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Cody Watkins avatar image
Cody Watkins asked

Sizing advice MPPT 100/20

Looking at upgrading my RV solar to a 3 panel system on a new MPPT 100/20. Existing is 2*100W on an older controller. The older panels are discontinued so I am purchasing a 3rd panel that is not identical. Battery bank is dual 225Ah golf cart (in series = 12V)

New system would be:

2x Renogy 100W Panel modules
Voc 22.5V
Vmp 18.9V
Imp 5.29A
Isc 5.75A

1x Renogy 175W Panel module
Voc 21.6V
Vmp 18.1V
Imp 9.67A
Isc 10.31A

I've calculated these 3 panels can be run in parallel with only a 2.68% loss due to the difference in panel specifications. Panels will need to be individually fused due to the max fuse rating in the event of a short.

The updated array will output 365W at 17.95V @ 20.33A.

The Renogy calculator shows it's 129% oversized. Should be a good amount due to primary usage in the northern hemisphere, and they will be flat mounted.

My primary concern is: I've calculated the array short circuit (5.75A + 5.75A + 10.31A) = 21.81A. This number is tripping me up with regards to the 100/20 recommended max short circuit.

Does this prevent me from using the MPPT 100/20? Do I need to be looking at the MPPT 100/30?

Thanks.



MPPT Controllerssolar sizing
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2 Answers
snoobler avatar image
snoobler answered ·

Officially that's a hard limit. Essentially, with more than 20A, the unit's built-in reverse polarity protection circuitry may not be able to protect the unit; however, you can find Victron folks playing down that limit on this forum.

Given that flat mounted panels rarely output max current (if ever), just don't hook the panels up backwards, and the 100/20 should be good.


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ahtrimble avatar image
ahtrimble answered ·

Have you looked into this calculator: https://www.victronenergy.com/mppt-calculator

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Cody Watkins avatar image Cody Watkins commented ·
Unfortunately that calculator only assumes that the solar panel modules are identical. You can't mix and match with that calculator. But I think Snoobler's answer above answers my question.
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snoobler avatar image snoobler Cody Watkins commented ·
You could fake it by adding up the numbers yourself, i.e., 1 big panel with an average Voc/Vmp and sum up the Imp/Isc.


Personally, I wouldn't hesitate to run that config.

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ahtrimble avatar image ahtrimble Cody Watkins commented ·

I hear you and understand.

To mix and match panels I did multiple inputs and then combined them on paper.

Glad you were able to get your question answered. @snoobler is a wealth of knowledge.

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