When I plug in my car, I see an initial voltage of 240V at 0A, when the charge current climbs to 16A, it drops to 233V. That is a drop of 7 V, That means there is a resistance of 0.44 ohm. I know from this table(American wire gauge - Wikipedia, the free encyclopedia), #10 AWG has a resistance of 1 ohm/ 1000 ft. It is hard to visualize a 440 ft wire round trip to the panel. My guess is a 80 ft max routing distance from subpanel to the main panel. that leaves 0.28 ohm resistance unaccounted for. Is that the contact resistance in the system ? what is the voltage drop do you see ?