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UPS external battery hook up

Discussion in 'General Electronics' started by rick stanford, Dec 9, 2003.

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  1. I have a Tripplite SU series UPS that allows external batteries to be
    connected to get longer run time in case of power outage. It gives
    specs. of 30 amps, 36 volts on the external battery connection area
    (so I assume Tripplite has 36 volts total from internal batteries).

    I want to use some godzilla batteries, like three 12 volt car
    batteries in series (which often give 13 volts or more each fully

    I'm nervous about connecting though, because I already toasted my
    DELTEC UPS connecting a 12 volt car battery to the internal battery
    leads (I removed 12 volt internal battery first and then connected
    using proper polarity, of course).

    It seems such a powerful battery(s) makes like a large inrush upon
    connection ?
    It toasted two 30 amp fuses hard soldered on the circuit board.

    Should I use a diode, or a resistor, or something to control/buffer
    the "inrush" ? What specs would you suggest ?

    Please cc to my private email at


  2. Electrode

    Electrode Guest


    I gave this some thought - and think you may have induced the problem
    in the Deltec UPS. The charge circuitry should protect the UPS
    against a low-battery, high-inrush condition, as SLA batteries exhibit
    the same phenomenon as a car battery.

    The design of the inverter is quite marginal in low cost units. When
    you removed the internal battery - you added lots of distributed
    inductance to the switching circuit - which expected the battery to
    function like a really large, low impedance capacitor. Lack of a
    local, large capacitor probably resulted in severe over/undershoot of
    the switching FET supply voltage as it was hammered down by the FET
    switch, and rang for relative days afterwards.

    Long leads also induce voltage drop - which will INCREASE the current
    drawn from the batteries - aiding in the above, and in causing other

    You should have no problems with a UPS that features an external
    battery connector - with the internal battery present. Play it safe,
    and confirm this with the manufacturer. They will have harness and
    battery guidelines to follow.

    ---- HINTS TO ALL ----

    DO NOT remove internal batteries from a UPS, and fail to provide a low
    ESR path (local battery or bank of low ESR capacitors, that can handle
    the high ripple current) for the switch current.

    Verify any modifications with a scope and a minimal load, such as a
    25W light bulb. Verify there is not excessive 'ringing' on the DC
    supply lines at the battery, or on the drain of the switch FET(s).
    Verify this again at a slightly higher load (100W) - then at your
    desired load. Use an isolation transformer to power the scope while
    doing this test! (The battery negative FLOATS -and can smoke the
    probe in an instant if the scope is grounded!)

    Remember that longer run times were not envisioned in most UPS designs
    - and if you modify a low-cost unit ADD AT LEAST ONE FAN to blow air
    directly over the FET(s) and switch diodes. The fans should be fused,
    connected directly to the battery, and operate at the battery voltage.
    Occasionally, a larger heat sink may also be required for the FETs
    and diodes.

    Current sharing is difficult in some conditions. When paralleling
    batteries, the current path for all batteries must be equal in length,
    and join together in one point (a heavy terminal block) for each
    polarity. Clean the block carefully, and use a thin grease to prevent
    oxidation. Check voltage drop with a DVM to ensure the drops are
    equal. Series connected batteries do not have this restriction.

    USE LARGE WIRE AND SHORT RUNS. Wattage in equals wattage out - plus
    10% or so. THE INPUT CURRENT IS OVER 10X THE LOAD CURRENT in the case
    of a 12 volt battery-supplied UPS - given this formula. If you have a
    200W load, this is 25A or so from the battery. If the battery voltage
    drops, the battery current increases proportionally - and can create a
    runaway condition - destroying the FETs in seconds. 4GA wire is a
    minumum for a 300W inverter - given a 12V battery, a full 2.5A-120V
    load, and three feet of battery leads.

    Never use 'modified sine wave' UPS's with line filters on the OUTPUT.
    The output waveform on these units resembles a square wave - which
    the line filters will try to correct (short) - resulting in the
    destruction of the inverter.

    You can see that inverters are not simple devices - given the physics
    of what they do - and what people want to pay for them. If you are
    unsure about any of this diatribe, - BUY AN APPROPRIATE INVERTER for
    your needs.

    Good Luck!

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