When we connect the Batteries in Series, we will get sum of voltages of both the Batteries.
Here we can see that, among the 6 electron bags, 3 is provided by one 3V battery and another 3 is provided by the other Battery.
|Fig: Batteries series|
When we connect the Batteries in Parallel, we get the same voltage, but sum of the currents.
Here we can see that, both the batteries are contributing Energy bags to the bulb. We can observe that both the batteries are providing only 3 energy bags.
|Fig: Batteries paralle|