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phatmatt avatar image
phatmatt asked

MPPT Sizing

Hi everyone new to this and currently looking at adding panels to my existing 12V system (caravan). To avoid any of my own confusion I thought I would ask you experts. I currently have a 175W panel Optimum Operating Voltage (Vmp) 18.1v Optimum Operating Current (Imp)9.67A Open Circuit Voltage (Voc) 21.6V; Short Circuit Current (Isc) 10.31A This is wired up to a Victron 100/20 MPPT controller. I want to add another identical panel (175W) wired in parrallel as the current set up isnt replenishing/recharging the battery quick enough for the use/amperage drawn from it. My question is will my 100/20 MPPT controller be suitable for the new set up or will I need to buy a higher spec one? Thanks in advance.

MPPT Controllers
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3 Answers
nickdb avatar image
nickdb answered ·

This is what the MPPT calculator is for

https://www.victronenergy.com/mppt-calculator

Using your specs, and you're missing a few, it looks like that mppt will be fine.

Under perfect conditions it may clip a bit or run near its maximum, but perfect conditions rarely exist.

Run it and see how it goes, if you're hitting the max 20A out too often or it is getting too warm, then you could size one up.

Headroom is always a good idea, but there is no point over-sizing either if you don't need to.

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JohnC avatar image
JohnC answered ·

@Phatmatt

You should really wire the panels in series. The Isc limit on that mppt is 20A and 2x parallel panels will exceed that. The doubling of Voc and Vmp will be better in low light or sun angle, where the one panel might shut down anyway.

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nickdb avatar image nickdb ♦♦ commented ·

The calculator is actually happy with it on the 100/20, depending on Impp (which wasn't provided), but yes, series is a better call.

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phatmatt avatar image
phatmatt answered ·

@JohnC @nickdb Thanks both for your comments. Much appreciated. Dont think this will ever reach optimum in terms of perfect conditions. Still getting my head around parrallel versus series. @JohnC are you suggesting that an in series set up would be better for low light conditions? My caravan is sited in a clearing in piece of woodland, so does get shaded at certain points in the day.

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JohnC avatar image JohnC ♦ commented ·
@Phatmatt

Yes. Bear in mind that your mppt will track for Vmp, and in poor light (particularly dawn and dusk), Vmp can be much lower than panel spec. Voc must be 5V higher than Vbat to start, and Vmp must remain 1V above Vbat to continue producing. Your mppt has a 100 Voc limit, that's a feature you can use to avoid getting near those limits. You mightn't see much power produced at dawn/dusk, but some is better than none at all.

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