To provide a C/F converter having a large dynamic range and a good linearity, and to provide a chargeable battery and a chargeable battery pack capable of highly accurately calculating the present discharge current value, the amount of an accumulated discharge and the like using an A/D conversion circuit by means of this converter.

A correction circuit is provided for a current to current conversion having a characteristic for correcting a non-linear area by a straight line or by an approximately straight line for a current frequency conversion characteristic having the non-linear area for an increase in an input current to a mirror integrating circuit. Inputting the current for the frequency conversion into the mirror integrating circuit through the correction circuit allows the frequency conversion having a high accuracy in a wide range of detecting current values. For the rechargeable battery or the rechargeable battery pack using such a C/F converter, the number of pulses corresponding to the number of completion of integration in an integrating capacitor in the mirror integrating circuit is indicated as the count value for a counter, so that it is possible to calculate the current value corresponding to the amount of a charge including a transient current.