Open circuit voltage and state of charge online estimation for lithium ion batteries

Open circuit voltage (OCV), as a nonlinear function of state of charge (SoC) of lithium ion battery, commonly obtained through offline OCV test at certain ambient temperatures and aging stages. The OCV-SoC relationship may be inaccurate in real application due to the difference in operation conditions. In this paper, the OCV is identified by H infinity filter (HIF) in real operation conditions. Due to the no need to derive the jacobian matrices with unscented Kalman filter (UKF), the identified discrete OCV points are propagated to state estimation process instead of the traditional OCV-SoC nonlinear function. Additionally, the polarization voltage across the polarization capacitor is also passed to state estimation in the form of discrete data points. The experimental results demonstrate that the HIF-UKF can obtain the OCV and SoC in real time.