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Help running a UPS on capacitors

Discussion in 'General Electronics Discussion' started by shiekh, Oct 22, 2010.

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  1. shiekh


    Oct 11, 2010
    One can get used 3000 Farad, 2.7V capacitors
    for $20 - $30 each, depending on quantity.

    My thought was to use them in a standard UPS, assuming they will stay balanced when in series... one might put a resistance across each to ensure they are equalized (chosen to carry significantly more than the leakage current of 5.2mA); this equalizing bridge would be dissipating a little less than 1W, somewhat wasteful. 2.6V Zener diodes would make for a better solution.

    In a traditional UPS things are charged to around 14.4V, so 6 capacitors ($150) should do; now if the UPS shuts down at say 10.2V, then one has 2.4V on each cap which drops to 1.7V, so

    Energy available = 0.5 3000(2.4^2 - 1.7^2) 6 = 26,000 J
    (50% of what is actually carried)

    This would hold up a 250W load for about 100 seconds; not so great but it might still be useful. The UPS may also be happy when load testing the 'batteries' as such tests last a good bit less than a minute.

    Now the question, will this actually work as anticipated above? as it is a bit expensive for a project.

    Problem, if one of the resistors/zeners in the equalizing bridge failed open, then that capacitor would be exposed to the full voltage and fail... I anticipate that the failure mode of such a capacitor is not pretty...

    I have had two APC UPS units suffer from the batteries going into thermal runaway, and wonder if the above might save me from having to keep buying batteries.


    Seems it has already been done
    Last edited: Oct 22, 2010
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