When two or more sources are arranged so that the positive terminal of one is connected to the negative terminal of the next, they are said to be in series and their e.m.f.s add up.
This arrangement gives increased e.m.f. because, the charge flowing round a circuit will pass through more than one source and gains electrical potential energy from each of them.
Cells can also be arranged in parallel. In this, all the positive terminals are connected together and all the negative terminals are connected together. The combined e.m.f. in parallel connection will not increase like in the series connection. But the battery will last longer before going flat.