A couple of months ago I had exactly the same problem. What it was in my case was the main white wire coming out of the converter was not tight. If you can, reach around the output wires, and if you feel a hot one, you might have a loose connection like I did.
When I wiggled the wire, suddenly the lights inside came on brightly as the converter voltage was suddenly connected to the battery and charging.
If you have a laser thermometer, point it around the connection points, if there is a loose wire, it will likely be hot.