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I use three 65Ah/12V VRLA batteries connected in series to a 36V UPS to power my computer. I will soon have two 200W/24V solar panels. What do I need to charge the UPS batteries with these panels? It's not clear to me if normal charge controllers can handle this situation, and I've received conflicting answers from the dealers I've consulted.

In the long term, my plan is to get a solar inverter and four larger batteries, but right now I'm looking for a stopgap that allows me to reuse my existing batteries and UPS for a while longer.

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3 Answers 3

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You could connect your battery string directly to the solar panel: the voltage is about right. However, your VRLA batteries are sealed, so you should never over-charge them.

Your "24V" panels probably have a peak power point around 36V, and an open circuit voltage around 48V. This is a suitable voltage range for running a "24V" power system, which actually runs at around 28V, charges at 30V, needs around 32V before losses, and needs some overhead so that it works in low-light conditions.

Running a "36V" system at about 42V, with charging around 44V, you wouldn't get very good solar efficiency out of your "24V" panel, and you wouldn't get peak charge current out of them, so they would not be 200W each (400W total) except perhaps under exceptional conditions when cold and new, running into a flat battery.

A normal charge controller can "handle" this situation, that is, it won't die, it will charge your battery, it won't overcharge your battery, it won't destroy the UPS. *Assuming the charge controller voltage levels can be configured for sealed batteries

The question is, can you "handle" the fact that you won't get 200W (each) out of your 200W (each) panels if you configure it that way?

Having said that, it's a UPS. You may never use it. You probably won't need 400W. And it's a stop-gap: your expectations don't have to be high.

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  • \$\begingroup\$ Do you mean the two panels connected in series would have an open circuit voltage of 48V? (I ask only because you say "200W" later on, where the series configuration should have a theoretical peak of 400W, right?) The manufacturer's site doesn't give much detail. \$\endgroup\$ Commented Aug 16, 2013 at 9:04
  • \$\begingroup\$ A clarification: I am in a rural area with unreliable mains supply (at least 3-4 hours without power daily), so the UPS is going to be used, and heavily. But the UPS itself will charge the batteries from the mains when it can. (I know I'll need to manually switch off the input from the panels when charging from the mains, unless I get a solar inverter that does it for me, or unless there's some easier solution that I've missed.) \$\endgroup\$ Commented Aug 16, 2013 at 9:14
  • \$\begingroup\$ Good answer David. You know, while OP is drawing power from the system, the battery voltage will be pretty close to optimum for the 24V panels, so in his case it is a pretty good stop-gap. \$\endgroup\$ Commented Aug 16, 2013 at 12:22
  • \$\begingroup\$ @Crocodylus porosus, 48V o/c is typical for a "24V" system. The manufacture has an equiry button -- they will be able to tell you what the o/c voltage is for their panel. Use a diode to prevent leakage back through the panels, unless that is already included. You need to disconnect to prevent overcharging, but not just because an inverter is connected. \$\endgroup\$
    – david
    Commented Aug 19, 2013 at 2:21
  • \$\begingroup\$ If the panels are 24V each, for a "24V" battery system, you should not put the two panels in series. The O/C voltage is too high for direct connection to a battery or to a "simple" 36V nominal charge controller. Typically, "24V" means "for a 24V system, which is actually a 28V-30V system". If it turns out that in this case "24V" means something different, you can direct connect if you don't overcharge, you can use a simple charge controller if the o/c voltage is not too high, you won't get good efficiency unless you match the panels to the batteries. \$\endgroup\$
    – david
    Commented Aug 19, 2013 at 2:34
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The correct way would be to connect the solar panels in series, to get 48v and then use a MPPT charge controller which can match the input voltage to the batteries correctly.

Most charge controllers are designed for 12, 24v or 48v but you may find one which is able to support 36v.

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There are a number of things to consider. a) If you have two 24volt panels you still need a 24v solar controller. Assuming you have this and the panels are about 240watts each I would suggest you put the panels in series which will give you an open circuit voltage of around 90volts. Most good controllers can handle this input voltage. This will allow the panels and controller to produce output in challenging times like overcast or cloudy days. b) what you will then need is a 24v dc to 12 dc converter that can act as a battery charger as well, which some don't. This will need to be rated at around maybe 20amps to be able to charge your three 12v batteries. Wire them in parallel. They will normally come with overcurrent protection.This will keep all your batteries charged evenly. c)then you can run any 12volt load from these batteries like a fridge lights etc. Hope this helps

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