There is provided a battery pack system with enhanced estimation accuracies of a polarization voltage and a remaining capacity of a secondary battery. A polarization voltage calculation section 108 calculates a polarization voltage Vpol from a filtered variation LPF (.DELTA.Q) of an accumulated capacity with reference to a look-up table (LUT) 1081. An electromotive force calculation section 113 subtracts the polarization voltage Vpol from an effective no-load voltage V0.sub.OK to determine a battery electromotive force Veq. A remaining capacity estimation section 114 estimates a remaining capacity SOC from the battery electromotive force Veq with reference to a look-up table (LUT) 1141.