Abstract: Methods and apparatuses for determining the charge of a battery are disclosed. Embodiments include determining the charge in batteries that exhibit mass diffusion during charging or discharging, such as by evaluating the response of the battery to vibration. Alternate embodiments include evaluating the amplitude response of the battery and the phase shift response of the battery. Still other embodiment include evaluating the H11 response of the battery to vibration. Further embodiment include vibrating the battery, for example, with a chirp frequency that may be in the acoustic range. Still further embodiments include performing a Fourier analysis of the battery response to vibration.