Power batteries, which voltage typically ranges from few tens to several hundreds of Volts, are made connecting in series many cells (each having 3.2V or 3.7V typical nominal voltage), and these should be controlled and managed individually to ensure that none of them will work beyond normal operating conditions, thus avoiding their early decay, which would affect the whole battery.
Without single cell management:
– In discharge phase, the duration of the battery is limited by the cell that first reaches the minimum operating voltage
– During charge phase, as soon as a cell is fully charged, no more current can be injected in the battery and the charge must stop, even if most of the other cells still didn’t reached 100% of charge
The combination of both these effects with cycling charges and discharges increases the difference between the cells and heavily reduces the amount of energy the battery can source.