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.


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.

  • \$\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\$ – Crocodylus porosus Aug 16 '13 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\$ – Crocodylus porosus Aug 16 '13 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\$ – Bobbi Bennett Aug 16 '13 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 Aug 19 '13 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 Aug 19 '13 at 2:34

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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