I have a little portable solar system I will be redesigning to add a bit of electronic features to it, basically a LCD screen with voltage/current/watts and a very basic UI. I want to be able to measure the voltage of the actual panels, the amps being drawn, and ditto for the batteries. A voltage divider will be used for voltage and then connected to an analog pin of the MCU. For current I'll use a negative side shunt and op amp and also connecting to analog pins of MCU. The issue is that I'm not too sure how I would go about measuring the panels as I am not sure if it's safe to just bond the negative to the system "ground" so that it's all at same potential as not sure how the charge controller will handle that. Is this generally safe to do?
What is the best way to verify with my current charge controller? If I take my volt meter and probe between solar negative in, and the battery positive out (with battery connected) I get 12 volts, but from battery positive to solar positive in I get like +0.4v. If I use my ohm meter I get about 3k. So the grounds are NOT connected internally, but also not floating, which seems a bit alarming.
If I can't connect them, what is the best way to go about measuring that voltage then? I could have a separate MCU and then use i2c or other communication with optoisolators to talk to the main MCU I guess, but that seems overkill and I might be overlooking something.