My friendly local PV installer let me have an Ulica UL-250M-60 panel that had come off worst in an encounter with an iron bar. According to the data sheet, open circuit voltage is 37.6V and short circuit current is 8.7A. The glass is cracked across the entire surface of the panel. Where the bar hit, the glass, cell and backing are bashed right in and there is a small hole right through the panel. Opening the junction box, there are 4 leads, W, X, Y and Z, which I assume connect the 3 strings in series. A pair of bypass diodes D sits between each lead and the next one. Lead W corresponds to the negative connector, Z to the positive. I hope this illustrates the layout.
W X Y Z
I I I I
I--D---I--D--I--D--I
I I
- +
I stood the panel facing bright sunshine, and tilted it as nearly perpendicular to the sun as I could make it, at about 13:00 last weekend. Location is the middle of the UK. Putting my multimeter clips onto the lead terminals, I measured as follows: W-X 12.7V; W-Y 25.3V; W-Z 37.9V, which corresponds nicely to the rated Voc. I changed to the proper holes on the multimeter, set it to measure current and measured as follows: W-X 2.1A; W-Y 3.4A; W-Z 3.5A.
Isc falls a long way short of the rated figure. I haven't been able to trace the wiring of the panel, but as far as I can see, the W-X leads showing 2.1A Isc correspond to the string with the hole in it. Strangely, when I measured the other way round (Z-Y, Z-X, Z-W), each of the 3 showed 3.3A.
Please excuse the basic questions – this is my first venture into solar PV. I've done a number of searches on this board and found answers to some of my questions. These are the questions to which I couldn't find answers. Should I expect weak winter sun to produce that much less current? I know that you get less power on a winter day because there are fewer hours of sunshine, but I haven't been able to find any figures showing how much less than the 1000W/m2 test conditions you really get. If the reduced current in the damaged string is in fact a result of the damage rather than weak sun, then shouldn't I expect the other two strings to show closer to the rated Isc? Or could that be a result of the cracked glass blocking out a lot of the sunlight? In either case, if standard Isc for the panel is 8.7A and standard current at Pmax is 8.2A, can I expect to get a similar proportion of my Isc – perhaps 3.1A - when the panel is loaded and properly running? I had originally calculated that I needed a 40W panel for my project, so even if I get only 3A or so, that should give about 90W if I understand correctly. Even after I've lost half of that through a charge controller charging a 12v battery, I think it should give me enough power. Any suggestions on maximising the power and/or preventing further damage would be gratefully received, though. For example, is it worth trying to test (how?) and then if necessary bypass the damaged cell, and if so, do I simply pick off the glass over the connecting ribbons and cut them, then solder on leads to bypass that cell and connect to the next in series? And do I need to do so anyway in order to prevent further damage to the panel? Presumably there's a risk that the hole in the cell has shorted it and/or neighbouring cells. I'll seal the panel as a whole either with varnish as described on one of the threads here, or with a sheet of glass and some sealant.
Thanks very much.