looks like a similar problem as this one > Solar Panel; Toast?
basically the solar panel output was dropping off when it heated up.
The conclusion was that there must have been micro cracks on the foil bond wires between the individual solar cells due to the continued expansion/contraction that was causing high resistance or just total failure.
end result was new panel required.
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got any pics of your solar panel and setup for us Photo Geeks.?
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Thanks, I have come to the same conclusion, now just got to persuade the manufacturer.