OK cool. I could use the 10gauge line that is currently there for the sense but that is a large amount of impedance for the sense line. But the stock FJ60 alternator to battery line is a 10gauge...additional thoughts?
I did a quick look at the Isuzu FSM and they also use the equivelant of #10 but in another section they state that the maximum allowable amperage is 37 amps. Go figure! If you want to continue using the #10 as the primary feed to the battery, then I would recommend running the sense wire to the battery. A #18 wire would be more than sufficient as the current in the sense wire is negligible.
For whatever it is worth, impedance is defined as AC resistance and, since you are dealing with DC, does not apply here. Also, if there was an AC component, the impedance of a 6 ft. #10 wire running between two points would be negligible also.