kimsails avatar image
kimsails asked

Choosing Victron controller(s) for new system


I am planning to install a new small solar system on a sailboat. There is no existing equipment or installation.

The plan is for 4 100W renogy compact panels and a 400 or 600 Ah 12v LiFePO4 house bank. Panels will be mounted on a hard bimini with a curve from port to starboard. I expect that there could be some shading on the two port panels or on the two starboard panels, but most of the time I should have one pair that has no shading. The curve of the bimini will likely cause each panel, shaded or not, to get a different amount of sun.

I am trying to figure out the best setup of Victron controllers - one big one? two smaller controllers? four controllers? I'm presently thinking two 100/20 MPPT controllers, each attached to two of the panels wired in series. Being new to solar, though, I am not sure if I have thought through all of the potential implications.



MPPT Controllers
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3 Answers
wkirby avatar image
wkirby answered ·

You seem to be thinking along the right lines in terms of shading.
The ultimate in performance for shaded panels is for each panel to have its own controller. This setup is OK for 12V systems like you are planning, but if you did ever upgrade to 24V a single panel per controller would not work. Multiple controllers is obviously more expensive of course and then there are additional runs of wiring etc.
I think your best option is to have two controllers, one for port and one for starboard. Wire the panels in parallel to each of the charge controllers to minimize shading effects between those two panels.
A 100/20 for each side would be adequate with 240W -280W of charge capacity on a 12V battery system.Two 100W panels on each controller (200W) will be well catered for.

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JohnC avatar image
JohnC answered ·

Hi Kim. I suspect those panels will be '12V' nominal, and if so I'd wire them in series, so 2x mppt's, 2 panels each, series wired.

And to save a few $, I'd choose a pair of 75/15's. If your batts are rated (say) 14V under charge, then 15A x 14V = 210W. Enough..

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wkirby avatar image wkirby ♦♦ commented ·

Good point John, nice thinking.

I'd actually had a quick peek at the specifications of the panels and they were around 22Voc, so a 75/15 controller would be perfect. Better value than a 100/20 but nearly no room for upgrade in the future.
Kim, it's for you to decide.

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Kim Young avatar image
Kim Young answered ·

Thank you JohnC and WKirby for your responses.

I have tried using the Victron MPPT sizing calculator. It seems to say that two 75/15 MPPTs would be acceptable whether the two panels on a controller are in parallel or series. At least, it shows this configuration as "Accepted", however, it notes that max current will be limited to 15A, and is "power limiting". Changing to a 100/20 MPPT, the calculator raises the current passed up to 17-18A depending upon temp. I don't really know how big of a deal losing those last couple of "power limited" amps is to the batteries, But being on a sailboat I am going to want to eek out as much power as I can from the panels that I can fit.

The calculator seems to be all green and happy with four 75/10 MPPTs, two 100/20 MPPTs, or one 100/50 MPPT. The calculator seems to be happy with these controllers whether the panels are in series, or parallel, or 2S2P in the case of the single 100/50.

Thinking about series or parallel, my understanding was that the controllers want enough series to ensure they have high enough voltage to be able to charge the battery, and the rest in parallel to reduce shading impacts. My initial thought was that I should keep at least two panels in series to keep voltage up. But the calculator seems to suggest that this is not necessary, in which case maybe all parallel is a better option.

Four 75/10's would be $320, two 100/20's would be $314, one 100/50 would be $294. The controller cost difference does not seem to be real factor. It would seem that the bigger issue is amount of wiring vs. whatever efficiency is gained from two or four MPPTs over one.

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kai avatar image kai ♦ commented ·

Don't really have anything technical to add beyond the existing two excellent responses - just wanted to upvote the comment on upgrade path. Worthwhile thinking about how you can upgrade without replacing a whole lot of equipment (and all the mounting, and wiring, and semistructural mods on the boat that goes with it).

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JohnC avatar image JohnC ♦ commented ·

All good. I don't use the 'calculator', First Principles for me. Some points:

There's both a Voc and an Isc limit on Victron mppt's. Voc is multiplied out by the number of panels in a string, and for a mppt cc (as distinct from a pwm), a higher Voc (& Vmp) will provide better response to dawn/dusk/cloudy conditions. With 22Voc panels I'd expect this to be very noticeable if all in parallel or 1 per mppt. That's why I'd series at least 2 of those panels. Poor light doesn't show in panel spec sheets, you gotta allow for it yourself..

Panel Isc doesn't increase by longer strings, it adds when you parallel them. So for those panels (6.24A ?), four in parallel would be ~25A. Too much for a 75/15 (see the spec sheet), but within spec for 2x of those in 2S2P panel configuration.

Fine too to allow for future expansion, for the buyer to decide. Comes with a need to match the extra panel specs strung suitably, and if you expect to be able to do that, great.

Cheers too WK for your graciousness. :)

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