Google tells me that a battery can be treated as a simple resistor with respect to voltage drop. And so can all the wires and connectors. Apparently then Ohm's law applies.
Example:
A single 12V battery with 50A load (600W) has Vd voltage drop
Two 12V batteries in parallel, with 100A load (50A each, 1200W total), also has Vd voltage drop. Half the internal resistance but twice the current so Vd remains the same.
Two 12V batteries in series, with 50A load (@ 24V, 1200W total), has 2xVd voltage drop? Twice the internal resistance of a single battery but the same 50A current, so TWICE the voltage drop.
But, percentage-wise the Voltage drop of two batteries in parallel is the same as two in series. And the wattage (VA) drops is the same.
No idea if this happens in practice though.