A system and method estimates the state of charge of a battery comprising at least two modules each provided with at least one cell. A sensor senses a series current flowing through all cells of the modules connected in series. A switching device selectively disconnects one of the modules, the other modules remaining connected in series. A calculator calculates the state of charge of the cells through which the series current flows from the integration of the series current and calculates the state of charge of each of the cells of the disconnected module from an open circuit voltage thereof.