A battery charger capable of correcting the deterioration of a secondary battery left unattended in a charge state and capable of accurately grasping a residual capacity. A battery pack having a nonvolatile memory and a secondary battery cell is attached to the battery charger. Full charge capacity data indicating a chargeable capacity of the battery pack at full charge and a left-charged battery deterioration correcting table to correct the full charge capacity data in accordance with the number of charge cycles are read out from the nonvolatile memory. The charged state data is counted each time it is stored in the nonvolatile memory. A battery cycle deterioration correcting value is specified based on the number of charge cycles, and full charge capacity data is corrected using the specified correcting value. The full charge capacity stored in the nonvolatile memory is rewritten.