Even when building a battery pack with identical and well matched cells, the individual state-of-charge of a long string of cells will diverge over time and charge cycling. This is due to small variations  in cell  characteristics  and  localized operating conditions,  which can  lead to  small  differences in  self-discharge and load current.

Equalising or balancing a multi-cell pack helps to maximize capacity and service life of the pack by working to maintain equivalent state-of-charge of every cell, to the degree possible given their different capacities, over the widest possible range. Equalising is only necessary for packs that contain more than one cell in series. Parallel cells will naturally equalise since they are directly connected to each other, but groups of parallel wired cells, wired in series ( parallel-series wiring ) must be equalised between cell groups.