Li-Ion Battery Pack Characterization and Equivalent Electrical Circuit Model Development

This paper outlines the characterization of a Li-Ion Iron Phosphate battery pack with nominal voltage of 700V as well as the modeling of this pack as an equivalent electrical circuit (EEC) for the purpose of vehicle simulations. For a higher level of fidelity and accuracy, the equivalent circuit is initially modeled as an R-2RC circuit which consists of a voltage source with one resistor (R) and two resistor-capacitor (RC) branches. In this modeling effort, first, several open circuit voltage (OCV) determination methods in the literature are benchmarked and state-of-charge (SOC) dependent OCV curve which is used in the voltage source of the EEC model is derived. Then, two methods of parameter estimation of the EEC are developed for both step current and dynamic current profiles. The first estimation method is applicable to discharge or charge step currents and relies mostly on the relaxation portion of the battery response and involves some manual calibration. The second estimation method utilizes online parameter estimation techniques and learns the EEC parameters automatically by processing the battery response to some special designed dynamic current profiles. At the final stage, the battery model is validated against actual battery test results for a current profile experienced in a heavy duty vehicle drive cycle. According to these test results, the necessity of R-2RC models and the sufficiency of R-1RC or R models are elaborated for the purpose of vehicle simulations and some modifications are proposed and validated for the standard R-2RC battery EEC model.