Online Monitoring Method for the Overall Safety Status of Battery Packs Based on Electrochemical Impedance

A safety-monitoring method based on the overall impedance of a battery pack is investigated to address the inefficiency and invasiveness of traditional disassembly based cell impedance measurements. By measuring the impedance values of individual cells via the AC injection method and conducting a superposition analysis, the relationship between the overall pack impedance and electrochemical states of the individual cells is established. An online measurement method for the overall impedance is developed, incorporating a weighted scoring system for the characteristic frequency points and local overcharge warning method based on the optimal single frequency point. Experimental results using a 280 A·h 1P16S lithium iron phosphate battery showed that the overall impedance effectively reflects individual cell characteristics under normal operating conditions and accurately inherits high-frequency impedance features during overcharge. The weighted scoring system identified 700 Hz as the optimal characteristic frequency, enabling a warning approximately 3 min earlier than those of traditional voltage-based methods. This noninvasive online impedance detection approach provides a new solution for early intervention to prevent the overcharging of battery energy storage systems.