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Bigger UPS Battery

Discussion in 'Home Power and Microgeneration' started by Rob, Oct 31, 2004.

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  1. Rob

    Rob Guest

    I have a 1 kV Best ups (old). The sticker inside says to use 4 x 31ah
    12v batts
    (it runs off 48v). Can I connect say, 8 x 6v golf cart batts (@48V) and
    still have
    it operate? Would the charger still function having to recharge such a
    huge battery bank? I noticed some batt chargers don't like to charge
    too big a bank, and they trip off. I have a 1 kV yamaha (is) generator
    that will run the charger in the Best.
    Can you put too much batteries in a ups?
    Thank Rob
  2. That may be a bit too much load for the charger, as it's 10X the
    battery capacity. There may also be some concern for the batteries,
    as the original batteries are very probably gelled type and you are
    proposing changing them to flooded, and IIRC there are some subtleties
    in charge voltage setpoints to be considered.

    Unequalized FLA batteries may die an untimely death, due to charge
    imbalances caused by electrolyte density stratification, trapped gas
    bubbles, etc.

    A slightly stranger effect of an underpowered multi-stage charger is
    that the batteries will use an inordinate amount of water, because the
    finishing charge will never terminate, and you'll be trying to float
    charge the batteries at the equalization voltage. I've seen this with
    an Accumate battery charger (rated for 2.5-33AH batteries) trying to
    charge an Exide golf cart battery (225AH or so). Other than my
    attempts to use it way outside its' specs, the isn't a bad

    I'd reccomend an external charger sized appropriately for the battery
    bank, _AFTER_ you confirm that the UPS will survive running 24x7 at
    full load. Many UPS inverters are only (thermally) designed for a
    short run at anywhere near full power, and will quickly go up in smoke
    if run for long periods of time.
  3. Ecnerwal

    Ecnerwal Guest

    Depends on the UPS. I have an "old clunker" that's specifically designed
    to work with external batteries to extend the length of operation time -
    complete with a place to connect up the wires from such.

    Many of the newer, more "cost engineered", less robust units will not be
    happy with extra batteries. They've crapped the design down to the point
    where the built in charger can just barely charge the batteres it comes
    with (and if they get it just right, it will be so stressed that it will
    burn out, fry the batteries, or both 2 days after the warrantee gives
    out). You might be able to work around that with a separate charging
    system, but that runs to money. On the other hand, a better battery
    charger may also reduce your generator runtime - some of the cheap
    chargers are horribly inefficient and slow, and the typical
    grid-connected application of a UPS does not call for spending much on
    making the charger swift and efficient.
  4. Rob

    Rob Guest

    Thanks for the reply, I didn't figure it would work, but it never hurts to
    know why. I use the 1Kw Best to run our fridge or the furnace, it earns its
    keep when the motors start. I use a 1Kw gen to power the ups, that way the
    gen can stay on its econo setting saving fuel. (After the motor start, the
    gen kicks up as the internal charger comes on in the ups).

    My next question is how big a 12v charger can I run off the 1Kw generator? I
    have a kill a watt meter that tells me my 15 amp charger draws 280 watts. (I
    also have a 1750w inverter to run the kids night lights and a tv or so when
    needed). I am in a position to purchase some (2 or 4) golf cart batteries
    (T-105's). If we have a repeat of the black-out from 2 summers ago, I'd like
    to take the gen off the ups as required to recharge the 12v system. If I
    read this right, a 15 amp charger would take 10 hours to put 150 amps into
    the battery bank. Also am I right in assuming that I could only run an
    approx. 45 amp charger off the 1Kw generator?

    It all seemed so simple.....till I started. Thanks for your
  5. That's not a bad paradigm, I'm assuming it works well?
    I'm confused, your UPS has a 48V input, yes? Or are you saying that
    you want to design a system around your 1750W inverter?

    Yeah, a 12V system with a (say) 75% efficient charger could charge a
    12V battery bank with something like 55 amps from a 1KW generator. A
    full system design isn't quite as simple as "I can get some batteries
    and I have an inverter" though, you have to know something about your
    load, your DOD on your batteries before charging them, and the care
    and feeding of batteries when you aren't using them, which can be
    "most of the time".

    It's not an insurmountable problem, but an integrated charger/inverter
    sized for your needs with an appropriate battery bank is going to be a
    lot easier to deal with. I'd reccomend looking at some of the
    offerings of for instance... The FX series
    inverters can be set to limit the AC input to between 5 and 60 amps,
    so your 1KW generator wants to be set at 8 amps.
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