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State of charge estimation based on the extended Kalman filter (EKF) has recently received attention for applications that apply rapidly changing current profiles to batteries such as in hybrid electric vehicles. A new battery model is proposed for use with such an estimator in an attempt to further improve its accuracy. The model is based on physical principles underlying the behavior of valve regulated lead acid batteries, taking into account nonlinearities that have been neglected in other models. The model is compared to a linear circuit model consisting of two parallel RC circuits, often used in EKF based state of charge estimation, demonstrating improvement in accuracy.