Grounding Arduino with solar charge controller

Hello everyone.
I am trying to power the Arduino project with a solar system and I have a grounding problem. Solar system has one solar panel, one battery and a solar charge controller. Solar charge controller has connectors (+ and -) for panel, connectors (+ and -) for battery and connectors (+ and -) for attaching load (relays, sensors, motor drivers...). On the solar charge controller, I can turn the load power ON or OFF via button.
Arduino is powered via 5V/2A USB connector on the solar charge controller.
This is the very basic scheme where I will describe the problem:

So, my idea is that Arduino GND and - from the solar charge controller should be connected to have a common ground.
The problem is that there is 12V on the load exit of the solar charge controller even when it's turned OFF. So, when Arduino GND and - on the solar charge controller are connected, the load power is ON even though the solar charge controller is set to OFF.

The solar charge controller load power ON/OFF works fine when Arduino GND and - are NOT connected.
But as soon as they are connected, there is load power and it does not matter if it is turned ON or OFF.

Does anybody have any idea how to work around this?

Without further investigation DO NOT connect to the solar controller GND as well as the USB. It may not be the same as USB GND.

The solar charge controller that I have has a common + between input and output rather than a common GND. I think that allows them to use cheaper Mosfets

Why do you think your Arduino needs to connect to the solar controller power GND ?

...R

Robin2:
Why do you think your Arduino needs to connect to the solar controller power GND ?

...R

Not exactly sure about the theory behind it, but as I understand, common grounds need to be established in systems like this so that there are complete circuits and current can pass through them.

NemanjaPN34:
Not exactly sure about the theory behind it, but as I understand, common grounds need to be established in systems like this so that there are complete circuits and current can pass through them.

You need to explain what part the Arduino will play in your system.

If the only purpose is to power the Arduino to do some task completely unconnected with the solar system then all you need is the USB connection. I am assuming that the USB connection is only for charging and does not have any data connections.

...R

No, the USB connection is only for powering the Arduino.

Arduino's role in this system is to control several sensors, a 2-channel relay module, 3 motor drivers and a GSM module.

Sensors and a GSM module should be powered via HW-131 power source (12V from solar controller -> 5V on the breadboard). Relay module and motor drivers should be powered from the solar charge controller.

Use your multimeter to check whether the GND of the USB socket on the controller is directly connected to the GND for the 12v power output. If it is then you are good to go.

If not and if you want to use the 12v power from the solar controller for things that will be connected to the Arduino then I suggest you get a 12v to 5v converter (the USB chargers for use in cars are very cheap) so that the Arduino is also powered from the 12v output rather than from the USB connection on the solar controller. That way there will be no difficulty with a common GND.

...R

Or I can use this:

Connect red wire to positive connector of the solar controller and black wire to negative connector of the solar controller. And then plug it in Arduino power input port.

Do you think it would be safe?