winnie avatar image
winnie asked

MPPT sizing qustion

I just want to be sure about the correct Victron MMPT solar controller to choose. I will have 4 solar panels at 320W and 33.7V Imp wired in series. [Full specs below.] I believe I need to go with the 250/100 MPPT because I would have 40.1V Short Circuit Current (40.1V x 4 = 160.4V). I did use the Victron MPPT Calculator, but I just want to verify. Thanks.
MPPT SmartSolarvictron productssolar sizing
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1 Answer
seb71 avatar image
seb71 answered ·

That's open circuit voltage, not short circuit current.

Yeah, in case you wire 4 of those panels in series you would need a 250/xx solar charger. And those are expensive to be used for a small 1280W PV array.

Do you already have those panels?

What is the battery voltage?

You could wire strings of only 2 panels in series and then put those strings in parallel. But this would not be optimal for a 48V system (48V battery), because the PV array voltage would not be much above the battery voltage.
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winnie avatar image winnie commented ·

Thanks a lot, seb71.

I got the panels for less than half-price through a friend, so I need to build the system around them. I'm probably just going to go with a 12v system (my friend also has access to six 100Ah Battleborn 12v batteries). My power needs aren't very high, so 600Ah should be a nice start. Just have a DC fridge/freezer, some DC lights, and I'll go with a smallish inverter to run fans, computer, etc. Heat is supplied via a wood burning stove. It's a small home in the shade with plenty of ventilation, so hopefully a few fans will suffice. The panels won't be in the shade obviously.

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seb71 avatar image seb71 winnie commented ·

So you could not buy two more of exactly the same panels, I guess. And maybe you would not have space for two more panels anyway.

I think you can connect those 12V Battleborn batteries in series (strings of two batteries in series, then 3 strings in parallel), for a 24V system (just make sure they are charged individually before that - they must be at the same level. You can use the solar charger to charge them in parallel and then rewire the batteries in series/parallel).

If you go that route, for DC devices, either get them for 24V DC (if you don't already have them) or buy some 24V/12V DC-DC converters.

And then get a 24V inverter.

The advantage of a 24V system would be that you deal with lower currents. And also cheaper solar chargers because of that (for the same PV array power: instead of a 100A charger if using a 12V battery, you only need a 50A charger if using a 24V battery). Voc restrictions remain the same in both cases.

Instead of getting one big charger, you could also get two smaller solar chargers (make sure they are SmartSolar, and not BlueSolar) and use two panels on each. Then use VE.Smart Networking to synchronize the charging. Compare the local prices to see which variant would be cheaper (taking into account the extra wiring). With two solar chargers you also can orient the panels in different directions, to better match your power needs during the day. Also you have some redudancy. On the other hand, with one charger it's a simpler system.

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Related Resources

MPPT product page

VictronConnect manual


MPPT calculator

MPPT codes

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