Series PV for RV?

[Jake]

Need help deciding!?
Have been wondering how I should configure my four 120 watt solar panels on my new RV. Each panel when connected to the MPPT produces ~18 volts @ 6.67 amps. In Parallel that would be a maximum of ~ 26 amps @ 18 volts to the 40A MPPT. In Series that would be ~6.7 amps @ 72 volts to the MPPT with a max input of 100V.
This brings me to the LARGEST Pros & Cons of each configuration. Parallel PV’s will require larger Gauge Wire and more wire. Series has the “Shading” problem! I could get around the Shading Problem by installing Bypass Diodes (100v 15A Schottky).
Both configurations seem feasible but I’m calling on the DIY Experienced Followers for their Knowledge Bank! Maybe I’m not seeing all the Advantages but mostly the Disadvantages of each, or even maybe a combination of 2S2P?

 

[gnubie]

Your panels should already have bypass diodes in the connection shell on the back so that may not even be something you need to consider.

MPPT controllers do like a bit of headroom to play with. Victron MPPT controllers need the PV voltage to go 5v over battery volts before they will start to charge and once charging the voltage must stay 1v over battery. Epever controllers often need the PV to stay 2 volts over battery. You've got your head around the issues really.

My preference would be 2s2p. You get less wiring to deal with, lighter wiring, more voltage for the MPPT controller to work with, some defence from shading, and less connections to combine.

 

[Jake]

Gnubie.... Thanks so much! My only question now has to do with the 2S2P configuration. Yes, all PV’s have bypass diodes but when one panel of the 2S is shaded and the bypass diode does its thing, the voltage from that leg drops to ~18V from the normal ~36V. Does that mean the other unshaded 2S panels is all the MPPT sees Due to its max Volts from 36??? Seems I’d be loosing 120 watts from the shaded panel and the reverse biased non-shades panel would not add power to the MPPT...
Okay, I just read what I wrote and I think I may be looking at it wrong! E x I = P..... both legs are supplying P? Do they add even if they both aren’t at the same Voltage?

 

[TCgreg]

Every test I have seen connecting panels in series that have bypass diodes including my own have shown that shading even a small part of one panel still has a dramatic effect on production. For that reason I installed my panels in parallel and cabled up to compensate for voltage drop. When there is no shading on the panels series connected produced a small advantage but in those conditions I am not wanting for production anyway.

Since Will has a backyard full of panels and this is a pretty quick and easy test Maybe he will do a series connected test with shading and see what results he finds.

 

[Jake]

Can Hardly wait for Will To do the YouTube Analysis. Will do my own test and see if we both agree ?! Will adjust my configuration braised on the results!
Thanks TCgreg for the Quick feedback! ?

 

[TCgreg]

Can Hardly wait for Will To do the YouTube Analysis. Will do my own test and see if we both agree ?! Will adjust my configuration braised on the results!
Thanks TCgreg for the Quick feedback! ?

Here is an interesting write up on the subject of shading and bypass diodes.

Here is a experiment between series and shading with some pretty decent controls. Note how even a very small shadow drops the production down below the output of a single panel in series. You can see a similar test done by the youtubers Gone with the Wynns.

My feeling is that larger panels with more bypass zones and higher voltage would probably do better. 12v panels like you (and I) have are typically only 2 zones. Of course if you seldom camp where there is shade and there are no other shadows from vents, air conditioner etc.. aside from clouds series is the obvious best choice for a few reasons. For myself though we like parking in the shade when it is hot out and having all 6 panels each contributing what they can worked far better for us. If the panels you are using have MC4 connectors you could always build the system for parallel with larger cables and play around with them and see what yields your best production. Heck you could even run them all in series when you are in sunny locations and switch them to parallel when you get into areas with trees and such. Someone should build a combiner box that could be switched.

 

[Off Grid Circuits]

I built my 1800 watts @ 3S2P configuration. All-in-all, you will need to keep withing the minimum and maximum voltages of the charge controller. On an RV with such short runs of wire, I never second guess 10 gauge. There are considerations to all ways to do it and most are not wrong.

My next build is coming soon and that will be an interesting design of six 200 watt panels in a 3S2P configuration and four 300 watt panels in a 2S2P configuration to separate charge controllers in parallel (Victron, of course). I'm doing this for many reasons to consider here including available roof space, wire routes, battery Bank voltage (24V) and more. Often, you have to make considerations of what works best for the individual situation. I do system "designs". These are different for every single client and no client is the same.

 

[Jake]

I have a problem with the 2S2P configuration. If one panel in the 2S gets shaded, the output from that section drops to 1/2 voltage, Right? (ie ~18 volts) The other side still produces full voltage of ~36 volts. What happens at the Charge Controller with 2P inputs? The 18 volt is over powered by the 36 volt and is blocked by reverse biased diodes? It seems the entire solar array drops to 1/2 power...
Okay now consider a 4S configuration. One panel is shaded and the bypass diode drops that panel offline. Voltage drops from ~72 volts to ~54 volts but the current stays the same. Power drops to ~75% vs ~50% of the 2S2P...
Am I missing something??