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Can one parallel two identical batteries and connect to the UPS, and will it act as a single battery of double the capacity? Or is there a problem electronically with it?

According to this answer, and commonsense battery knowhow, the only thing that matters technically about a UPS battery is that it's got the same (or compatible) chemistry and voltage output, and an appropriate charge capacity (VA rating).

However the latter's only effect is probably how quickly it runs down. I'm guessing at basic requirements - they run completely in parallel and they have similar capacities so that we don't get charging loops between them.

I'm also guessing that it's much like any other issue of parallel batteries of identical characteristics. SO if there is a problem it's one that basic electronics can handle - maybe diodes or other rectifiers? But not sure what.

Yes it's not what manufacturers would recommend. No that doesn't mean automatically it's a bad idea :) It might be a bad idea for other more serious reasons though.

Can it be safely done? if so does it need any other circuitry to ensure it goes well, and if so what's needed?

  • In addition to the considerations you mention, I would be concerned about the battery charging current. Two heavily discharged batteries in parallel will want twice the initial charging current and the UPS components might not be rated for that. – fixer1234 Aug 1 '17 at 22:04
  • There are UPS out there that allow you to attach more batteries. I have one you can daisy chain 2 batteries onto. They have to charge the batteries separately as otherwise it would draw too many amps and trip the breaker. However, the separate charging is automatic and requires no user intervention. – cybernard Aug 2 '17 at 0:41
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It is not acceptable. For runtime purposes you would actually get better behavior than you expect as the batteries will not be able to deliver the requested power before they actually run out--I've got a setup like this and the increased runtime is far more than the increased available power.

The problem comes with charging the batteries, though--I have 3x the stock batteries which means 3x the draw while charging. My system is specifically designed for this, though, and thus it is acceptable. If your system isn't thus designed you're likely to overheat the charging circuit.

Likewise, you can substitute batteries with a lower rating than the originals (I give up 12% of the performance in exchange for cutting the replacement cost in half) but you can't increase it.

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