A Battery Management System (BMS) inspects a battery pack using AC impedance. A controller on the BMS drives a Pulse-Width Modulation (PWM) output signal to an on-board excitation regulator such as a synchronous buck converter that modulates a limited energy unit such as a capacitor with a swept frequency of a PWM input signal. The capacitor modulations are applied to a terminal of the battery pack as an AC excitation signal. Synchronous sampling of the battery pack provides responses to the AC excitation signal. An AC excitation signal current and a battery response voltage are processed and Fourier-transformed to generate a Nyquist plot of the excitation-response data. Curve shifts can indicate worn battery cells. The capacitor generating the AC excitation signal draws little energy from the battery pack so AC impedance inspection can occur during all modes: charging, discharging, and idle modes without an external power supply.