Abstract: To accurately estimate the state-of-charge of battery pack in hybrid electric vehicles,to reduce the experimental work before application of the Ampere hour counting method,and to eliminate the accumulation of errors,the Kalman filter is developed to application.Based on the equivalent circuit model of the battery pack,the dynamic state and measurement equations are established,by processing the data collected from the current and voltage of battery pack in real vehicle testing.The algorithm developed by using Kalman filter can estimate the open circuit voltage of battery pack,then state of charge can be calculated.It indicates that Kalman filter has potential for estimating state of charge of batteries in electric vehicles.