Impedance Measurement Technique for Vehicle Battery Health Assessment Using On-Board CAN Signals

Assessing the State of Health (SOH) of on-board battery pack is imperative to ensure optimal vehicle performance and safety. Amongst the non-destructive methods, one approach for calculating SOH involves extracting the battery’s ohmic resistance value. This process is conventionally done with the use of electrochemical impedance spectroscopy in a laboratory setting, necessitating specialized equipment. In this article, we introduce an online impedance measurement technique that utilizes only the existing sensors and accessible controller area network (CAN) bus data from the battery management system in an electric vehicle. The AC harmonics present in standard driving cycles are employed in conjunction with the Fourier transform to generate a Nyquist plot with a frequency range of 10 mHz to 0.5 Hz. The method is tested on three different standard drive schedules and the findings indicate that the ohmic resistance value can be determined from the Nyquist plot using as little as 360 seconds of sensor data sampled at 1 Hz. The value of the ohmic resistance is subsequently validated through the hybrid pulse power characterization (HPPC) test, performed using a state-of-the-art laboratory battery tester. The ohmic resistance value obtained using the proposed methodology exhibits remarkable consistency with that derived from the HPPC test results. A maximum difference of 7.4 percent is observed when a short segment of the drive cycle is employed, which decreases to a maximum of 2.7 percent when the entire drive cycle data is considered.