# Is it possible to charge 3 parallel batteries without sending charge to the UPS?

I have a UPS (uninterruptible power supply) with a 220VAC input designed to be used with 12V batteries. It has a rated capacity of 400 watts and I've placed three 12V car batteries in a parallel arrangement so it will last a long time.

The drawback is when used too long the UPS can't charge all three 12V batteries so I'm using a car charger to charge two batteries in parallel while one battery is still connected to the UPS. If possible I'd like to use a diode (or something else) to switch so that I can charge all three of them without sending the charge voltage back up to the UPS.

• Why? What's wrong with letting the UPS charge one of the batteries? – Dave Tweed Oct 3 '13 at 11:56
• Ignoring the various reasons it's a poor idea to use/charge three un-matched batteries in parallel... Why can't the UPS charge three batteries? Do you need to limit the current to them perhaps? – John U Oct 3 '13 at 12:05
• The UPS probably charges the batteries far too slow for it to be feasible to charge three batteries in parallel. Therefore it makes sense to charge via an external charger. – Gunnish Oct 3 '13 at 13:17
• Is there a way guys to put a diod,resistor or probably a switch for the battery charger and not to affect the ups charge. Thanks guys – shc csh Oct 4 '13 at 1:47
• Diode is not a good idea in the sense that even if you could use one (or a couple in parallel) that would support the current flowing to the battery, the voltage drop across it would make the battery charger overestimate the battery's voltage level. – Doombot Sep 23 '14 at 0:08

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