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Testing an uninterruptible power supply (UPS)

Discussion in 'Electronic Basics' started by John Doe, Jun 6, 2005.

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  1. John Doe

    John Doe Guest

    I would like to buy a battery for my UPS which has failed. But there
    is no clear indication that the sealed lead-acid battery is dead. How
    do I test?

    Should I hook a multimeter to the positive battery terminal? The 10
    amp DC multimeter setting should do? Put the black multimeter probe
    to the battery positive terminal and the red multimeter probe to the
    UPS charging output? I expect that will measure charging current
    flowing into the battery and provide an indication of whether the UPS
    is attempting to charge the battery.

    Any idea of how much charging current there should be?

    This is the 12 V sealed lead-acid battery model number:
    BP7-12 7AH/20HR
    I guess "20HR" means that it takes 20 hours to recharge.

    Thank you.
  2. Chris W

    Chris W Guest

    The 7AH/20HR means that the battery will put out 7AH if it is discharged
    at a 20 hour rate, 7/20 = .35 amps. If you discharge the battery with a
    ..35 amp load, it should be dead in 20 hours. In a UPS though they
    normally drain them at a much higher rate. As for charging I would
    estimate something less than 2 amps. maybe slightly more. As far as
    testing the battery goes, if you want to spend less money than a new
    battery would cost (about $15 for a 7AH), I would suggest taking a 12V
    light bulb and hook it up to the battery and monitor the voltage till it
    drops to about 10 volts. Then do the math and see how many amp hours
    you got out of the battery. Then compare that to a similar energy curve
    on the spec sheet for a new battery like this one....

    If you don't know the wattage of the light you are using, you could use
    your 10A amp meter to measure current draw and another meter to monitor
    voltage. The current will also change as the voltage drops.

    If you really want to know how all your batteries are doing you could
    get this thing

    or this thing

    The second one is cheaper but you need to provide the load to discharge
    the battery.

    Chris W

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  3. John Doe

    John Doe Guest


    That was my best wild guess.
    I messed with it for a while, but the charging light in my Tripp
    Lite UPS does not even come on (blink) now, like it did before.

    Oh well.

    It is being replaced with a line conditioner (APC LE1200). I think
    that is what I want. I am not very concerned about loss of data (I
    do backups frequently in Windows). I am concerned about possible
    damage to my hardware caused by brownouts.

    Ideally, I would like something without a battery that would
    sustain my system for a few seconds. I am sure the LE1200 will not
    do that, but hopefully it will help.

    Thanks for helping me get on with replacing the thing.

    As far as
  4. John Doe

    John Doe Guest

    No, I was thinking it might mean 20 hours to charge, as in "hour
    recharge". Thanks for the correction.
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