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 in which bypasses said energy storage device; and an inductor provided between the positive terminal and the negative terminal. The inductor provides a means by which to monitor current changes in the rest of the circuit. The inductor can thus be used to detect current changes as seen by the smart cell and this sensed information can be used to control the smart cell, e.g. to control an energy source (energy storage device) that forms part of the smart cell. As the inductor is part of the smart cell, it forms part of a decentralized controller strategy to regulate the state of charge of cells within a larger system. For example, it may form part of a decentralized controller strategy to regulate the state of charge of serially connected cells of varying capacities.