Passive impedance spectroscopy of EV Lithium-ion batteries in transient operation

• Battery impedance estimated from natural harmonics in driving cycles. • Passive EIS performed under dynamic conditions. • Resilient to dynamic conditions via drift compensation. Impedance characterization of a Lithium-ion cell is a commonly used method to assess its state of health or internal temperature. However, Electrochemical Impedance Spectroscopy (EIS) requires a dedicated excitation system and can only be performed under steady-state conditions, making it challenging to implement onboard. The objective of this work is to develop a passive EIS method, which does not require an external excitation device and allows impedance estimation even under dynamic operating conditions. This paper proposes an impedance measurement approach based on the analysis of the naturally occurring harmonic content in electric vehicle driving cycles. To achieve this, a signal processing algorithm was developed to extract and analyze these harmonics while compensating for non-stationary effects related to current, State of Charge (SoC), and temperature. The methodology is validated through experimental tests reproducing realistic operating conditions.