A battery pack circuit controls the charging and discharging of a battery pack including a first standard cell having a first standard cell capacity coefficient/voltage characteristic curve and a shift cell having a shift cell capacity coefficient/voltage characteristic curve. The shift cell capacity coefficient/voltage characteristic curve mirrors the standard cell capacity coefficient/voltage characteristic curve but is offset from the standard cell capacity coefficient/voltage characteristic curve by a predetermined amount along a reference axis of the shift cell and standard cell capacity coefficient/voltage characteristic curves. A memory stores a potential difference characteristic curve for the standard and shift cells. A processor detects a present potential difference between the standard cell and the shift cell. A processor determines a present capacity coefficient value of the standard cell as a function of the present potential difference detected by the processor and the potential difference characteristic curve.