Fuzzy logic modelling of state-of-charge and available capacity of nickel/metal hydride batteries

Ni/MH batteries are playing important roles in many applications such as power tools, and a dominant role in hybrid electric vehicles (HEVs). In the case of HEVs it is particularly important to be able to monitor the state-of-charge (SoC) of the Ni/MH batteries. We have previously reported on the use of a fuzzy logic (FL) methodology to estimate the SoC of various battery chemistries, including lead-acid and lithium sulfur dioxide. In the present work, we have measured electrochemical impedance spectroscopy (EIS) on 2.7 Ah Sanyo Ni/MH cells and two- and three-cell strings of these cells at different SoC's and over 100 cycles. We have been able to select features in this data to develop fuzzy logic models for both available capacity and SoC estimation, simply by measuring the impedance at three frequencies. The fuzzy logic model estimates the SoC to within +/-5%. In this paper we will present the details of the experimental measurements, the details of the fuzzy logic models themselves, and the resulting accuracies of the developed models. (C) 2004 Elsevier B.V. All rights reserved.