Batteries in a series provide a higher voltage at once. Batteries in parallel provide a lower voltage but last longer.
Work Step by Step
See Figure 19-37. With the batteries in series, the sum of their emfs is provided to the load. It doesn’t matter if they are identical. With batteries in parallel, the current they each provide adds together. The combination provides a smaller current over a longer time period. In this case, the batteries should be nearly identical, or the one with the larger emf charges the other battery.