Lipo batteries contain 2,3,4,5,6 cells... 2S,3S etc etc...
Each cell has a voltage at 'rest' of 3.7 volts.... You hook them together in a pack like 3S, and you get 11.1 volts , 'at rest' when the pack is drained. A charged 3S is like 12.6 volts.
Apparently even though the cells are in series, and the current flow through each cell on its journey to the motor, a cell can become unbalanced.
That is one can be 3.7, another 3.9 volts and from what I've read it is not a good situation.
Some chargers take the time to balance charge each cell to the same voltage and I think its through a special connector that is on the side of the cell...

Anyone? How close am I ?
Tony