I'm confused a little bit about whether or not a DC return line carries any current. the reason is that in our design we have 2 batteries 24V and 8V, they supply ~4A and ~2A respectively to different parts of a circuit. Both (-) terminals are tied together on the PCB on a plane which acts as the system ground or reference. this PCB is coneected to the rest of the circuit thru a connector. The design allowes 4 pins for the (+) terminal of each battery, and 2 pins for ground (whcih connect to the PCB ground plane) The connector manufacturer specs each pin to withstand 1A current. I think 1 pin for ground should be enough but added a 2nd pin for redundancy because the ground should not carry any current. also I thougth it would be better to have less gound paths to eliminate ground loops. My question is am I write or wrong? and we should add more ground pins? thanks in advance!!
For any circuit to operate correctly, there must be a closed loop for current to flow. Therefore, current does flow through the return path (system ground, in your circuit).
As you can see, current flows around the entire circuit (dissipating most of its energy across the load) and eventually back to the negative terminal of the source (batteries in your circuit). So, you must consider the maximum current specifications of the pins, given the high currents in your circuit. All that current is definitely flowing in system ground.
If the current limit is 1A per pin, you should definitely include more pins to ensure reliable power delivery without overheating.
To further address your grounding concerns, I would recommend either purchasing a different connector with a higher amperage rating, or re-designing the PCB to have large and thick pads for connecting to the terminals of the battery.
Remember, all that current is flowing both from the positive pad terminal (+), through the circuit, and back to the negative terminal (-). Therefore, you must ensure the entire path along which the current flows has been designed to handle this current. If your traces are too long and/or thin, you risk wasting significant amounts of power along the delivery path. There's no such thing as an ideal wire, and having long, thin wires conducting large amounts of current is not good design!
Adding a few extra pins/pads for a localized negative (-) terminal for your battery will not cause any ground loop issues assuming your circuit was designed properly in the first place.