The batteries were originally in the observers compartment. They are now in with the ballast bags right up against the seat backs.
The space gained by moving the batteries is amazing on my boat.
I now have golf cart batteries and a custom built hdpe battery holder. This was with a 4d deep cycle.
My starter/ backup battery sits on the drivers side and then my golf carts sit on the port side.
I ran the stock 4AWG and then added a 00AWG ground and positive wires to the observer compartment where the fuses are. My battery switch sits under the corner seat by the batteries.