As long as the conductor has intrinsic over ampacity the length issue is of minimal concern. One 8-D connected to a 12" length of 2/0 cable is essentially going to see the same delivery as an eight foot length of 2/0. Where ampacity comes into play is at high volumes of amperage. But four 8-D batteries may feed a 40MT 12-volt starter that may demand 600+ amperes. Then length will amplify voltage (resistance) differences. But high amperage is not the issue in this case.
Point of illustration: My 12 2-volt cells furthest connections are 26' from the inverter and 29' from the chargers. I had to calculate voltage drop not only to the furthest (weak sister) cell, but the total length of the circuit. Fifty eight feet. Only 500MCM cable could meet that spec. I had forgotten I used 500 MCM. It's been so many years. I have been incorrectly thinking I had used 250 MCM which itself is 30% larger than 4/0. 500 MCM is close to double the circular mil. The cable sheathing is white braided telomer fluorocarbon (Teflon). I remember the spool weighing close to 440 lbs. Stiff. I had to bend the wire mechanically.
