Hello Everyone,
I have been working on a small off-grid solar power system for a little while now. The system consists of 4 100W monocrystalline panels, a 30 amp pwm charge controller, a 100 ah 12v battery, and a 1000w inverter.

Charge Controller Link: https://www.amazon.com/gp/product/B00ADKOMDI/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

What I have been trying to do is to use an Arduino Nano clone to measure the voltage of the panels, and cut power to the inverter when the sun goes down. I have some lights for a fish tank and a frog tank that I just want to run during the day, so I created the attached circuit to try to control the system automatically. (I excluded the relay portion of the circuit because I know that is not causing the problem, I am focused now on getting a voltage reading from the panels using my Arduino.)

I am using a simple voltage divider to drop the voltage down to a readable level for my 5v arduino. This works perfectly when I am testing it on AA batteries and 9V batteries, but as soon as I connect up the solar panel leads, something starts to short and the battery begins overcharging. What I believe is happening is the charge controller somehow sets a different ground for the panels and the battery, because when I check the voltage between the battery negative and the solar negative, the battery reads about 5.6 volts higher. Thus, when I connect the negative end of the solar panel to the ground pin on my arduino, it flows right back through my buck converter and into the battery.

Buck converter link: https://www.amazon.com/gp/product/B01GJ0SC2C/ref=oh_aui_search_detailpage?ie=UTF8&psc=1

Can someone give me some insight into how I might solve this problem? What I need is for the arduino to read the solar panel voltage, and any way that can be accomplished will be good enough for me. I was thinking about using an optopcoupler to isolate the arduino, but I am hoping to just use a voltage divider and keep the circuit very simple.

Let me know if my drawings or explanations are unclear, I appreciate any help I can get.
Thank you

 

Pretty simple to me, just use a relay across the Solar panel & it will automaticly switch the inverter on when the sun rises & off when the sun goes down. No need to make the system over complex.

 

You're a genius! More like I'm just slow, but this is perfect! I can even use this to interface with the arduino, since the circuits will now be separate. Thank you so much, I really appreciate the help!