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whitewhale avatar image
whitewhale asked

Looking for suggestions - Best way to hook up 12, 100 Watt - 18v panels to my MPPT 150/85

Folks,

I have an off-grid installation with 12 each of 18 VDC Solar PV panels producing 100Watts in direct sunlight (in the American Southwest, if that matters to you). My battery system is 4 each of 6 volt Duracell Ultra batteries which are wired series-parallel (producing 12VDC). I already own the MPPT 150/85 MCA, the feed wires are run, I have some 4-to-1 parallel cables which I was considering hooking 3 banks of 4 panels in parallel (there are 3 MCA input pairs on the unit)...

I believe that will produce 18-21V at about 22 amps per pair... which should be more than enough in theory to do what I need it to do if I understand my requirements correctly.


My question to you: Is there a more efficient way to charge these batteries with the current setup I have? I.E. - Should I do 6 banks of 2 panels in series (~36V), then parallel 2-to-1 down to the 3 inputs with 24V charging?

What is the best way to handle this considering my low wattage panels and the multitude of them?



(Panel Specs)

12 of the following panels -

Specification:

Optimal power [Pmax]: 100W(±5%)

Working voltage [Vmp]: 18V

Working current [Imp]: 5.62A

Short circuit current [Isc]: 5.97A

Open circuit voltage [Voc]: 21.6V

Dimensions: 105 x 54 x 0.25 cm/41.3 x 21.3 x 0.09 inch

Net weight: 2.6kg/5.73 lbs.

Working Temperature：-40~+85℃

Solar Panelsolar sizingdesign
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4 Answers
wkirby avatar image
wkirby answered ·

Thank you for the detailed information, it's very helpful.
18 cell panels' Voltage range is a bit too close to that of a 12V battery bank. In cloudy conditions the MPPT would not produce optimally and may even drop out. You wouldn't harvest the full potential. Only in full sunshine you'd probably be fine.
I'd suggest wiring four of the panels in series to make three strings like this. Then you can simply wire each string into the MPPT directly.

Six strings of 2 in series is also OK, and recommended if you have shading during the day, although the wiring is a little bit more complicated.

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whitewhale avatar image whitewhale commented ·

Thanks for the input. I'll give the 4x3 in series a try. I get about ~1 hour of shade on one bank of two panels currently (this season, but it clears up a bit as the year carries on and the sun changes position) so that should be fine. Today it's a bit overcast, but I may get out there anyway and finish the connections.


BTW, do you happen to know if there is anything I'd need to change from default settings within the controller to allow for that configuration? From what I'm reading I only set the battery voltage (or it senses it) and then it's pretty much plug and play unless i need to use relays or have other special needs. Does that logic seem legit to you?

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wkirby avatar image wkirby ♦♦ whitewhale commented ·

The PV side has no settings. The only strict rule is to keep below the 150V limit, which you are.

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whitewhale avatar image whitewhale wkirby ♦♦ commented ·

Thanks again!

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petajoule avatar image
petajoule answered ·

"4x3 wiring" - for preciseness it's called 4s3p.

And while we are at it - "although the wiring is a little bit more complicated" - if you are a true diehard, you can muse on how exactly to do the "p" part of the wiring:

https://hal.archives-ouvertes.fr/hal-00485197/document

The research is already 10 years old - should be common knowledge by now.


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whitewhale avatar image whitewhale commented ·

Thank you!

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whitewhale avatar image
whitewhale answered ·

Folks, can someone please confirm the polarity of the MC4 connections on the bottom of the unit. I've dug through all the areas of the manuals and technical docs.

I wired this back to the mounting area as black negative, red positive, but the MC4 connections (male/female) do not specify which is which (an oversight?). To make matters worse, the cables I see available online are a mix of positive male, positive female and now I'm scratching my head as to what to do.


Thanks for all the help!

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whitewhale avatar image whitewhale commented ·

Here I was thinking that using MC4 connections would make my life easier but in reality... not so much.


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whitewhale avatar image
whitewhale answered ·


Thanks for the allegory with the response. Entertaining :-D lol.

So by your statement above, the red-cock ring should be POS +. I am sitting here with a black wire with a red cockring = NEG -.

I bought two pairs for each bank of these and basically confued the hell out of myself by making a very long gender changer by mistake - I have resetable fuses in line and I wired them back following your diagram and common sense (ie, Red = Positive = +) all the way through. Except I failed to consider the connection on the Charge Controller fully, hence the issue. I have some replacement MC4 ends coming today. I'm glad I asked and I wished I just bought the black cables all around. Lesson learned.


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