State of health (SOH) estimation plays an important role in battery management system. This problem is addressed by introducing a second-order RC equivalent circuit model (ECM) and square root extended kalman filter (SREKF) in this work. According to the ECM, the ohmic resistance (R0) which represents the SOH is molded as a state vector, and lithium battery SOH is obtained by using the internal relationship between ohm resistance and SOH. Then the state of the resulting nonlinear dynamic system is estimated in real time by the SREKF which can guarantee the symmetry and non-negative characteristics of the state variance matrix by the square root method recursively. The verification results show that the SREKF algorithm can estimate the SOH parameter more accurately and reduce the maximum error of the ohmic resistance by 6% under 1C-rate constant current test compared with the EKF algorithm.

SOH,Lithium Battery,SREK,ohmic resistance