A patent search will reveal numerous strategies for 'Lithium battery balance chargers'. These all read the voltage of individual cells in a series string and ensure any cell get whatever it needs to charge to the same voltage as the rest of the cells in the battery. This is essential for all high capacity rechargeable Lithium systems and is used in most laptop computers.

A very cute system places a large capacitor across one cell. It then puts the same capacitor across the next cell. Any voltage difference results in a small charging/discharging of that cell. The switching process cycles indefinitely and all cells end up at exactly the same voltage. Being capacitive, the system is essentially lossless and very efficient.


HTH
Brian