current only flows one way.
in a serial circuit the
batteries connect to each other (nearly) directly because the output from the first battery just enters through the other one and continues on to the resistor. practically no wiring needed, because you could just stack all parts one on the other.
battery > battery > resistor > Grounding
in parallell on the other hand, the
batteries are put, as the name suggests, parallell to each other and the two
batteries are not connected to each other at all, but both are equally connected to the circuit putting out shared amounts of Voltage to the resistor. to achieve this you can not simply connect the batteries to each other in any way, because then it would be serial again, or it would end in nothing at all. therefore in achieving a full circuit in parallell more wiring is needed, to fill the empty space between the electric source and the resistor (coil) and back. But there are also a few mod examples, that show parallel builds without to much wiring
battery
battery > resistor > Grounding
the main difference between these two is the voltage/amperage output.
serial doubling your Voltage and parallel doing it similar with your amperage.