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Modified UPS - can it work? Why doesnt it?

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  • Modified UPS - can it work? Why doesnt it?

    Hey all

    From dissasembling tens of UPS's in my line of work to replace their worked out Sealed lead acid batteries (they come into our PC store all the time) I know a lot of the theory behind a UPS.

    basically, i was wondering whether i can use a UPS as an inverter (600w) and since i get UPS's from $40, it's a lot cheaper than a 500w 12v inverter at $162.

    So i ordered a couple, stripped one appart en lengthened the wires from where the Sealed Lead Acid battery was, to attach to my power connector block in the trunk.

    It works, it seems, but once the PC boots past POST, the voltage across the UPS at the battery momentarilly drops to 10 - 8 volts and the UPS disconnects itself, powering the PC off

    If i substitute the car battery for the Sealed Lead acid, during same phase (no mains connected, at POST time) it drops to 11, and back to 12.4v, but can only provide some 18 minutes of working time.

    So now i need an answer to the q:

    Why can the small 7aH lead acid battery handle the load, and my 45aH Car battery + alternator (even tried it with friend's foot on the gas, to keep revs up, and while idling,) cannot keep the voltage high enough?

    I was wondering it there is a way to hook up so that it draws most power from the car's electrical systems, but have a Sealed lead acid somehow in the circuit to compensate for the small 1 - 3 volt drop.

    Please help!!!

    Regards
    Peter

    PS: It's a Duron 1GHZ desktop PC on a 250w Power supply.

  • #2
    Why can the small 7aH lead acid battery handle the load, and my 45aH Car battery + alternator (even tried it with friend's foot on the gas, to keep revs up, and while idling,) cannot keep the voltage high enough?
    You are seeing the line loss from wherever your connecting to the cars electrical system. When your machine is POSTing, its drawing alot more current then at most other times. You can lose more than you think in a 20' length of 8-10 guage. thats why you might see 13.6v with no load but pull 5-7 amps and the voltage will drop if you dont have large enough guage wire.

    Also, a UPS is made to respond very quickly to voltage fluctuations, in milli seconds. Your cars electrical system isnt that stable. Just watch the headlights dim when your bass is really hitting or turning the blower all the way up.

    I would suggest either mounting a car battery thats run through a divi charge type device close to the UPS or running the UPS off a DC/AC inverter with the small battery installed. I have read that some UPSs dont like to run off the modfied sine wave that inverters produce. I have not had that trouble with my APC UPS.

    Hope this helps,
    Pete

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    • #3
      Put the SLA across the car's electrical system, with a diode so that it won't try to power the accessory system when the car's off.

      So the battery powers the PC during cranking etc, and then just gets charged up for those few seconds of draw.

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      • #4
        Pete the Geek:

        You must be abloa to read minds or sumthing.

        UPS in boot, battery up front, runs to the boot via 10A Main cable, i think i'll order a proper wiring kit like tho one powering my amps, and use that. As a quick test, i hooked UPS with 20cm of wire directly to battery and seems to work.

        George00:
        Can you maybe make a quick circuit diagram/description of the location of the diode? Also, i guess it needs to be a fat *** diode, how many amps?

        If i understand correctly, this circuit makes the alternator charge the SLA and the SLA provides additional power to the UPS along with car electrical system.

        Thanks peeps

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        • #5
          bump

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