UKF Based Estimation of SOC and Core Temperature of a Lithium Ion Cell Using an Electrical Cell Model

Energy optimization in Hybrid Electric Vehicles (HEVs) relies on the instantaneous battery pack capacity and power limits which can be impacted by a single degraded cell. Further, often battery cooling by a single fan induces a temperature gradient on individual cells that may cause a differential degradation among them. Strategies for cell balancing need to use not only the estimate of State of Charge (SOC) but also estimate of the core temperature of each cell. Here an Unscented Kalman Filter (UKF) based SOC estimator is designed using an electrical equivalent circuit model of lithium ion cell where the model parameters vary with SOC, core temperature and charge-discharge modes. These model parameters are obtained from pulsed charge and discharge test done on the cell. Outputs of the UKF based estimator, which are SOC and polarization voltage, are fed as inputs to a cell core temperature estimator. Core temperature of cells can be estimated from its surface temperature and electrical cell model. However, only a limited number of thermistors can be put inside the battery pack which makes it difficult to measure surface temperature of each cell. In that case it is required to know the temperature gradient inside the pack which will be used to estimate the core temperature of all the cells. The data used for this work is obtained from NASA Prognostic Data Repository.