Solar module arrangement question

Thread Starter

ischonfeld

Joined Jun 22, 2019
15
I'm not sure if this is the right place to post this, so please advise if this needs to be moved.

The question deals with 4 mis-matched solar panels on an RV roof. Yes, it is a bad design, but the RV mfg is no longer in business, so there you go.

I am somewhat knowledgeable on solar array design for a "normal" system. In simple systems, modules get arranged in strings, which get tied together in parallel, and then brought into an inverter, The number of modules in each string needs to be the same or inefficiencies develop quickly. Each panel design has a short circuit current rating and an open circuit voltage rating. The inverter than attempts to find the maximum power point where current draw and voltage output of the array produce the most wattage for the amount of sunlight at the time

Now to the question. There are 4 panels on the RV roof. 1 large (6 cells wide, 10 cells long), 2 identical mid-size panels (4 cells wide, 9 cells long), and 1 small panel (2 cells wide, 6 cells long). I cannot find part numbers or specs on the panels due to them being up against the RV roof. I believe (a guess, but that’s all I have to go on at this point) that they are wired as:

LARGE -> 2 MIDSIDE in parallel -> SMALL

since open circuit voltage is around 100V DC. The quoted output of this arrangement is 470W, which likely means that the RV manufacturer took the absolute max wattage rating of each panel and added them up. As installed, on a cool (better for solar output) bright sunny day with sun directly overhead, output has never exceeded 100W. I don’t expect anything near 470W, but certainly more than 100W.

With this mis-match of panels, is there a way to wire this up so the solar inverter has a chance of finding a maximum power point? Thanks.
 

nsaspook

Joined Aug 27, 2009
8,533
If you have a MPPT controller that can take a wide range in DC input voltages then it might be possible to optimize the power in a series string by removing the small panel. As the string open circuit voltage is ~100vdc that's about what I would expect from 4 series connected nominal 12vdc panels with a individual panel open circuit voltage in the range of 22 volts.
 

Thread Starter

ischonfeld

Joined Jun 22, 2019
15
I think that's part of the problem/question. The large panel is very similar to those that I have on my roof, with an open circuit voltage of approx 45VDC. And without seeing the specs, I'm guessing that the large panel alone is rated at 250W or maybe even better. The other 3 panels appear to be confusing the MPPT controller. Dropping the small panel, or even the two midsize and one small, would in theory provide more real-world power than I'm getting now with all 4. But that's kind of wasting the 3 panels. Seems like there should be a way to use at least 2 more (without adding a second MPPT controller). Maybe not.
 

nsaspook

Joined Aug 27, 2009
8,533
What model MPPT controller do you have? What are the open circuit voltages on each of the three larger panels? I would just eliminate the small panel from the equation for now.
 

Thread Starter

ischonfeld

Joined Jun 22, 2019
15
The MPPT controller is an Epever Tracer 3215BN. I don't have the specs of any of the panels since they're mounted too close to the roof to even get a pic of the spec tag or model number - which makes this much harder to figure out.
 

nsaspook

Joined Aug 27, 2009
8,533
Do you have a monitor (display, computer, etc...) connected to the RS485 MODBUS port on the charge controller?

What's the battery bank voltage? The solar panel input need to be at least battery voltage + 2dc volts. I assume you do have access to the individual panel wires from each panel, right?
 

Thread Starter

ischonfeld

Joined Jun 22, 2019
15
Yes, I have the optional display/programming module which is very informative. Battery bank voltage is 12V nominal, 800Ah capacity, 13.2-13.5 actual depending on state of charge. Controller settings are such that at these voltages the controller will be in its Bulk charge state. Solar panel input is 100+V open circuit. No access to the individual panel wires - if I can determine a good wiring approach (maybe even eliminating one or more panels), I will disassemble and re-wire. But I don't want to tear into it without a plan ahead of time.
 
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