A method for measuring energy cell impedance, the method comprising: generating a plurality of time sequences comprising: a time sequence representing a repetitive signal at a characterizing frequency; a time sequence representing an orthogonal-phase-repetitive signal at the characterizing frequency that is ninety degrees out of phase with the repetitive signal; and at least one time sequence corresponding to a respective term of a power series polynomial, each respective term of the power series polynomial having at least one respective exponential value; applying, to an energy cell, one of an electrical current or an electrical voltage corresponding to the time sequence representing the repetitive signal; measuring, contemporaneously with the applying, a time sequence of measured values of a response waveform corresponding to another of the electrical current or the electrical voltage across the energy cell; determining a set of correlation values comprising a respective correlation value between the time sequence of measured values and each of: the time sequence representing the repetitive signal; the time sequence representing the orthogonal-phase-repetitive signal; and the at least one time sequence corresponding to a respective term of a power series polynomial; and transforming the set of correlation values into a set of fitted coefficients comprising: a fitted repetitive coefficient corresponding to a magnitude of a repetitive signal component within the time sequence of measured values at the characterizing frequency; and a fitted orthogonal-phase-repetitive coefficient corresponding to a magnitude of an orthogonal-phase-repetitive signal component within the time sequence of measured values at the characterizing frequency that is ninety degrees out of phase with the repetitive signal component; and determining an impedance of the energy cell at the characterizing frequency based on a ratio of the fitted orthogonal-phase-repetitive coefficient to the fitted repetitive coefficient. generating a plurality of time sequences comprising: a time sequence representing a repetitive signal at a characterizing frequency; a time sequence representing an orthogonal-phase-repetitive signal at the characterizing frequency that is ninety degrees out of phase with the repetitive signal; and at least one time sequence corresponding to a respective term of a power series polynomial, each respective term of the power series polynomial having at least one respective exponential value; a time sequence representing a repetitive signal at a characterizing frequency; a time sequence representing an orthogonal-phase-repetitive signal at the characterizing frequency that is ninety degrees out of phase with the repetitive signal; and at least one time sequence corresponding to a respective term of a power series polynomial, each respective term of the power series polynomial having at least one respective exponential value; applying, to an energy cell, one of an electrical current or an electrical voltage corresponding to the time sequence representing the repetitive signal; measuring, contemporaneously with the applying, a time sequence of measured values of a response waveform corresponding to another of the electrical current or the electrical voltage across the energy cell; determining a set of correlation values comprising a respective correlation value between the time sequence of measured values and each of: the time sequence representing the repetitive signal; the time sequence representing the orthogonal-phase-repetitive signal; and the at least one time sequence corresponding to a respective term of a power series polynomial; and the time sequence representing the repetitive signal; the time sequence representing the orthogonal-phase-repetitive signal; and the at least one time sequence corresponding to a respective term of a power series polynomial; and transforming the set of correlation values into a set of fitted coefficients comprising: a fitted repetitive coefficient corresponding to a magnitude of a repetitive signal component within the time sequence of measured values at the characterizing frequency; and a fitted orthogonal-phase-repetitive coefficient corresponding to a magnitude of an orthogonal-phase-repetitive signal component within the time sequence of measured values at the characterizing frequency that is ninety degrees out of phase with the repetitive signal component; and a fitted repetitive coefficient corresponding to a magnitude of a repetitive signal component within the time sequence of measured values at the characterizing frequency; and a fitted orthogonal-phase-repetitive coefficient corresponding to a magnitude of an orthogonal-phase-repetitive signal component within the time sequence of measured values at the characterizing frequency that is ninety degrees out of phase with the repetitive signal component; and determining an impedance of the energy cell at the characterizing frequency based on a ratio of the fitted orthogonal-phase-repetitive coefficient to the fitted repetitive coefficient.