Mixed solar panel install questions

[tonygoncalves]

I currently have two BougeRV 170W solar panels hooked in series, connected to a Victron SmartSolar MPPT 100/50. Panel specs are Vmp:19V Imp:8.91A Voc: 22.5V Isc: 9.44A. The panels are installed on the roof of my 2400BH, at the front on either side of the bedroom vent, so one on the driver side, and the other on the passenger side. Due to the curvature of the roof, unless the sun is directly overhead, the panels do not receive the exact same amount of sun. I purchased these panels about 2 years ago, and of course as soon as I installed them, this model got discontinued and replaced with the 180W model.

During the Black Friday sale last year, I bought 2x Renogy 175W panels for cheap. I tried getting BougeRV 180W panels, as they were the closest in V and A size to my 170W, but they were out of stock. The Renogy 175W don't match as well to my 170W, but they're the closest I can find that was actually available. Renogy 175W specs are: Vmp: 18V Imp: 9.75A Voc: 21.6V Isc: 10.35A

So the panels are close enough to match (in my opinion) but not exact.

My question is how should I mount and wire them? It will have to be a combo of series/parallel, or parallel/series. But should I have the same panels on the same side, ie: 170W x2 on passenger and 175W x2 on driver sides? And should those be wired in series or parallel? Which will then be the opposite of how I connect both sets of panels together. Will the curvature of the roof affect the sun exposure enough to make a difference?

I'm trying to avoid having to install another charge controller, the 100/50 was expensive enough!

My calculations are as follows:

Option A: 170W+175W wired in parallel, on the same sides, the both sets then wired in series. The sets will run at the lower voltage of 18V, and amps will add up to 18.66A. Both sets wired in series together, the voltage adds up to 36V, but the amps will be decided by which panel set gets the most sun, up to 18.66A, so a max of 670W.

Option B: 170W+175W wired in series, on the same sides, both sets then wired in parallel. The sets will run at the stacked voltage of 37V, but at the lowest panel amp of 8.91A. Both sets wired together in parallel, the voltages stay at 37V, and the amps stack up to 17.82A, for a max of 660W.

Option C: 170W x2 on one side, 175W x 2 on the other. Each set wired in parallel, then both sets wired in series. 170W set will run at 19V and 17.82A. 175W set will run at 18V and 19.5A. Connected in serial, the voltage stacks up to 37V at the lowest amp of 17.82A for a max of 660W.

Option D: 170W x2 on one side, 175W x 2 on the other. Each set wired in series, then both sets wired in parallel. 170W set will run at 38V and 8.91A. 175W set will run at 36V and 9.75A. Connected in parallel, the amps stacks up to 18.66A at the lowest voltage of 36V for a max of 671W.

Maybe I'm just overthinking this? The easiest for me installation wise is option A or B, as they won't involve moving an already installed panel. I do have a SmartSolar 100/20 controller that I use for my portable 2x50W panels, that I can install instead and have each set of panels connected to their own controller, but the extra labour and cost of installing and running another set of solar cables from the roof to the storage bay, just makes me cringe. Especially since at most, will gain me only few extra watts under ideal cirumstances.

Thanks for any advice.

[Country Campers]

The wire size from the roof down to the solar controller will effect how you wire. If the wire can handle it I would go with option A as it seems to be the best of what you have. I am no solar genius by any means but just going off of what I have learned.

Brian

[bironacad]

As you well know, panels in series drop to the lowest amperage in the string? So personally I would keep the similar panels in series and then combine into parallel. Just purchased a renogy 175 watt panel to add to the 165 watt furrion panel, I will have them in series, this ups the voltage and keeps the amperage therefore no need to change wire size. New 200 MK Trandscend picked up two weeks ago. Adding a battery an panel. How do you like the renogy panel?

[howson]

tonygoncalves

In this reply let's tackle the solar controller question.

Using Garret Towne's methodology (AMSolar engineer) outlined in his YouTube video (embedded below) there are three separate calculations required.

Q: Does the Open Voltage exceed the controller's rating?
A: I used the highest possible output voltage when all four are tied together in series. The lowest Open Voltage of a single panel (thus the highest the system would see) is the 175W panel's which is 21.6. 21.6 x 4 = 86.4. That's less than the 100V spec of the controller so that one's good.

Q: Does the Short Circuit Amperage exceed the controller's rating?
A: For this one add all the amperages together (9.44+9.44+10.62+10.62)=40.12 That's also less than the 50 amp rating so that one's also good.

Q: Is the total wattage appropriate for the controller?
A: Garret uses the equation (Total Wattage of array *.9) / Battery Voltage = max output amperage from controller For your system using the max wattage and minimum battery voltage values for worst case scenario, the equation is (690 * .9) / 12 = 51.75. While slightly over the 50A spec of the controller, if you watch the video it's really not an issue.

In reality, due to how the panels will be wired, IMO the controller is fine for your configuration.

Howard

[Country Campers]

Do not forget when adding panels to the factory set up you will run in to the issue of the Furrion roof port with the awesome Furrion plug connections (sarcasm there), so you will have to deal with that when adding the other brand of panels.

Brian

[howson]

tonygoncalves

I had to "see" the configurations and look at the numbers. It was good practice to learn (again) how to figure this out.

First, the resource for others to get more information (with great diagrams and examples): https://solarpanelsvenue.com/mixing-solar-panels/

Second, to do these calculations the values used were the optimal operating voltage and amperage values of each panel. I got them from https://www.renogy.com/175-watt-mono...e-solar-panel/ and https://www.bimobject.com/fr/bougerv...ne-solar-panel (One of the values was slightly different from what you'd posted...forget now what it was.)

There were six different possible combinations of combining and wiring the panels together. The result of wiring and mixing the panels together is posted below. A yellow highlight means there was a loss due to the configuration. Comibinations II, III, IV, and V are self-explanatory. After the pic I'll explain more about the values entered for I and VI.

Note: Ignore the gray fill...that's just a visual reminder to me that the value is calculated by the spreadsheet.



Configuration I

The total voltage can not be higher then the lowest maximum, so that's 18 * 2 = 36. The amperage is tricky, because the 170W panel will not generate 8.91A at 18V, so I guestimated 8 amps maximum through the circuit. (Maximum amperage will be the lowest generated by either panel in the circuit.)

Should be obvious this is not a good option.



Now let's look at VI closer.

Maximum voltage again is limited to 18, so 18 * 2 = 36. Amperage again is tricky due to the 170W panel will not generate the full 8.91A at 18V, so I guestimated again by adding 9.73A + 8 = 17.73A



Hopefully above makes sense and I didn't screw up the math. (Someone here will check !)

Now consider each wiring combination above as a single panel and let's wire them according to your four options. Here's the results:



Option C and D yellow highlights are because both maximum voltages and amperages are affected. I again used a similar assumption for the 170W panel operating at a lower voltage by assigning 8A.

So what would I do?
Option C is an easy "no".
Option B is also a "no" due to the loss of output.

So what's the difference between A and D? The actual results will all depend on the angle of the sun as it hits the combination of panels, where and how the panels are mounted, and the affect of shade on where they are on the roof.

I think I prefer D if the similar panels are grouped close together. The sooner the voltage of the combination is >5 voltages over the battery voltages the sooner the controller will start charging the battery bank. (Remember the controller doesn't start outputting charging amperage to the battery bank until the solar input is 5v greater than the battery.)

I attached the excel spreadsheet I created so everyone else can double-check my numbers and conclusion. Look forward to reading what you end up doing.

Howard

P.S. It is more likely than not that something is not 100% correct...so please...double-check and don't assume what I've posted is error-free (though I tried hard not to goof it up!).