Batteries are composed of a series of cells from one to several and may be series or parallel connected.
Individual cells or a single cell battery in Lithium Ion or LiPoly is anywhere from 3.2 to 3.7 volts depending on chemistry.
Although cell voltage remains identical for a given chemistry current capacity of cells can be different.
All cells in a given battery will, however, have the same current capacity.
If two cells are connected in series (positive to negative) voltage will double but current capacity will stay the same.
Two cells in parallel (positive to positive and negative to negative) doubles current capacity but voltage stays the same.
The batteries we use are usually a set of cells of the current capacity we need wired in series to produce the voltage we need.
For Example a LiPo 3S1P designation refers to three 3.7 volt LiPo cells in series producing a nominal 11.1 volts.
It is possible to combine two or more batteries in parallel to double or further increase our total energy capacity.
It is also possible to combine two or more batteries in series to double or further increase the voltage.
When combining batteries in parallel (or series) they must have identical characteristics (voltage and current capacity).
For Lithium Ion we may make a battery with series and parallel cell interconnection for both the voltage and current we need.