In a two-cell flashlight, the batteries are usually connected in series. Why not connect them in parallel? What possible advantage could there be in connecting several identical batteries in parallel?

They are connected in series because the voltage that supplies the flashlight cell increases when they are connected in series. As a result, as the voltage across the flashlight cell increases, so does the brightness.

However, in parallel, the voltage remains constant while the current flowing to the flashlight cell increases. In this case, the flashlight cell consumes more power and thus becomes brighter, but it may burn quickly.

Hence, the brightness of the flashlight cell increases in series.

Advantage: If you connect several identical batteries in parallel, the voltage remains the same but the available current increases. The number of batteries x amp hours = total amp hours available but the voltage is the same.