 A robust extended Kalman filter, EKF, has been developed for accurate state of charge SOC, estimation of lithium ion batteries used in hybrid electric vehicles, HEVs. An equivalent circuit model of the battery was used to analyze the effects of the EKF gain coefficient on SOC estimation. An optimal gain coefficient was determined to minimize the variation in battery terminal voltage. Experimental and simulation results were presented to demonstrate the accuracy and precision of the proposed robust EKF algorithm for SOC estimation. The standard EKF was compared with the proposed robust EKF algorithm to show the superiority of the latter for accurate SOC estimation. This article was authored by Suleiman M. Shark, Weigazhan, Jochun Zhang, and others.