Abstract: There is provided a method and system for monitoring the condition of one or more cells within an electrochemical energy storage device. The electrochemical device is operated at a current comprising a predefined time-varying component for a first period of time. Voltage across and a measure of heat generation for each of the plurality of cells is sensed during the first time period. An estimate is then calculated of the open circuit potential of the electrochemical storage device from the predefined time varying component of the current and the sensed voltage during the first time period. Then, for each cell, a function can be calculated which relates a ratio of the time derivatives of the sensed measure of heat generation and voltage to the open circuit potential. From this, a measure of the state of health for each cell can be determined.