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.

  • \$\begingroup\$ Why? What's wrong with letting the UPS charge one of the batteries? \$\endgroup\$
    – Dave Tweed
    Oct 3, 2013 at 11:56
  • 1
    \$\begingroup\$ 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? \$\endgroup\$
    – John U
    Oct 3, 2013 at 12:05
  • \$\begingroup\$ 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. \$\endgroup\$
    – Gunnish
    Oct 3, 2013 at 13:17
  • \$\begingroup\$ 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 \$\endgroup\$
    – shc csh
    Oct 4, 2013 at 1:47
  • \$\begingroup\$ 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. \$\endgroup\$
    – Doombot
    Sep 23, 2014 at 0:08

2 Answers 2


My thought would be to leave all 3 batteries connected to the UPS at all times plus add a 'proper' lead-acid charger as well. This charger would be a proper 2-stage or 3-stage charger with automatic float - equalization switching.

However, you need to be aware of a probable safety issue: the batteries are most likely LIVE with respect to ground when the UPS is connected to the incoming AC Mains. That's because there is NOT any isolation from the AC Main in most low-power UPS units.

That's one of the reasons that the battery terminals are so well insulated in small UPS units.

You can use an external charger but you must ensure that the charger can withstand being at a voltage significantly above ground and you must ensure that no one can touch the battery terminals while the system is live.


Connect the 2 batteries you want to charge from car charger through a DPDT switch. V1 UPS charging wires. V2 car charger supply

 Click the "edit" link below the image in the preview instead. -->



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