Cell balancing is the process of equalizing the voltages and state of charge among the cells when they are at a full charge. No two cells are identical.
There are always slight differences in the state of charge, self-discharge rate, capacity, impedance, and temperature characteristics.
This is true even if the cells are the same model, same manufacturer, and same production lot. Manufacturers will sort cells by similar voltage to match as close as possible,
but there are still slight variations in the individual cells impedance, capacity, and self-discharge rate that can eventually lead to a divergence in voltage over time.
Most typical battery chargers detect full charge by checking whether the voltage of the entire string of cells has reached the voltage regulation point.
Individual cell voltages can vary as long as they don’t exceed the limits for overvoltage protection.
However, weak cells (ones with lower capacity / higher internal impedance) tend to exhibit higher voltage than the rest of the series cells at full charge termination.
These cells are then weakened further by continuous overcharge cycles. The higher voltage of the weaker cells at charge completion causes accelerated capacity degradation.
If the maximal recommended charging voltage is exceeded even by as little as 10 percent, it will cause the degradation rate to increase by 30 percent.
On the discharge side, the weak cells tend to have lower voltage than the other cells, due to either higher internal resistance,
or a faster rate of discharge that results from the lower capacity.
This means that if any of the weak cells hits the cell under voltage protection limit while the pack voltage is still sufficient to power the system,
the full capacity of the battery will never be used as the pack protector will prevent over discharge (which would damage the cell)
by stopping the discharge of the whole pack when one cell voltage goes below the cell under voltage threshold (usually around 2.7 V).