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Old 12-14-2021, 07:55 AM   #57
Behemot
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City & State: Prague, 504'52.22"N, 1423'30.45"E
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Default Re: Powerware 9120 Online UPS

than you can only solder piece of wire to it as a last resort

that pretty much looks like linear regulator for each accumulator, burning the excessive voltage on the bulb; while that works, it is not very efficient, as the lamp will be drawing about 250 mA, that's twice what accumulators do without this circuit on float (which also tends to equalize - a bit), that is going to stress the charger a bit more (no problem for things like Smart-UPS which output several amps; a problem for thing like Back-UPS RS which outputs about 1 A on the charger)

plus it will consume electricity which could be a concern these day, if it was 1 W per accumulator, with two its over 100 CZK a year just for that, and new Yuasa SW280 costs me about 500 with 5+ year rated life; so pretty much no benefit here, it will cost same or more per year of lifetime than a new accumulator!

I think what would be enough, if the float voltage is not too excessive, to just provide a parallel path for current, similar way as in old half-bridge PSUs for balancing input filtering caps - but some large (10k+ depending on voltage) resistor in parallel with each accumulator, and a schottky diode in series with each such resistor (so it does not discharge them as much when the UPS is turned off; it will still do a bit because of leakage current over the diode).
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