A smart cell, comprising: a positive terminal; a negative terminal; a switching circuit which is arranged to select between a first switching state in which an energy storage device is connected between the positive terminal and the negative terminal and a second switching state which bypasses said energy storage device; an inductor provided between the positive terminal and the output of the switching network; and a controller arranged to monitor the voltage across the inductor and arranged to control a duty cycle of the switching circuit based on the magnitudes of voltage changes detected across the inductor. By monitoring and analysing the magnitude of voltage changes across the inductor, the controller determines the states of charge of other series connected smart cells without any communication between cells. None of the smart cells need to transmit information on their states of charge to other smart cells in the string as each cell can sense information about the other cells from the voltage changes on the inductor. By analysing the voltage across the local sense inductor, the average state of charge of a series string of smart cells can be obtained and compared to the state of charge of the local smart cell to determine how the duty cycle of the local smart cell should be modified to synchronize its state of charge with the series string. The magnitude of the voltage change across the inductor is related to the state of charge of the cell that just switched in or out of the string.