Synthetic methods for the evaluation of the State of Health (SOH) of nickel-metal hydride (NiMH) batteries

The State of Health (SOH) of a battery is important to know the maximum energy that a battery can release while is operative and to plan the correct maintenance. In this work, we have implemented a diagnostic method to evaluate the SOH of single nickel metal hydride (NiMH) cells, that have an important role in power tools applications. Single NiMH cells of 1.2 V and 1.3 Ah have been characterized with Electrochemical Impedance Spectroscopy (EIS) technique to evaluate the SOH with synthetic methods. Through the study of an equivalent circuit model, we determine that three free parameters synthesize the information contained in an impedance spectrum. A new simplification allows us to construct a diagnostic diagram with only two degree of freedom. A mathematical approach based on the Dempster-Schafer Theory of Evidence has been implemented to interpret the diagnostic diagram. Combining the points obtained by the impedance spectra (IS) at different State of Charge (SOC) and SOH, the Theory of Evidence can improve the estimation of the SOH iteratively, a great advantage compared to the classic Theory of Probability.