Troubleshoot low watt panel

[sfog]

I have two Jinko 405W panels in parallel (rated Voc=50.1V, Isc=9.56A) and they are connected to a Victron 100/50 charge controller. Sunny weather (NC, US) and the panels are side by side with no shade, producing ~300W. Battery is 24V 100ah, currently in bulk charging mode.

However if I connect them individually instead (directly from SCC to MC4 connector of panel) and check the smartsolar app, one them shows a very low output fluctuating around 5W or less (from screenshot with just this panel plugged in: 4W, 29.01V, 0.1A) while the other is doing all the work. Since everything up to the panel is the same, I assume the problem is specifically the panel rather than the SCC/battery or their connections. Is there anything I can do to fix this panel? Any other troubleshooting steps I can try? It was a used purchase, so my chance of refund/return is probably pretty low.

(p.s. I went back to get a screenshot for the other panel but it's partly shaded now: 75W, 41.93V, 1.8A; I added a 2nd screenshot that shows the switchover from the bad to the good panel)

 

[sunshine_eggo]

Definitely operating at lower than expected voltage. I would assume that a bypass diode has failed.

Have you confirmed the Voc and Isc values on the panel with measurements?

 

[sfog]

I just tested the voltage through the MC4 connectors and got ~28.6 with the panel face down and ~44.6 with it upright.

I can't test Isc without putting a load in, is this correct?

 

[sunshine_eggo]

I just tested the voltage through the MC4 connectors and got ~28.6 with the panel face down and ~44.6 with it upright.

I can't test Isc without putting a load in, is this correct?

Many multimeters have a 10A setting where you move the (+) to a different port. You simply touch the leads to the panel wires (spark!) and measure the short circuit current.

 

[sfog]

(as follow-up to first post, here is a smartsolar screenshot of full sun with only the good panel plugged in; diagnosis continues in next post)

437W, 36.88V, 11.9A

 

[sfog]

I angled the "bad" panel away from the full sun to make sure it read <10A.

From the multimeter: 47.5V, 4.51A

Once plugged in (again, I just unplugged the other panel from MC4 and plugged in this one, so wiring is all the same), smartsolar shows* 8W, 29.73V, 0.3A

What should I look for next?

(*I took a few other screenshots and saw it as high as 20W, 30.28V, 0.7A)

 

[sunshine_eggo]

Bad bypass diode(s). With no current flowing, you can measure Voc, but when current can flow, the voltage collapses.

 

[sfog]

The diodes should be inside the junction box(es) on the back of the panel, right? I pried off the cover and there is some kind of white material covering the entire inside (the same as what is around the outside of the box).



Do I keep digging?

edit
I decided to go for it, but I don't know where to go from here.