A method for characterizing electrical impedance of an energy storage device includes the following steps: (a) controlling a first switching power converter to transfer electric power between a first energy storage device and a load, (b) controlling the first switching power converter to generate a sinusoidal perturbation on electric current flowing through the first energy storage device, (c) determining an alternating current (AC) component of the electric current flowing through the first energy storage device, (d) determining an AC component of voltage across the first energy storage device, and (e) determining a complex impedance of the first energy storage device based at least in part on the AC component of the electric current flowing through the first energy storage device and the AC component of the voltage across the first energy storage device.