Batteries: Two-6V In Series, or Two-12V In Parallel ???

So maybe I need to go through my rig to make sure I have now, or change the positive wire to a much larger diameter. I have real nice thick cables (as thick as a finger) going to the batteries now, but I don't know if they are that thick all the way to the isolator & alternator.

Here is a picture of what I see at the batteries today.