The parallel circuit still generates 12 volts but doubles the amperage output. Charging only some of the batteries will result in batteries attempting to equal out the power and charging each other.
Charging Two Lead Acid Batteries Connected In Series
Charging two 12 volt batteries in series. Two 12 volt batteries wired in series produce 24 volts but the capacity remains the same as one for a 12 volt battery. Batteries may be wired in parallel or in series. Two 12 volt batteries wired in parallel continue to produce 12 volts but the capacity doubles. You need to overcome the total voltage of the series batteries 12 v 12 v 24 v to drive electrons. If the batteries are not each at the same voltage this means that one battery is charging higher than the rest and one is not charging as high as the rest. Charge all the batteries at the same time.
In the second example to charge a 24v system simply use each bank of the charger on each 12v battery terminal leads. Charge more than five batteries by connecting one 12 volt battery charger across each battery in the series as if each battery were the only one being charged. In basic form here is the difference between wiring two 12 volt batteries in series versus in parallel. As in the diagram above two 6 volt 45 ah batteries wired in series are capable of providing 12 volts 6 volts 6 volts and 45 amp hours. To charge two 12 volt batteries in series youd need a 24 volt charger. Thats the pat answer.
When you charge batteries in series you are charging the entire group of them all at the same time. In the examples below two 6v batteries in series 12v. The charger will stop charging when the entire group reaches a certain voltage. Charging 12 volt batteries wired in parallel requires a 12 volt battery charger. The basic concept when connecting in series is that you add the voltages of the batteries together but the amp hour capacity remains the same. This requires only one bank of a 12v charger to be attached to the positive terminal of the first battery and the negative terminal of the second battery.