The problem I had with 6 volt JD batteries in series is that one is on the right side of the tractor where it grounds to the engine block. A wire, 1/0 or less gauge goes across the tractor to the other and from it you go to the starter solenoid input. The resistance of both batteries is in series. It takes 200 amps give or take to crank a moderate hp diesel at a decent temperature. 12v/200 amperes = a max resistance of .06 ohms in the circuit. When you figure that you need at least 10 volts across the starter to spin up a diesel, then the circuit resistance can only be 2v/200 amps = .001 ohm and that includes the internal resistance of the battery (ies). With 2 batteries in parallel, you halve what resitance they have (resistance in parallel halves if equal). Using a single battery located adjacent to the starter is the best bet with the battery wiring going directly to the solenoid input and ground under a bolt holding the starter to the engine block.
i have used all 3 systems and if space allows the single 12v is best for me, second is parallel if you have to leave one on either side but to do it right you need to run parallel wires from the battery on the right side to the battery on the left then wire it to the engine as stated for a single battery.
On my 400 cu in JD diesel it made the difference between having to use ether in the winter and not having to. But that worked for me!
