Two or more 12 volt batteries wired in seriesthe positive terminal of one. In parallel the voltage stays the same and in series it is doubled.

Trolling Motor Batteries With 4 Wire System
Two 12 volt batteries to make 12 volts. The series connection adds the voltage of both 12 volt batteries for a total of 24 volts. Two or more 12 volt batteries wired in parallelpositive to positive negative to negativeis still a 12 volt system. Wiring two 12 volt batteries in series combines the voltage from each unit to double its output thereby allowing the electric current to flow evenly through every component in the circuit. To produce a 12 volt output two 6 volt deep cycle batteries must be wired in series positive to negative. You can use the same electrical technique to create your own battery packs at home. Each 6 volt battery pair operates as a single 12 volt battery.
Wiring two 12 volt batteries in parallel provides longer battery life but the output will remain 12 volts. There are two ways to wire up two batteries. How to make 12 volt battery charger easy way diy homemade battery charger duration. To make batteries with higher voltages manufacturers link identical batteries in a series circuit. In a parallel connection the voltage remains the same but the overall amperage of both batteries is added together. Connect a battery cable from the second batterys negative terminal to the starters ground connection.
Add a second 12 volt battery to the circuit. There are two methods to connect two 12 volt batteries. In this way the voltages of the individual batteries are added together so six 2 volt battery cells become one 12 volt battery 6 x 2 12. Disconnect the cable from the starters ground connection and connect it to the second batterys negative terminal. You can then connect each 6 volt pair in parallel with another pair to create a larger 12 24 or 48 volt battery bank. One way doubles the volts of the two batteries and the other keeps the same amount of volts.