Online Impedance Measurement of Batteries Using the Cross-Correlation Technique

Electrochemical impedance spectroscopy (EIS) is a powerful test technique that is extensively applied to electrochemical cells for determining the frequency response of the cell impedance. The results from EIS may be used for many applications, such as state of charge (SOC) and state of health. This article presents an approach for online EIS based on cross-correlation technique applied to a boost-type dc-dc charge controller for batteries. The theoretical background is explained and some improvements specific to battery applications are suggested. Preparation of noise datasets, postprocessing of results, and model fitting of the measurements are discussed in detail. The validity of the approach is experimentally confirmed with a digitally controlled boost converter that is charging a 12-V, 7-Ah sealed lead-acid battery: First, the perturbation magnitude is determined experimentally and then the battery is tested via the proposed method at 50% SOC. It is shown that the results of the present approach coincide with those obtained by a commercially available, laboratory-type, high-precision instruments. Finally, the tests were also repeated for 25% and 75% SOC values. It is shown that the proposed approach can be reliably used to analyze the impedance of batteries over a wide frequency range during battery charging process.