The sorted data:
Voc Isc(A)
1 30.0 8.2
2 29.7 7.2
3 29.9 6.2
4 20.2 8.0
5 20.6 7.3
6 20.3 7.2
7 19.9 7.4
8 10.3 6.5
Number 8 is pretty dead, the others all seem not bad, and quite worth using.
Figures taken with the panels manually tilted up to approximately point at the sun. But a few minor shadings - long grass, clothes hoist, etc.
I just scored 6 panels like yours today for $100 and one panel was faulty. I have been able to resurrect it by replacing the failed diode in it. The panel was only giving out about 2/3 it's normal voltage because the diode had gone short.
Thanks for the tip! At the time I didn't open the terminal boxes, and had forgotten there were diodes in there. (To take the current past cell strings that are shaded, so they don't get reverse biased and damaged.)
I had noticed that the voltages were all close to 30V, 20V or 10V, so wondered if there was some 'rule of thirds' happening. In my defense I would have figured this out once I got around to opening the junction boxes up. Was just in a hurry that day, and stacked the panels away under cover.
I PM'd you. Said I'd check my panels next week when current urgent chores done. But then, I couldn't wait. Just now opened up panel #8 (the worst one.)
Could tell by eye there's a diode problem - one of the three has leads discoloured slightly by heat, and has an 'extrusion'.
They are only clipped in. Pulled them out and measured them:
Diode 1 (discoloured) 364 ohms both ways.
Diode 2 dead short
Diode 3 dead short
So the output from this panel was the result of two shorted strings, and one string driving a resistive load of 364 ohms. (Heating it up quite a bit.)
Also cleaned the lichen off the glass. Mold-killer spray, scourer pad, then hosed off. Easy.
Too bad it's evening now. In room fluro lighting without the diodes, the three cell strings (each 18 cells in series) produce 5.87V, 5.99V, 6.4V.
Pretty close. Differences likely mostly due to uneven lighting across the panel.
The diodes are 12SQ045 ( Schottky, 12A 45V, Vf=0.55V@10A ) Commonly available.
Ordered 50 for US$15 via aliexpress
With luck, maybe I'll have 8 perfectly good panels. Woo hoo!
20160420 Edit to add: Had some free time and it's a sunny day, so I took panel #8 outside in the sun and measured output without the dead diodes. Voc=31.9V, Isc=8.03A Excellent!
I can't wait till the 50 new diodes arrive. I'll leave retesting all the panels till then, since I don't want to open the junction boxes twice. They use plastic lid retaining clips; I'm afraid of breakages if I keep playing with them.
Once they are all fixed I'll measure them under load at max power operating point.
Edit on 20160510 to add: Started a thread over here:
https://www.eevblog.com/forum/renewable-energy/solar-electric-panels-vs-lightning-emp/ to update on those panels. Some nice pics of EMP arc-tracking on the cells. But the panels all work!