Play with fire and you get burned...
Play with a nice commercial PV panel & batteries without taking the time to put a fuse inline first ... and you'll eventually screw up by reversing polarity to the batteries and frying your panel. 
The panel is a new Sharp NE80EAEJ - 80w module, 21.6 open circuit voltage. Got it relatively inexpensively on eBay because the frame was damaged after manufacturing, but the cells & glass were still OK.
So enough back story. Here's how I SNAFU'd it:
This afternoon I mistakenly hooked it up BACKWARDS (reverse polarity) to a nearly fully charged 12v battery and the instant I connected the lead I knew I done a bad thing from the size of the arc.
Now the panel is putting out exactly half its rated voltage. Fine. So I guess I blew out a connection somewhere between the two strings of cells, right?
I identified which string seemed to be isolated, and dug the potting material out of the junction box (see photo) and found what looked like a "blown" connection. After cleaning the material out and soldering it back together I now have ZERO volts output.
So I disconnected the join and I'm back to half panel voltage. And... now I'm a little confused.
I should add: the blown out connection seemed like it may have had some crumbly material in it between the copper traces - maybe silicon wafer? Was it maybe a diode?
Any ideas for me?
(I'll be away this evening - will reply tomorrow or later tonight if anyone has any suggestions or questions....)
