Sensorless On Board Cell Temperature Control for Fast Charging

Fast charging capability is one of the key requirements for the success of electric vehicles. Considering the growing energy storage capacity of automotive batteries, fast charging can only be achieved using high-power charging systems. This leads to increased power dissipation inside the battery cells. The resulting heat generation inside the battery cell is a critical effect, as cell safety, performance and life time strongly depend on cell temperature and current. This must be considered by a simultaneous current and thermal battery management strategy, which requires reliable information about the individual cell temperature. Sensorless cell temperature can be derived from the cell impedance, where the charging current profile is superimposed by an excitation current and the resulting cell voltages are observed by the battery management system (BMS). An efficient algorithm for the impedance and temperature calculation can be implemented in actual BMS. In this work, this concept is verified by fast charging experiments. The thermal properties of a prismatic cell for electric vehicle energy storage are investigated under real boundary conditions, including effects of active fluid cooling. For a more detailed thermal analysis and modeling, cell surface temperature distribution is monitored by a temperature sensor array. The internal and external cell temperature increase is analyzed for different fast charging profiles. 3-dimensional thermal modeling is used to determine the internal peak temperature from the average measured temperature for a given cell type and assembly. The results can be used to define fast charging and thermal management strategies that are optimized for safe operation and long life.