Leveling Li Cells
A stack of Lithium batteries in general can't be balanced from the outside. The stack therefore requires a balancing of their individual cells prior to assembly. So-called "balancers" are used for this task. The level the the cells by charging them to a certain identical voltage.
Lithium battery stacks of a higher quality are often equipped with a protection circuit that not only monitors the individual cell voltage, current and temperature. Furthermore these protection circuits have a built-in state-of-charge leveling device. Because the cell voltage of each individual cell is known, it is quite simple to bypass a little amount of carging current to the cells with the highest voltage. This keeps or restores the balance between the individual cells.