Analysis of Open Circuit Voltage and Internal Degradation under External Factors for EV Battery Considering Hysteresis

The open circuit voltage (OCV) tests and internal potential degradation diagnosis for vehicle application under different external factors has significance in understanding the relationship between the state of charge/ state of health of the battery and the external factors. In this paper, an analysis method is proposed for the OCV hysteresis of battery (cathode: LiFePO4 under the different external factors, including mechanical stress and temperature under driving conditions. Firstly, the main phenomenon inside the battery and impedance calculation methodology (ICM) is introduced. The ICM is used to describe the impedance conditions of ohmic resistance, charge transfer resistance, and Warburg resistance under external factors. Then, many experimental tests are conducted to investigate the OCV hysteresis and internal resistances under external factors. Finally, the results demonstrate the OCV hysteresis is mainly affected by low temperature especially. The electrochemical impedance spectroscopy (EIS) is influenced by both the mechanical stress and the temperature. The -10 °C temperature effect and extrusion stress have a huge impact on ohmic resistance; the 40°C temperature effect and 6-DOF vibration stress have great impact on charge transfer resistance. The 10°C and 0°C temperature effects have tremendous influence to Warburg resistance. In addition, according to the relationship between internal resistances or external factors and potential aging mechanisms, potential aging effect and pertinent effect, the internal pertinent degradation can be diagnosed qualitatively.