    disk and in case of finding a blank offer the user to restore from
    there? Also, the BIOS is in flash so why not store there instead?

    It would behove the industry to think about this because there is one
    major reason why PC sales are slumping: The things became to darn
    complicated for ol'Leroy. He does not want to face a pricey Geek Squad
    call every time some obtuse "unrecoverable error" has occurred. So he
    invests his money into a smart phone instead. Because that's not
    complicated. A PC is complicated.
  18. miso

    miso Guest

    Internet updates are prone to latency errors , so do not expect better
    Yep, though on windows you need Meinberg for NTP. Windows Time service
    isn't as good. Linux real real NTP.

    Tuning up NTP is a bit of work if you want to be optimal. Even though
    people claim the accuracy is independent of the time reference, I make
    it a point to find servers close to me (less hops) and that experience
    low jitter, which can be determined via logging features of NTP.

    I have found that running the antivirus check will effect the NTP
    accuracy. It pays to plot the time error and then correlate it to known
    time dependent programs.

    You should be able to get the time error under 20ms.
  19. SoothSayer

    SoothSayer Guest

    Get it to synch to your phone and you'll get better accuracy than that.

    Call NIST directly with a hard connected modem and you can get within
    2ms. Regularly. Every time.. repeatably.

    Your claim of 20ms is falsely based. Do it 25 times, and I'll bet you
    get 25 different offsets from the real. So more likely plus or minus
    about 80ms. You could probably get closer by hand synching to your phone
    with the mouse click.
  20. Jasen Betts

    Jasen Betts Guest

    11000km in <2ms ! I don't think so.

    25 times? ****! do it a million times the offset is still always under 5ms,
    and that's using third tier timeservers.

    These graphs are from someone else's server but are typical of a
    carelessly installed NTP setup:
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