State of health estimation for battery modules based on EIS analysis

Sammanfattning

The usage of lithium-ion batteries has enhanced a lot in recent years much due to today’s technical society. In step with the increased demand are higher requirements set regarding of the battery’s performance and its lifespan. The status of a battery is known as the battery’s state of health. Electrochemical impedance spectroscopy is an established method used for measuring the impedance at different frequencies of a battery. The generated impedance, the EIS-data, could be visible in a Nyquist plot were the electrochemical processes occurring inside of the battery could be identified based on frequency. The Nyquist plot could therefore be used to give indications about the status and performance of the battery. There are no general methods about how information of state of health could be generated directly from the present EIS measurement. The purpose of this study was to investigate how measured EIS data could be utilized to determine the state of health for battery modules. To determine the state of health of a battery module the degradation process and the capacity loss must first be established. This is required since both degradation and capacity loss are directly connected to the state of health. An increase of degradation and capacity loss entails a decrease in state of health estimation. Based on the degradation process and capacity loss, different parameters connected to these could be used to determine or give an estimation regarding the state of health. By using EIS-measurements of different battery modules the generated EIS-data could be displayed in a Nyquist plot. Based on the Nyquist plot different parameters corresponding to several electrochemical processes occurring in the battery could be determined. These parameters were connected to degradation or capacity fade and could therefore be used as indicators for estimations regarding state of health. The parameters chosen from the Nyquist plot to determine the degradation process and the capacity fade was ohmic resistance and arc diameter. It was established that an increase of these parameters resulted in an increase of degradation and capacity fade. This result could then be used to indicate the determination of state of health.