Battery state-of-health assessment using a near real-time impedance measurement technique under no-load and load conditions
Christophersen, Jon Petter
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The reliability of battery technologies has become a critical issue as the United States seeks to reduce its dependence on foreign oil. One of the significant limitations of in-situ battery health and reliability assessments, however, has been the inability to rapidly acquire information on power capability during aging. The Idaho National Laboratory has been collaborating with Montana Tech of the University of Montana and Qualtech Systems, Incorporated, on the development of a Smart Battery Status Monitor. This in-situ device will track changes in battery performance parameters to estimate its state-of-health and remaining useful life. A key component of this onboard monitoring system will be rapid, in-situ impedance measurements from which the available power can be estimated. A novel measurement technique, known as Harmonic Compensated Synchronous Detection, has been developed to acquire a wideband impedance spectrum based on an input sum-of-sines signal that contains frequencies separated by octave harmonics and has a duration of only one period of the lowest frequency. For this research, studies were conducted with high-power lithium-ion cells to examine the effectiveness and long-term impact of in-situ Harmonic Compensated Synchronous Detection measurements. Cells were cycled using standardized methods with periodic interruptions for reference performance tests to gauge degradation. The results demonstrated that in-situ impedance measurements were benign and could be successfully implemented under both no-load and load conditions. The acquired impedance spectra under no-load conditions were highly correlated to the independently determined pulse resistance growth and power fade. Similarly, the impedance measurements under load successfully reflected changes in cycle-life pulse resistance at elevated test temperatures. However, both the simulated and measured results were corrupted by transient effects and, for the under-load spectra, a bias voltage error. These errors mostly influenced the impedance at low frequencies, while the mid-frequency charge transfer resistance was generally retained regardless of current level. It was further demonstrated that these corrupting influences could be minimized with additional periods of the lowest frequency. Therefore, the data from these studies demonstrate that Harmonic Compensated Synchronous Detection is a viable in-situ impedance measurement technique that could be implemented as part of the overall Smart Battery Status Monitor.