Long-Term Validation of Rapid Impedance Spectrum Measurements as a Battery State-of-Health Assessment Technique

The objective of this study was to assess the long-term capability and impact of a rapid, in-situ impedance measurement technique known as Harmonic Compensated Synchronous Detection. This technique consists of a sum-of-sines excitation signal that includes a targeted selection of frequencies and only requires one period of the lowest frequency. For a given frequency range of 0.1 Hz to approximately 2 kHz, the measurement duration would only be ten seconds. The battery response is captured and synchronously detected for impedance spectra measurements. This technique was compared to laboratory-based performance degradation measurements using commercially available lithium-ion cells. The cells were aged for 150,000 cycles at accelerated rates using temperatures of 40 and 50°C. Every 30,000 cycles, cycle-life testing was interrupted to gauge degradation at the reference temperature of 30°C. The results demonstrated that growth in the ohmic and charge transfer resistances during aging strongly correlate with the corresponding changes in discharge capacity, pulse resistance, and available power capability that were independently determined from standardized test methods. Additionally, the rapid impedance spectrum technique appears to be a benign measurement that does not impact battery aging. Consequently, this technique appears to be useful as an onboard sensor that, when combined with passive measurements, may be used for enhanced diagnostics, management, and control.