Application of electrochemical impedance spectroscopy to monitoring discharging process of nickel/metal hydride battery

This paper presents the possibility of applying impedance analysis to cell diagnostics. In order to obtain characteristic curves of both electrodes of a NiMH battery simultaneously, a reference electrode was added into the circuit. The authors analysed the cell under three cases, i.e. when its properties are determined only by the state of the positive electrode, only by the state of the negative electrode, or when both electrodes determine operation of the cell to the same extent. Impedance characteristic curves of the entire cell in the function of variable state of charge do not allow for a conclusion as to which electrode determines cell capacity. This is not possible until an analysis of impedance graphs is carried out for each electrode. In view of the obtained results equivalent circuits for the positive electrode, the negative electrode, and the entire cell were selected. Further, a correlation between charge transfer resistance, calculated for the entire cell, and the values obtained for the positive electrode, as well as the negative one, is presented.