Electrochemical impedance spectroscopy is an important parameter to characterize the performance of the battery and evaluate the state of health. However, the current methods of EIS measurement take too much time and require specific equipment of high cost. In this paper, an automatic method through three-section measurement for online EIS measurement is proposed, which can be obtained directly through cross-correlation and digital control. In the battery charging system, the impedance spectrum of the system can be obtained directly by integrating the frequency response measurement capability to the digital controller of Buck converter. This paper demonstrates the validity of this method by mathematical formula derivation and proves the feasibility by simulation on MATLAB.