A follow-up:
Thank you everyone in this thread for good
advice.
For one, I was not aware that without load, it will always show the voltage from the battery.
It is impossible to meassure at the end of the cable that goes into the device, while plugged in, so I decided to go back to the panel, and follow the cable from there, and man... it turns out that it get it's
power from a USB
plug, that is powered by a cable from the busbar that also has one or two places where it is assembled.
Just by twiddling the connections I got the drop down to 0.2 to 0.3 volts, so I now know that I have a
project on my hands. That mess (the other stuff look almost equally ..... up) needs to be dealt with soon.
I do meassure full battery voltage on the
panels busbar itself though, so I take that as a good sign, that when I clean up that panel-mess, I should be in much better shape.
Am I right, or just optimistic?