Batteries serial & parallel
If batteries connected as series, all of the battery voltage collected, but it will give the same current amount.
E.G.- If you connect four 1.5V batteries, output is 6V.
Parallel – If batteries connected as Parallel, it will give the same one battery voltage. But it will give more current amount, because the all current amount collected.
E.G – If you connect four 1.5V batteries parallel, it will give only 1.5V. But will give high current amount.