The series connection adds the voltage of both 12 volt batteries for a total of 24 volts. In parallel the voltage stays the same and in series it is doubled.

Charging 6 Volt Batteries In Series With 12 Volt Charger
2 12 volt batteries in series. A 6 volt battery is often three 2 volt cells and a 12 volt battery is usually six 2 volt cells. One way doubles the volts of the two batteries and the other keeps the same amount of volts. Connecting batteries of different voltages in series in theory a 6 volt 5 ah battery and a 12 volt 5 ah battery connected in series will give a supply of 18 volts 6 volts 12 volts and 5 ah. Cut a piece of copper wire around 6 inches long and using wire strippers remove 12 an inch of insulation from each end. Connect the positive terminal of one battery to the negative terminal of the other. In a parallel connection the voltage remains the same but the overall amperage of both batteries is added together.
There are two ways to wire up two batteries. There will be 24 volts about 28 volts with an alternator if two 12 volt batteries are wired together in series. Check out the diagram for more info. For example these two 6 volt batteries joined in series now produce 12 volts but they still have a total capacity of 10 amps. Series and parallel methods means that charging 12 volt 12 v batteries in line can use either a series or a parallel circuit. In series circuits the current is constant throughout the circuit and the voltage changes across each element of the circuit.
For example a 6 volt battery could be produced by connecting four 15 volt cells together in series. You need to overcome the total voltage of the series batteries 12 v 12 v 24 v to drive electrons. How to wire batteries in series. There are two methods to connect two 12 volt batteries. Connecting batteries in series adds the voltage of the two batteries but it keeps the same amperage rating also known as amp hours. To charge two 12 volt batteries in series youd need a 24 volt charger.
Thats the pat answer.