Nonlinear acoustic resonance spectroscopy (NARS) for determining physical conditions of batteries

A method of determining one or more physical conditions of a battery, the method comprising:

• driving two or more acoustic signals of two or more amplitudes, each acoustic signal having two or more frequencies, into the battery, wherein the two or more acoustic signals comprise corresponding two or more chirp signals, the two or more frequencies being the same and in the same frequency range for all of the two or more chirp signals;
• detecting vibrations generated in the battery based on the two or more acoustic signals;
• determining nonlinear response characteristics of the battery for the two or more acoustic signals, from the detected vibrations, wherein the nonlinear response characteristics of the battery for the two or more acoustic signals comprise two or more resonance frequencies corresponding to the two or more amplitudes of the two or more acoustic signals;
• determining a slope or curve between the two or more resonance frequencies;
• comparing the nonlinear response characteristics of the battery to a reference nonlinear response characteristics, based on comparing the slope or curve to a reference slope or curve;
• and
• determining one or more physical conditions of the battery based at least in part on comparing the nonlinear response characteristics.