Connect with us

How do APC UPS's detect low battery condition?

Discussion in 'Electronic Basics' started by AC/DCdude17, Jul 1, 2004.

Scroll to continue with content
  1. AC/DCdude17

    AC/DCdude17 Guest

    X-No-Archive: Yes

    I got an old APC Smart UPS that was tossed out apparently because the
    owner thought it was broken because it would only run for a few seconds.
    Being as old as it is, the battery was shot. I put a new battery in
    and it runs to manufacture's specified run time. Although there seems
    to be a problem with the low battery indicator and software's runtime

    The unit is rated to run 650VA 400W.

    The interface port is connected to a test computer and a simulated load
    of 200W(computer power supply with resistive load to simulate computer
    like load)applied under which condition manufacture claims to give 15min

    The low battery alarm goes off only one minute into battery operation
    and tells the computer that is connected to the interface to turn off,
    but the simulated load continued to run for 20 minutes, well beyond the

    Other than not being able to tell when the battery is actually low, this
    is not a problem when used as a dumb UPS. However, it is a major
    problem when I use it with a management software, because it thinks the
    battery is low and signals the computer to shut down almost immediately
    after power goes out with 90% of battery capacity remaining.

    I don't understand how the UPS is detecting low battery. The software
    estimates 7 min of run time(which is way off). The estimate changes
    with load, so it's measuring the amount of load some how.

    Below are the test results of continuously monitoring battery voltage at
    the circuit board:

    13.95V float voltage at 25°C ambient (isn't this kind of high? I wonder
    this is why the installed battery was dead as a brick only after 3 yrs)

    Low voltage alarm came on about a minute into battery operation and the
    battery voltage was around 12.2V, very health reading for a battery
    under load.

    UPS cuts off at 10.3V, seems about right.

    Voltage drop at the terminal was already eliminated since readings were
    taking at the board. I suspected there's something wrong with the
    voltage detection, although a reasonable cut off votlage seems to imply
    that internal voltage measurement is working fine.

    So, I pulled the board out to see if I could somehow change the low
    voltage alarm threshold. There is no adjustment trimmer of any kind.

    The possible causes I can think of are: Something was programmed
    incorrectly through interface, firmware is retarded, out of tolerance
    passive components or failing semiconductor parts.

    Anyone have any idea what might be wrong with it and how to fix it?
  2. It's based on the battery's capacity during the last calibration
    run. You need to force the UPS do a calibration run, where it runs
    the load off battery until the battery's nearly flat and then
    switches back to mains. It will remember the energy the battery
    supplied. The figure it currently has will have been set when
    the previous battery was in the unit.

    What I didn't ever work out was how you reset the battery fitted
    date, but that wasn't used for anything important.
  3. Guest

    The float voltage does seem high, and the cut-off voltage (10.3) is too low.
    Check the battery manufacturer's specs for the "care and feeding" of their
    battery. The load you are using may have an affect, too - I don't know.
    A PC uses a switching power supply - that might make a difference to
    the UPS. It will draw current in spurts while the resistive load draws
  4. Guest

    See and look down near the end for
    the answer to "Q: I replaced the battery in my APC Smart-UPS and now it
    thinks the battery is low all the time. How do you fix this?"

    Matt Roberds
  5. Run "inititate run time calibration" in APC's Powerchute software. This
    then stores the run time in the UPS.
    It's slightly high, but I wouldn't expect it to cause a problem. Is
    this at the board or the battery terminals?
  6. AC/DCdude17

    AC/DCdude17 Guest

    X-No-Archive: Yes

    Mike Tomlinson wrote:

    Battery terminals
  7. bushbadee

    bushbadee Guest

    Who the hell wants to scroll down to the bottom to read the attachments on
    every post.
    That is the cause of about 90% if carpal injuries.

    Low voltage detection is quite a problem sometimes.
    Particularly when you have 3 phase power.

    On one UPS I built for some very sensitive equipment that operated on 3
    phase this was a problem that we had to overcome.
    The Voltage itself was not as important as their not be a transient on the

    I built what I called a slow switch.

    The power to the unit was supplied by the 3 phase mains.
    But in parrelel with that voltage was a regulated power supply coming off a
    The regulated voltage was set to about 0.05 volt above the DC voltage from
    the mains.
    So no current was drawn from the battery.

    In the event of the input voltage off the 6 rectifiers dropping, for any
    reason below a set voltage even for an a microsecond the slow switch would
    be activated.
    It would then begin to supply some of the load current.
    If the voltage remained at the proper point, inother words, it was just a
    drop out transient, then the switch would shut off and the normal current
    path would be used.

    But if the voltage dropped out, the regulated supply would supply the
    current to the load.
    Now the current was in the order of 25 or so amps and the switch could not
    handle that kind of power for long.
    As the peak came off the battery the regulator pass current would slowly
    satureate and raise the voltage to the load and reduce the power dissipation
    in the pass transistor.

    When the power came back on, the supply would detect this and the regulator
    would again come on and slowly drop the output voltage till it was equal to
    the supply voltage and then the pass transistor would cut off again.

    It worked quite well.

    I believe it was used on a test set for the SR 71 power supply which we
    also designed.
    That was a rather interesting supply.
    It had to operate at 100,000 feet and 85 degrees c.
    This was also before Silicon transtors.
  8. OK, fine. If you won't post properly, I won't be reading it.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day