You have that backwards:Parallel connected batteries increase voltage while keeping amp hours the same.
Series connected batteries increase amp hours while keeping the voltage the same.
Wire size and length add resistance to the circuit and increases voltage drop.
Parallel batteries (pos to pos, neg to neg) = same voltage double the amp hours.
Series batteries (neg1 to ground, pos1 to neg2, pos2 to unit) =double the voltage, same amp hours.