Why are batteries connected in series?
A connection in series is been made when the power, supplied to or taken from the batteries, becomes very high and resistance of the wiring could be a problem. For the correct functioning of the batteries connected in series it is very important that performances of the connected batteries are equal. This means the batteries have to be taken into service as a "set" of the same age and type. Mixing battery types causes trouble because the smaller one is overcharged and the bigger one is starved and will lose capacity.
2 batteries 12V / 120Ah in series become a 24V / 120Ah system.