The object is to have all of the cells run down at the same rate. This will allow the car to run the fastest up until the end of the race. It also allows us to fast charge the battery more efficiently. The key is that in a series wired battery pack, the same current flows through each cell regardless if the battery is being charged or discharged. By choosing cells of similar capacity and impedance (resistance to current), all of the cells will reach their maximum charge at the same time. Likewise they will run out of charge at the same time.
When charging a mismatched battery, some cells will reach their full charge state before the others. To get all the cells charged, we must keep the charge current flowing. When a cell is charged past its full charge state, it can no longer convert chemicals internally to store electrical energy. Instead the excess charge is converted to heat energy. This heats up the cell. If the current is low enough, as in trickle charging, this heat is dissipated as fast as it is generated.

You probably thought i wrote this..............i didnt, got it off the web.