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.
Best Answer
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.