Optimised Online Multi-Sine Battery Electrochemical Impedance Spectroscopy using a Three-Phase Neutral Point Clamped Converter

Electrochemical impedance spectroscopy (EIS) is a method that is commonly used for determining the state of charge and state of health of batteries and fuel cells. Historically, it was used offline in research laboratories, but the rapid increase of demand for fuel cells and battery storage to meet the accelerating growth of electric vehicles and renewable energy has stimulated research into online in-situ EIS. This paper presents an online EIS technique integrated within the control and measurement system of a neutral point clamped (NPC) inverter. This is achieved by adding zero-sequence perturbation voltages to the inverter’s reference three phase voltages. By connecting the battery midpoint to the neutral point of the inverter, a perturbation current flows into the battery. Measurement of the battery voltage and current and subsequent spectral analysis are used to calculate its impedance. To speed up the EIS measurement, the perturbation signal includes multiple sinewaves of 5 different frequencies, instead of a single frequency sinewave. Furthermore, the phase angles of the sinewaves are optimised to prevent inverter overmodulation thus preventing distortion of the inverter’s output voltage. Detailed simulation results show close alignment of the proposed EIS measurement with theoretical values in the range of 0.01 Hz – 4 kHz, which covers the typical range for batteries and fuel cells’ interest frequency on the spectrum.