An apparatus and method for measuring energy cell impedance. Time sequences representing each of a repetitive signal, an orthogonal-phase-repetitive signal at the characterizing frequency, and at least one term of a power series polynomial are generated. One of a current or voltage corresponding to the repetitive signal is applied to an energy cell. Contemporaneously with the applying, measured values of a current or voltage are measured. A set of correlation values between the measured values and the generated time sequences are determined. The set of correlation values are transformed into a set of fitted coefficients of a repetitive signal component and an orthogonal-phase-repetitive signal component at a characterizing frequency. An impedance of the energy cell at the characterizing frequency is determined based on a ratio of the fitted coefficients for the orthogonal-phase-repetitive component to the repetitive signal component.