UPS hack info needed

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Peter Kuykendall
Peter Kuykendall's picture
UPS hack info needed

Hi Mark -

A while back you mentioned setting up a comparatively small UPS and augmenting the modest battery with a gang of deep cycle lead acid batteries.  I'm thinking of doing something similar.  My load is small, average of 500 W (or possibly less, I need to measure it over 24 hours with my Kill-A-Watt), peak under 1,000 W, but I need to keep it alive for 8 hours.  This means roughly 500 * 8 = 4 kWh battery capacity, which is 333 AH @ 12 V, so call it 4 batteries @ 100 AH each to be conservative.  Walmart has giant AGM deep cycle batteries, and with my nifty Amazon Prime membership I can get them via free 2 day shipping, or get them overnight for $4, which is tempting just to say that I got a quarter ton delivered overnight for $4 LOL.

I can buy a 600 watt UPS for $120, and the 4 giant aux batteries for roughly $100 each, so I'd have under $600 in the entire setup.

Here's my questions.  First, did you need to do any modifications to the charging circuit, or is it happy slowly charging 10 times more AH capacity in the aux batteries than it normally would?  Presumably it handles current limiting etc. just fine, so I'm guessing that if the batteries are all in an equal state of charge (likely), then it's all good.

Second, how do you test the batteries to know when they are shot?  Does the local auto shop have a load tester that can load them appropriately, charge / discharge over time, etc?  Or do you simply pull the wall plug and see how it performs over some period of time?  Or just replace them on a calendar schedule, or something else?

Third, any lessons learned?  Anything I should avoid going in?  Is there a different approach you would take, knowing now what you didn't know then?

Thanks - Pete

Mark Cockrell
Mark Cockrell's picture
Battery hack

Hey Pete,

  What I found in my experimentation is that the trickle charging circuit of the off-the-shelf UPS could handle the larger batteries, but at several orders of magnitude slower.  I never did any hard tests, but my rough estimate was it would take 4 to 5 days to recharge the collection of batteries I had hooked up.  Once a year I'd take the batteries to my local auto shop and they'd test them for me, and when one was dead I'd replace it- typically every 3 years.  I kept it pretty simple; just wired them in series to the wires inside the UPS.

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