Experimental Aircraft Battery Management System - (BMS)

The faster the charging, the greater the imbalance that is occurring.
It is a common weakness with all lithium batteries that cell imbalance is a liability in every battery system. If lithium batteries are overheated or overcharged, those conditions will accelerate battery degradation and quickly shorten the batteries lifespan. Just as there are no two identical snowflakes, so too are there no two identical cells. There are always subtle differences in SOC, self-discharge rate, capacity, impedance and temperature characteristics. This is the case even if the cells are the same model, same manufacturer or same production batch.
Without a robust balancing ability the results are a large difference in voltage over time decreasing capacity.
In Passive balancing, the practical goal is to achieve capacity balance at the end of charge. However, due to the typical low balancing current, if the cells begin to diverge in SOC, it is virtually impossible to correct the charge imbalance at the end of charging. In other words, Passive balancing, while avoiding overcharging of the strongest cells does not allow a full charge of the weaker cells because extra energy is wasted in shunt resisters as heat... a LOT of heat!
With Active type balancing, 2 goals - achieving voltage parity at the end of charge and minimizing V differences among cells can be achieved at the same time. Energy is conserved and transferred to the less charged cells which results in increased safety, discharge capacity and life of the battery.
Unless the cells are well balanced, a ' weaker ' cell in the pack will limit the overall performance of the battery and eventually render the battery unusable. To avoid this, the cells should be balancing at all times  not just while being charged so that the differences between cells are as small as possible.
Operational wise; the Passive method is simple and straightforward - the BMS uses resisters to dissipate energy. It is cost effective and this method is used in all older tech aircraft batteries still available today. However, since 100% of the excess energy is turned into heat inside the battery, and it is incapable of keeping up with the incoming charging current, imbalance is assured and runtimes are diminished.
A typical BMS with Passive balancing comes with 50 - 200milliamps capability. When the charging current is high - such as from an alternator - the resisters are overwhelmed and the heat generated by them degrades the cell pack over time.
Active balancing on the other hand utilizes capacitive or inductive charge shuttling to deliver energy where it is most needed with minimal loss and heat generation.