Fast and Accurate Lithium-Ion Battery Impedance Measurement Using Multirate Quadrature Demodulation With Bandpass Filtering

Electrochemical impedance spectroscopy (EIS) is a noninvasive tool for characterizing battery states based on impedance across a specific frequency range. High-capacity lithium-ion batteries (LiBs) in electric vehicles (EVs) demand impedance measurements within a sub-mΩ resolution. While EIS systems widely adopt quadrature demodulation for its accuracy, it suffers from long settling time and high hardware complexity, limiting integration feasibility. This work presents a novel multirate quadrature demodulation scheme with complex bandpass filtering that simultaneously reduces measurement time and implementation costs while maintaining high precision. The proposed EIS system employs multiple spur suppression stages where complex bandpass filters (BPFs) suppress DC offset and image tones, enabling subsequent low-pass filters (LPFs) to operate with a wider cutoff frequency. Multirate signal processing using two distinct decimation stages enables adaptive LPF control, optimizing settling time across a 100-mHz-1-kHz frequency range. Compared to multirate architecture without complex BPF, the proposed approach achieves a 66% and 67% reduction in the impedance measurement time over the frequency range of 100 mHz-1 kHz and at the lower bound (100 mHz), respectively. Compared to prior single-rate architectures requiring multiple LPF pairs, this approach achieves 32% faster measurement over the overlapping frequency range of 1-1 kHz while requiring 26% fewer hardware resources (ALUTs, registers, and DSPs) through a single BPF coefficient set and a single infinite-impulse response LPF pair to cover the entire frequency range. Measurement repeatability shows standard deviation below 1.6μ Ω for known resistors (100 and 1 mΩ). The system is compared with a commercial electrochemical workstation (EW) for a high-capacity battery (70 Ah) across a 20%-80% state-of-charge (SOC) range at 0°C-45°C temperature.