Charge capacity parameter estimation system of electric storage device

A charge capacity parameter estimation system for an electric storage device capable of charging and discharging is provided, which can accurately estimate a charge capacity parameter indicative of a charge capacity of the electric storage device. The charge capacity parameter estimation system for a battery includes an ECU and a sensor unit. The ECU estimates that an actual charge capacity of the electric storage device is less than an estimated charge capacity, when a charging state of the electric storage device is detected, in a case where a generated voltage of a power generator is controlled such that the battery executes a discharging operation to an auxiliary equipment.