A device and associated testing method for empirically determining the state-of-charge of an electrochemical energy device, comprising: applying electrical excitations to the energy device at a predetermined electrical excitation frequency ωe; applying mechanical excitations to the energy device at a predetermined mechanical excitation frequency ωm; measuring an electrically-induced phase difference Δθe(ωe) between voltage (V) and current (I) within the energy device from applying the electrical excitations; measuring a mechanically-induced phase difference Δθe(ωm) between voltage (V) and current (I) within the energy device from applying the mechanical excitations; and deducing the empirical real-time state-of-health of the energy device by comparing the electrically-induced phase difference Δθe(ωe) with the mechanically-induced phase difference Δθe(ωm); and using the deduced state of health to determine the state of charge.