3 solar panels, 2 in parallel, 1 in series?

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Nickus de Vos asked Jun 20, '20

3 solar panels, 2 in parallel, 1 in series?

Hi all

I want to permanently mount 3 solar panels on a caravan roof in a 12V system.

The 3 panels are identical, 100W, VOC 22.7V 5.5A and I have a SmartSolar MPPT 100/20.

I don't want to connect all 3 in series because of shading on the panels, with a caravan you can't exactly plan shading, shading is a given.

I also don't want to mount all 3 in parallel because of the 5V rule, I have a Lifepo4 battery which voltage stays above 13V for a very long time.

I can't get clarity on this, I read mentions which says yes it's fine and others which says it's a big no. Would I be able to connect the panels 2P + 1S or the other way around 2S + 1P?

I know 2S + 2P is good, but what about odd numbers?

Nick

solar sizing

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Kevin Windrem commented · Jun 20, 2020 at 08:49 PM

I did some shading tests on my system with 4 panels and found the best overall topology was all 4 in series. The panels all have bypass diodes so if one panel drops out, the current flows around the cells via the diodes. Your mileage may vary. Only way to know the best approach is to experiment.

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ceriw answered · Jun 20, 2020 at 05:30 PM

All parallel strings need to have the same length so 2s in parallel with 1 is not going to work. Your options are 3s or 3p with those panels.

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Modifications Space

Node Red Space

Answer 1 · 2020-06-20T17:45:12Z

Nickus de Vos answered · Jun 20, 2020 at 05:45 PM

Ok plan B.

If I add a 4th panel to make it 2S 2P I'll be putting 36V 10A in to the MPPT which is perfectly within the specs, the max the 100/20 controller can utilize is however only 290W according to the manual.

Will it be fine to connect 400W to the MPPT even though it can only in effect utilize 290W?

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dazey77 commented · Jun 20, 2020 at 05:53 PM

Yes, you can do that.

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jwfrary commented · Jun 20, 2020 at 07:14 PM

99% of the time your panels won’t be putting out full power anyway once the mppt reaches its limit it just won’t allow any more power through. So you should charge earlier and later in the day but just miss off a little bit of panel efficiency. But I wouldn’t worry about that.

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Nickus de Vos jwfrary commented · Jun 21, 2020 at 08:54 AM

Thanks, this was my thinking as well, I'm not too worried about getting 400W in to the batteries, but getting a reasonable charge from earlier in the morning till later in the day is more important to me.

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Answer 2 · 2020-06-20T19:33:00Z

raymond-nicolas answered · Jun 20, 2020 at 07:33 PM

Idea and question to the pro:

What's about 2 + 1 whith Anti-feedback diodes?

- __/-------[__]--|>¦---\___. +

. . . . . \--[__]-[__]--|>¦---/

Is that possible?

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ceriw commented · Jun 20, 2020 at 08:54 PM

The 2s panels will have a maximum power point voltage that is always above the Voc of the single panel so the charge controller will follow the 2s and the single panel won't contribute any power as it will always be reverse biased.

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