Solar complexity- mixing different watt panels mounted on tow vehicle and trailer

rfoubi

Observer
So, long ago I mounted 2 x 30W Renogy solar panels on top of my ski box. The ski box moves between our vw van and our subaru. On the VW, the panels were plumbed via a quick connect into a 20A charge control to charge the house battery. Now, as a family of 4 we are more often using the subaru/trailer combo to travel.

The trailer has a 100W Renogy panel on a long cable to allow fixed or movable installation. Its wired through a 20A charge controller to the 2 trailer (house) batteries. The Car 1 normal starting battery, wired via 4 Ga wire and anderson plugs on the bumper to the trailer to provide full alternator juice to the trailer house bank while running. There is a 12V relay in that system to not allow the car/trailer batteries to draw off each other when stopped.

Basically I am trying to add the ski box 60W of power into the mix

Can i mix the 60W of panels on the car with the 100W panel on the trailer and plug both into the same charge controller? I have read not, or something about this being voltage or current limiting based on the smallest panel. Note: I do not have any fancy MPPT controllers, these are simple PWM.

Alternatively, I bought a second charge controller. I know I can use multiple power sources on the same system (shore power, alternator, solar) so i assume i could wire the car panels to one controller and then feed that into the battery bank separate from the 100W panel?

The other option is to put the second controller on the car, use it to charge the start battery on the car, and then assume that if either the tow vehicle or trailer solar panels were in the sun it would probably put the system over the combine voltage of the main relay, and then it would all act as the same system anyway?

Sorry this got long and complicated. Heres a diagram.


And a pic for attention

 

john61ct

Adventurer
Best if all the panels connected to one controller are identical.

Especially with MPPT.


Really the ideal is big enough panels selected so that each has its own MPPT, 1:1 ratio.

if you don't care about efficiency, using PWM at least voltages should match. But so long as total SC specs are not exceeded, experiment, likely not going to burn anything.
 

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FJR Colorado

Explorer
I have a 160w and 100w feeding into the same Renogy Rover MPPT controller. Runs fine.

The controller only knows how much you are pumping into it. It has no idea what is connected. It's not like a node on a computer/server system with an IP link, or even a serial link.

The multi-panel connectors are basically "Y-connectors."

All that said, you'd have a bunch of mounting and wiring to add not much more input. I would just add another 100w panel (or more if you have room for something larger say a 160w); cost is fairly minimal.
 

dreadlocks

Well-known member
diodes will cut em out if they cant keep up with a stronger panel and the'll get pulled down.. whichever panel is doing highest voltage at the time is gonna win, you'll store more power with a seperate controller for seperate panels..

you put multiple panels on a single controller when its an array of matched panels all mounted together with same tilt, all in the same environment and all more or less always outputting the same as everything else in the array.. in a two panel array with mismatched panels you'll likely see one or the other contributing but not the combined output of both of em.. if they were identical panels you'd still likely see one or the other mounted like that until both are in identical lighting conditions and then you'll see em both combined.
 

rruff

Explorer
diodes will cut em out if they cant keep up with a stronger panel and the'll get pulled down.. whichever panel is doing highest voltage at the time is gonna win
Diodes where? I've never used MPPT. Seems like the controller will only see one voltage... which the panels will "compromise" on. And the amps for each panel will be whatever the panel can produce at that voltage. I see what you mean about the panels seeing different sun and different output; the optimal voltage will then be different for each. But how much will you lose? Doesn't seem like it would be much based on the curves I've seen.

This one for instance. If you have one panel at 100% (optimal ~17v) and the other at 20% (optimal ~16v) and you run them at 17V, you'd lose maybe 5-10% of the weak panel's potential. Am I thinking about this right?

 

dreadlocks

Well-known member
the panels have bypass/blocking diodes built into em, sometimes many of em in a series but just one on a 12v panel.. if one is at 17V and the other is at 16V the diode on the 16V panel will not let current flow into it and it'll cut out.
 
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rruff

Explorer
Doesn't the controller determine the voltage though, and all panels connected to one input will have the same voltage? And the current will be the sum of what all the panels can produce at that voltage?
 

FJR Colorado

Explorer
Doesn't the controller determine the voltage though, and all panels connected to one input will have the same voltage? And the current will be the sum of what all the panels can produce at that voltage?
dreadlocks is a very knowledgeable dude :)

He knows a lot more than I do.

But I think we may be confusing a series and parallel architecture here....

Lots of good info and videos on these topics via web search. You'd be looking at a simple parallel connection via branch connectors.
 

dreadlocks

Well-known member
Doesn't the controller determine the voltage though, and all panels connected to one input will have the same voltage? And the current will be the sum of what all the panels can produce at that voltage?
not even close, all the cells in the solar panels determine voltage.. the number of cells and how they are wired.. Controller takes whatever it can get, and converts it into a usable charge.
 

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rruff

Explorer
not even close, all the cells in the solar panels determine voltage.. the number of cells and how they are wired.. Controller takes whatever it can get, and converts it into a usable charge.
Those things determine the panels *max* voltage and current, but not its operating voltage. In the chart I posted earlier the panel will supply 0 to ~22V (@100%). You can hook it up directly and charge a battery that has a voltage anywhere in that range. If it's a 1V battery you'd only get ~4.8W which would be a poor use of the panels capacity, but you could do it. If you attached it directly to a 12V battery it would supply 12V and ~58W. MPPTs adjust the input voltage to maximize the power from the panel. In this case that would be ~17V and 75W.
 

dreadlocks

Well-known member
nope, you'll fry your battery with too high of a voltage.. been there done that with those aweful 'attach directly to the battery' maintainers that just destroy your battery.

PWM and MPPT act as regulators that step the power down to the desired voltage.. I can go out right now to my 65v panel, cast shade on it and watch its voltage drop as I put more and more shade on it.. the MPPT is constantly adjusting to the constantly varying voltage coming out of the panels, not the other way around..

A power source (panels) determines the voltage it runs at, not the consumers of that power (controller)
 
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