Testing solar panels?

I have a new-to-me 2009 Freightliner step van that spent its previous life as a LDV-made tool truck. I'm still building basic knowledge and trying to understand what I have before I get on the road full time in a little over a year.

On the roof are four, factory-installed Go Power! GS RV 110 solar panels. The panels are rated 110W, 6.7A and 16.4V. I checked voltage at noon today (sunny and no shade) in Washington, DC, area and the voltage was ~18V and current was ~4A. Wouldn't the voltage output suggest the panels are wired in parallel, and if so shouldn't the amps be much higher?

I cracked open the combiner box on the roof last weekend and given what I see it would appear the panels are 2S2P instead of 4P, so the above output does makes sense to me. I know how to use a multi-meter and the one I am using is rated to measure DC current up to 400A, but I guess it could be a dud, or I'm just not understanding winter sun angles, etc.

The rest of the house battery system is:
Inteli-Power PD2160 Electronic Marine Converter/Charger (rarely will I be plugged in)
Cole Hersee Co. No. 48161 Schottky Battery Isolator Guard 250A 12-24V DC (step van alternator is assumed to be 200A+)
Morningstar ProStar-30 (PS-30) PWM Solar Charge Controller
Two 6V SBS S-6V225GC batteries (was four but two were dead and these don't have much life left)

I plan to put ~600W more of solar on the roof and move to LiFePO4 in the future.

I’m assuming your readings were taken while connected to the charge controller. The stated panel ratings are likely 16.4Voc (open circuit) and 6.7Asc (short circuit).

Thanks and correct. I measured the voltage at the input terminals of the SCC. I see where I went wrong on measuring the voltage--I wasn't measuring open circuit. It is slowly sinking in. To see if I understand:

1) I am seeing lower amps as my system (2P2S) is 24V instead of 12V and

2) there really is that much of a reduction from optimal output (~13A from all four panels) given the panels are mounted flat on the roof and it is winter?

From the picture I'm not understanding why they are not 4p how are each of them connected to each other with a y or in series looks like the connections are under the panels?


Either way the first thing I would do is make a new combiner box before that one corrodes.

Good eye @Craig on the combiner box. I looked at the pics again and noticed the corroded terminal and condensation on the wires. That is likely contributing to the low readings.

You've correctly interpreted my challenge as the rest of the wiring is under two panels and I don't have access to those unless I remove the brackets screwed into the roof. I was hoping measuring volts and amps would tell me how it is wired.

Yes, I saw the corrosion as well and will be addressing it (this weekend's focus was on the engine). When I do I guess I need to measure open circuit voltage and current. Thanks.

[I should add my first post should have said "so the above output doesN'T makes sense to me". Oops]

One trick to determining whether the panels are series or parallel connected is to cover one of them up with cardboard or other light blocking material. Parallel connected panels output current will be half, while series connected panels output will approach zero.

pat.kane said:

...and current was ~4A...

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How were you measuring this? At the CC? With the battery hooked up? If so, lead acid batteries will only draw what they can take, so the readings will not always be peak output.

@cass3825, thanks for the cardboard tip. I will definitely do this next weekend assuming we have sun.

@SolarRat, I measured the current at the combiner box and at the input side of the SCC. At the SCC I left the negative wire connected to the SCC and passed the positive wire through the multi-meter to the SCC. I assumed the SCC would only impact volts/amps out to the battery bank, not volts/amps in from the panels. Since I have to go back into the combiner box anyway it sounds like the easiest thing to do is measure as open circuit at that time.

I appreciate everyone's advice. Even though I'm reading a lot of the forum posts to gain basic understanding, the uptake is a little slow.

I was able to clean the combiner box and install a new rubber gasket, as well as take open circuit readings.

I have four panels. Each side of the vehicle's roof has two panels wired together in a combiner box under a panel that isn't accessible unless I remove the panel from the roof. From that box two wires go to a combiner box at the center of the roof. I disconnected both pairs of panels at the central combiner box and measured the open circuit voltage and amperage. Measuring one pair only I saw 20.6V and 4.0A which dropped to ~3.7A when I covered one of the two panels. The results were consistent for the other side of the roof. It was overcast and the panels are clean.

Is this a 4P setup? 2P2S? Something else? The numbers still don't make sense to me (seem low given four panels).

I'm trying to figure out my baseline for moving forward and whether these panels are worth keeping or should be replaced.

Thanks for any input.

That's a 4p setup (you can see that in the pic of the box in your first post, and the voltage confirms it).

pat.kane said:

It was overcast

Click to expand...

If you only get 4 amps when the sun is out something is wrong with the panels or wiring. With it being overcast, can't tell.

Only dropping from 4amps to 3.7 when one panel was covered makes me think there may be bad connections under those panels that are limiting the amps. Did you cover each panel one at a time with the same result? If so, they are each individually capable of at least 3.7 amps, so you should be seeing the combined amps with both uncovered. Something is limiting that.

First thing I would do is try another meter to make sure yours is correct. Then, I'd pull up a panel to see what's going on under them with the connections. If they look anything like in your first pic...they need to be cleaned up and sealed better.

Thanks @SolarRat. I've thought about another meter but this one test fine on the alternator amperage and I don't know anyone who has one I can borrow. I might try harder as I really don't want to pull up the panels. The numbers don't make sense to me, so I probably won't have a choice though.