I can never remember which is series and which is parallel, but what doubles up is the capacity (amp/hours) while the voltage stays the same. I have a boat that is wired with one battery primarily for starting and one for operating the boat's electrics (lights, water pumps, depth finder, radar, GPS(s), VHF radios, etc.) with the switching set up such that the "house" system can jump the starting system. Still, the system is 12-volt.