It's the heat, not the humidity
When I spent last week at Wrights Lake as a volunteer for the Desolation Wilderness, I left my solar charger on top of the dashboard of the van, along with my volunteer parking permit and the campground receipt.
And then I put up the reflectix panel to try to keep the van a bit cooled in the heat of the day.
Yeah, well, not so good for the solar charger. I don't know how hot it got. Actually, yes I do--it got hot enough to melt the solar charger. It still works, after a fashion, but I think this one is going to the recycling center.
And the next one is going to avoid that kind of oven. If I'd only known, I probably could have cooked dinner there.