System design....how is this?....

Estragon said:

1.  It should be ok to put 4 panels (or series strings thereof) in parallel, but each should have a breaker/fuse.  I'm not a big fan of flexible panels unless you really need the flex (eg boat), and imho you shouldn't need to install blocking diodes for modern panels.

2.  With 2 strings of 2 panels,  you'd have Vmp of ~35v, which should be ok for an mppt controller into a 12v nominal bank.  With 4 panels in parallel, you'd have Vmp of ~17v, marginal for an mppt controller.

3. I don't see any obvious problems with gear.  Seems to me you went through the arithmetic on loads in another thread.

BB. said:

The question about MPPT or PWM controllers... For small systems (short distance from array to charge controller+battery bank), a PWM controller+Vmp~18.0 volt array works fine.

HOWEVER, if you plan on installing the panels some distance from the RV, then it is possible that an MPPT controller with Vmp 2x or 4x higher voltage (two panels in series or four panels in series) would be an interesting option.

More or less, you want roughly 1% to 3% drop from the solar array to the charge controller (and for the controller to battery bank, you want a maximum of approximately 0.05 to 0.10 volts)... With low voltage/high current DC, the only two ways of addressing the voltage drop is short and/or heavy cables. For controller to battery bank, that is not bad (short cables are what you want anyway).

But for Solar array to Charge controller, long distances can be a big issue. to get a 0.18 to 0.54 volt drop (1% to 3% of 18 volts Vmp-array), you need short cables (you want long), or pretty heavy cables.... Just to give you an example of the math:

• Vmp: 17.1 Volts
• Voc: 21.4 Volts
• Imp: 5.9 Amps
• Vmp=17.1 volts, 4xImp = 23.6 amps; drop 0.54 (3% * 17.1v)
• Vmp-array=4*17.1v=68.4 volts; Imp=5.9 amps; (3%*68.4 volts=) 2.1 volt drop max

First system: low voltage array, high current array, 14 AWG "extension cord"... Using a simple voltage drop calculator:
Voltage=17.1; amps=23.6; 14 awg copper cable; max drop 0.54 volts:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=17.1&phase=dc&noofconductor=1&distance=4&distanceunit=feet&amperes=23.6&x=0&y=0

4 feet of 14 AWG:
Voltage drop: 0.48
Voltage drop percentage: 2.79%
Voltage at the end: 16.62

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.6465&voltage=17.1&phase=dc&noofconductor=1&distance=50&distanceunit=feet&amperes=23.6&x=61&y=16

50 feet of 3 AWG copper cable:
Voltage drop: 0.47
Voltage drop percentage: 2.72%
Voltage at the end: 16.63

Now, the same power (400 Watt array) with 4 panels in series, using 14 AWG cable--how far?
Vmp-array=68.4v; Imp-array=5.9a; Vdrop~2.1 volts max:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=8.286&voltage=68.9&phase=dc&noofconductor=1&distance=70&distanceunit=feet&amperes=5.9&x=0&y=0

70 feet of 14 AWG copper cable:
Voltage drop: 2.09
Voltage drop percentage: 3.03%
Voltage at the end: 66.81

Regarding "remoting" your panels... 1) Security (panels can walk, locking/driving stake in ground, etc. to keep panels safe. And 2) people can drive away and forget the panels... I suggest using a non-locking plug and receptacle (not use a twist lock)--If you drive off, the cord unplugs and does not drag your expensive panels behind you.

So--The desire to mount your panels remotely (park in shade, panels in sun)--It may justify the more expensive MPPT type charge controller over the less expensive PWM.

-Bill

BB. said:

You are very welcome 1911,

Please feel free to ask questions and challenge my assumptions.

From behind a keyboard, I am not always sure I was clear enough in my (rather long ) explanations.

-Bill