Although that may occur to some solar controllers, the one that came with our Renogy solar suitcase does not have this issue. There is no way you can connect the portable solar panel to your battery without prior exposure to the sun, our unit comes in a padded black case and the second you unzip it both panels are fully exposed.
Just start with a new controller, water proof would be better as I lost one to a passing flash storm common here in the Mountain West. Since you inherited it double check wiring before connecting new controller in case it has been fiddled with. I find our portable panel can really keep up with what we already have on the roof being that we can reposition it a few times a day maximizing periods of peak output.