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hovr avatar image
hovr asked

17v vs 18v Vmax panel with 100/20 MPPT

Hi all,

I am building solar system for a small Airstream trailer with very limited roof space, my goal is to get > 200w panels. Due to roof top space limitation, My panel selection is limited to the 2 options below.

1. Renogy 100w + 50w + 50w (Voc: 21.6, Vmax: 17.9) total 200w

2. Newpowa 75w x 3 (Voc: 20.3, Vmax: 17.0) total 225w


I am leaning towards #2, as it offers more watts and having the same panels are always a plus.

But since I need to install them in parallel due to roof top A/C unit shading, I am concerned that the ~1v Vmax drop in Newpowa might not work well with my Victron SmartSolar 100/20 SCC and LiFePo4 battery bank (13.4v fully charged)


According to SCC manual the

Voc needs to be 13.4+5 = 18.4v for SCC kickoff charging

Vmax needs to be 13.4+1 = 14.4v for SCC to staying in charging


This is very close to the spec of Newpowa panels (20.3v, 17v) and I am wondering if it's a bad idea to use these panels - panels won't be able to supply high enough voltage for SCC on a cloudy day? The Renogy panels would have a more comfortable margin and should be used instead?


Thanks in advance!




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2 Answers
launacorp avatar image
launacorp answered ·

Don't look at the Vmax only, check Vmp too.


Vmax is important (+5V spec) to initiate Charge, Vmp (+1V spec) is important to stay at Charge.


By the way, how much shadow do you have? Is it really so much that you must parralel the panels?


Think about that the panels most have bypass-diodes so it could be a possability to connect the panels in series and have no drop in energy harvested.

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hovr avatar image hovr commented ·

Thanks, sorry in my post I used Voc and Vmax, actually by Vmax I really meant Vmp.

attached is a picture of a similar Airstream, the AC unit on mine goes a few inches forward than the one on picture and the antenna at the top will be removed for another panel. The AC unit is a bit taller than the picture shows, so unless the sun is right above at 90 degree, there will always be 1 or 2 panels in partial shades(corners and edges). I have also read some folks' real world experience with RV panels in series and in parallel and they got pretty bad result when connect panel in series. as you don't have much control over how trailer is parked.

So with 17v Vmp, and 15feet 8awg wire, I think I will lose another 0.1v before voltage reaches Victron SCC. that gives me 16.9 which is still above 14.4(4s fully charged)+1 = 15.4

but that's Vmp and I don't know what voltage I will be getting when it's not in ideal condition?

so :

Renogy panel. 17.9vmp - 0.1v = 17.8v

vs.

Newpowa panels 17vmp - 0.1v = 16.9v


would this make enough difference to the Victron SCC that I should go with Renogy?


Thanks!


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jvan avatar image
jvan answered ·

Hey @hovr,


I notice there has not been a definitive answer here. I have the same dilemma as you. I have read a few reviews on Amazon by people who have bought Newpowa panels and used them with a Victron MPPT, and they are quite happy and generally give their system 5 stars.


I want to follow up to see what you ended up doing -- did you stick with Newpowa, and if so, how'd it turn out? Or did you go with another brand?


Thanks!

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