In-Situ EV EIS with a High-Density Flying Capacitor Multi-Level Converter Supercapacitor System

This paper presents the implementation of a novel Electrochemical Impedance Spectroscopy (EIS) system for battery pack-level perturbation, which leverages an optimized bidirectional hybrid switched-capacitor dc-dc converter. The system consists of a 400 V, 2 kW, 6-Level Flying Capacitor Multi-Level (FCML) dc-dc converter and a 209 V Supercapacitor (SC) module, all integrated within a total volume of 2.1 L. Simulations of the EIS measurement model are used to quantify the trade-offs between the number of perturbation cycles and the accuracy of the measured impedance. The air-cooled FCML prototype utilizes closed-loop control to apply sinusoidal current perturbations for a 400 V battery without a dc offset. Experimental results validate the operation of the SC module and the bidirectional FCML converter together for a battery voltage of 400 V with 2 kW peak perturbations across frequencies ranging from 40 mHz to 2 kHz. EIS measurements are performed on 44 Ah lithium Nickel-Manganese-Cobalt-oxide pouch cells at varying state-of-charge.