Development and test of a real time battery impedance estimation system

Electrochemical impedance measurement systems use the Bode analysis technique to characterize the impedance of an electrochemical process in a well-established and proven technique. The battery being evaluated is excited with a single frequency current and its response is measured. The process is repeated over a range of frequencies of interest until the spectrum of the impedance is obtained. The method is effective but time consuming, as the process is serial. A parallel approach using bandwidth limited noise as an excitation current can obtain the same information in less time. The system response to the noise is processed via correlation and fast Fourier Transform (FFT) algorithms and many such responses are averaged. The result is the spectrum of response over the desired frequency range. The averaging of many responses also makes this process somewhat serial. Another technique assembles the current noise waveform from a sum of sinusoids each at a different frequency. The system response as a time record is acquired and processed with the FFT algorithm. To reduce noise, multiple time records of waveforms are processed and their resultant spectra averaged. This process is also serial. There is interest in real-time acquisition of battery impedance for control and diagnostics over a limited frequency range. To support this need, a true parallel approach has been developed by Montana Tech of the University of Montana (Montana Tech) that uses a single acquired time record of the battery response with duration compatible to a real-time control process. The system uses custom-designed instrumentation integrated with a National Instruments Data Acquisition System. The system excites a test battery with current and acquires a time record of the voltage response. The time record of data is processed with a Montana Tech-developed algorithm to yield a frequency spectrum of the test batteries impedance