Using tandom UPS's with larger Ah ratings.

Discussion in 'The Projects Forum' started by Andrew Leigh, Dec 10, 2014.

  1. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008

    down on the southern end of the dark continent we are experiencing 220V supply issues with the utility now resorting to "load shedding" as it fails to meet demand. I need a cheap and short terms solution to the 3 odd hour periods that we sit without power. I need a couple of lights and a small TV to run.

    As it so happens I have two identical 650W UPS's , I was thinking that I would like to parallel these to create a backup supply. Now obviously 2 x 7.2Ah batteries will simply not cut the mustard so I was thinking of the following;
    So I am wanting to connect an appropriate charger to a 100Ah battery. The battery will be connected in parallel with the two UPS batteries. But as the UPS charging circuit is only suitable for the 7.2Ah batteries I plan on inserting a relay that will have the battery out of circuit while power is on and all three batteries will be appropriately charged or floated by their respective charging circuits. The minute power is lost the relay will return to it's N/C state and help to power the UPS's. I can make it manual and fumble around in the dark and throw a couple of switches but would prefer a seamless transition.

    So the questions are;
    When power is lost how will the current drain from the batteries, will it be equal, will the 7.2Ah croak?
    Will the relay require some form of protection in terms of sparking and if so what would I need to do?

    Thanks and regards
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    the 100ah battery will keep the 7.2ah batteries topped up,then as the 100ah battery begins to drop voltage the 7.2ah will discharge first, try running it with an ammeter on each battery.
  3. #12


    Nov 30, 2010
    You seem smart enough to make sure that forcing all the batteries to the same, "common" will work, and that is my only concern. If, "ground" is carried intact through the internal battery chargers, this should work. And, PS, you better use a whopper of a relay. You're looking at possible loads over 100 amps on the 12V batteries.
  4. Andrew Leigh

    Thread Starter Well-Known Member

    Sep 8, 2008
    Thanks chaps, that gives me clarity.

    Clearly there is a need to either build a low voltage drop out circuit to protect the 7.2Ah batteries or to monitor them manually through Ammeters. The former being the preferred as batteries can spoil real quick when overloaded.