I have been led to believe solar panels degrade over time. I have some panels in my array that are 20 years old. I tried to test them for current output, and I got very low readings. However, I am unsure of my testing method. I tested them inline, interrupting the positive lead from each panel, while they were contributing to the system. Also, my charge controller was in float mode when I tested them, so the current going into my batteries from the controller was minimal. Should they be isolated from the system when checking for amperage output?