Parallel Solar Panels - one fixed, one portable

[ekarlson]

I am considering expanding the solar power available to my trailer by adding a side port to attach a portable solar panel. The portable panel, when plugged in, would be wired in parallel with the fixed panel that is mounted on the roof. I have a concern about how to wire the panels, MPPT charge controllers and battery.

I've seen videos where someone simply hard-wired the side-plug in parallel with the wires coming from the fixed PV panel on the roof. This seems like a bad idea for at least two reasons:

1. If the portable panel happens to be wired in reverse, this will loop the negative and positive outputs of the two panels. All the current from one would flow through the other. At best, the charge controller isn't getting any energy. At worst, something is going to get damaged and/or melt.
2. If one panel is shaded or is otherwise generating a lower voltage than the other, then the more highly energized panel is going to drive current through the other panel instead of the charge controller. This again results in less power flowing to the charge controller, but is also going to heat up the other panel, which further reduces it efficiency.

One obvious way to fix this problem is to put in blocking diodes to prevent current from flowing into the panels. This assumes that the solar panel junction boxes do not include blocking diodes (difficult to tell from the specs as no one seems to discuss specifics of the junction boxes, but it seems that they only have bypass diodes to handle shading on the panel itself). While this is inexpensive and would work, the diodes incur about a 0.5V drop, which represents about a 5% loss of power not to mention a higher threshold for being able to drive the charge controller.

The other approach is to wire each panel to its own (lower amperage) charge controller, and then wire the battery side of both controllers in parallel to the battery. This is obviously more expensive, but would seem to be the best solution in terms of isolating the panels from each other with minimal energy loss to address all the issues associated with mismatched voltage outputs and reversed polarity (which I guess is an extreme case of "mismatched" voltages).

Actually, I guess there is one more possible solution. Wire the panels in series with a bypass and diode around the portable. Handles shading on the portal panel as well as outright removal. Only question then is how to handle reversed polarity on the portal panel.

Has anyone else tried to design a system where they had to address this sort of issue? Any thoughts on what approaches work best?

 

[Bud Martin]

So the panels do not have MC4 connectors on them which will prevent from reversed connection?

 

[Hedges]

Connectors should be polarized. MC4, MC3, or another.

Series connection isn't a good idea unless both panels have same orientation. Otherwise, you either get 2x voltage and lower current, or power only from the higher current producing panel.

Parallel is perfect, so long as they have similar voltage. Nothing significant flows back through an off-angle or shaded panel (except at Voc, when you aren't extracting power anyway.)

My home setup has multiple strings of different orientation in parallel. Tests by vendors have shown about 2% less output that having them on separate MPPT.

 

[ekarlson]

I was planning on using an SAE socket on the side of the trailer, but I kept seeing warnings that the polarity on the plugs was only a "suggestion" and that one had to test any given device to see what it was doing. But since most PV's seem to use MC4 connectors, the MC4 to SAE adapter should maintain a consistent polarity on the SAE plug, so I am starting to think I don't have to worry about that. Plus it seems that trying to accommodate a reversed polarity connection has too many challenges.

Given that PV voltage does not drop off significantly in low-light (well, not until there is almost no light) there seems to be little advantage to wiring in series in terms of meeting the MPPT's minimum input voltage requirement. Series wiring also makes things more difficult in terms of being able to unplug the portable PV.

Oddly enough, there isn't that much different cost-wise in terms of using two smaller MPPT's vs one larger one. Looking at the Victron Energy units, the 75/10 would handle the100W on the roof, and a 75/15 would handle a 200W portable. To handle both in parallel requires a 100/30, which is almost exactly the cost of the two smaller units. However, a single 100/30 MPPT would actually give me more headroom (440W vs 145W+220W). Also, the 100W PV on the roof does not fully utilize a 75/10 MPPT, so some of its capacity is essentially wasted if I use it to run a single 100W panel. In any event I will have to replace the Jaboni 10A PWM that comes in the trailer - doesn't even support lithium batteries.

So I guess I'm back to wiring the panels in parallel using a single 100/30 MPPT and don't worry about polarity, which makes the wiring easier and would allow me to use the fixed 100W panel plus a portable 300W panel if I so choose (using a 75/10 & 75/15 combo would limit me to the 100W fixed plus 200W portable).

Thanks for the input!

 

[ekarlson]

Tracked down the source of the warnings regarding SAE ports - there is no defined standard for which wire is positive vs negative (https://en.wikipedia.org/wiki/DC_connector#SAE_connector). The most common convention is that the bare prong is negative so that if it touches the frame of a car one does not short out the battery. However, Zamp devices are wired with the bare prong being positive (https://teardrop-trailers.vanillacommunity.com/discussion/128/zamp-solar-connections-explained). So if one were to hook up a Zamp PV to a "standard" SAE port, the polarity is reversed.

I guess as long as I stay away from Zamp I would be OK.

 

[ekarlson]

After some more thought, here is my solution to the SAE/Zamp polarity issue ->

Custom sticker to apply next to the solar port on the trailer:

 

[HRTKD]

I installed two solar charge controllers. One for the roof and one for the ground deployed panels. It was just easier that way (for me).

With regard to the polarity of SAE connectors, it doesn't matter. As long as you wire it up the way you need it, the polarity will be correct. Liberal use of a voltmeter during the install process will keep the polarity correct.

 

[Hedges]

If not already built in to your SCC, you can protect against reverse-polarity PV panels by installing a diode to conduct and clamp voltage to a diode drop (wired in parallel). Unlike a reverse-polarity blocking diode (wired in series), it won't affect voltage when panels are connected properly. Diode must be of sufficient voltage/current rating and well enough heatsinked. A fuse could protect against excessive current source.

Two panels (of same rating) connected backwards to each other is no big deal. That is identical to connecting them in series and then shorting to measure short-circuit current. Each panel experiences same as being shorted by itself (if in the sun) or has its bypass diode activated (if shaded.) But it will double the Voc available to sustain an arc when disconnected.

Panels of different ratings, that could overstress one.

I like the idea of single SCC over-paneled, better yet multiple angles so it is used more hours.
But two SCC gives redundancy, backup in case one fails.