This paper proposes a method of accurately estimating the state of charge (SOC) of lithium-ion rechargeable batteries in high fuel efficiency vehicles, such as hybrid electric vehicles (HEVs) and electric vehicles (EVs). Although it is important to accurately estimate SOC of the battery to maximize efficiency and safety, there exist many problems for conventional methods. To address this issue, a model-based approach using "Series Kalman Filters" is proposed and implemented in this paper. Its approach is verified with series of simulations under the basic HEV operating environment. A discussion on a limitation of the method is also included in this paper. The ultimate goal is to design a state estimator capable of providing accurate state estimation of batteries under any possible user conditions.