RS485 network ground pin - when to connect?
It is a myth that you can make RS485 interfaces work without the Common (C) ground wire connected up between the various devices on the bus. The receiver is only capable of measuring the relative potential between the A and B signals when the common mode voltage of the A and B inputs is kept within -7V to +12V of the GND reference of the receiver.

The idea that both sides of the interface being operated on batteries would somehow make a difference is also a myth. It all comes down to what the common mode voltage is between the transmitter GND and the receiver GND. The third wire connection keeps the common mode voltage under control. Without it any undue influence to either unit or to the bus between the two can lead to the common mode voltage going outside the -7V to +12V range. This influence could be due to coupling into other systems through EMI. It can also be common to see this show up as AC variation that follows the mains line frequency.