Connection to battery charger, ground reference

Question

My battery charger (Genasun GV-5-MOD) has a pair of "LOAD" terminals to which I have my single board computer (SBC) connected. I want to read the LTC2959 from the SBC.

Should the LTC2959's GND be connected to LOAD- or BATTERY-? 
 I think if connected to LOAD- then the battery voltage readings will be inaccurate, but if connected to BATTERY- then the I2C lines will have problems due to the SBC having different GND reference. What is best practice here?

kevintom98 · Accepted Answer

You could use a photocoupler but will need extra circuit or you can use a i2c isolator IC. LTC2959 GND should be connected to BATEERY- for accurate measurements.)

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Connection to battery charger, ground reference