Electrochemical and Thermal Characterization of Battery Modules Commensurate with Electric Vehicle Integration

A relatively simple mathematical representation of a nickel metal hydride traction battery is developed and implemented. The approach is based in part on an equivalent circuit comprising a resistor element in series with a parallel resistor-capacitor combination. Additional features include self-discharge, current inefficiency, temperature and state-of-charge (SOC) dependencies, and mass-transport limitations. An energy balance is coupled to the electrochemical problem; the energy balance incorporates transient heat-transfer to the battery surroundings as well as reversible and irreversible heat generation. The high-frequency and pseudo-steady-state impedance exhibit an Arrhenius temperature dependence. All the model parameters are constants or are described by continuous functions of temperature and SOC. Calculated results from the coupled electrochemical and thermal model are compared with charge and discharge experiments conducted over the time scales and current magnitudes of interest for electric-vehicle applications. The paper closes with a brief summary and a discussion of open questions.