INA3221 breakout board grounding detail.

In your tutorial you mentioned, "trust your mother...not your ground and ensure common ground of INA3221 breakout board and DUT" relative to experiencing incorrect voltage readings.  I am having this problem.  Note current readings are fine as tutorial states they would be despite voltage issues. 

Present wiring:  Groove connector to Raspberry PI3 pins 3.3 vdc (VCC), SDA, SCL and ground (GND).  A second ground pin on the Raspberry Pi3 is connected to the ground bus from negative terminal of battery bank and earth ground.  Voltage readings are off anywhere between 0.2 and 1.0 vdc (12 vdc battery bank), which seem to vary as a function of the battery voltage during charging.  Voltage readings from solar charge controller and voltmeter agree and are referenced from the ground bus as well.

My question is how should the ground be connected to eliminate the voltage error?  For example, should I connect the ground (GND) pin on the INA3221 breakout board directly to the ground bus from negative terminal of battery bank and eliminate the Raspberry PI3 ground connections in my present configuration altogether? 

Thanks in advance for your thoughts on the subject.