State of charge estimation for electrical vehicle batteries

Lithium ion batteries play an increasingly important role as high-rate transient power sources for hybrid electric vehicles, cycling around a relatively fixed state of charge (SOC). A crucial step in enhancing the performance of these batteries is the estimation of the state of charge as a function of the load. Most of the existing literature supports an empirical model - based on either an electric circuit, arbitrary pole placement or an analytical expression with an arbitrary set of parameters. Such empirical do not provide information on the physical limitations. Alternatively, an electrochemical cell model incorporating transport, kinetic and thermodynamic limitations can be used to estimate parameters that hold physical significance and hence provide a better insight into the cell performance. Estimating the SOC of a battery using physics-based models and filtering algorithms is illustrated. The relative merits and disadvantages of the approach are evaluated.