A resistance factor determining method of a secondary battery according to the present disclosure determines, when the secondary battery has a certain temperature and state of charge, a primary differential value regarding a charge initial I-V profile calculated from a current value at a point of intersection at which a charge final I-V profile according to changes in magnitude of the charging current meets a boundary line set to a charge upper limit as the resistance factor corresponding to the temperature and state of charge. Further, a charge power estimating apparatus and method according to the present disclosure estimates a charge power corresponding to the temperature and state of charge of the secondary battery while the secondary battery is being charged using the resistance factor pre-defined according to the state of charge and temperature of the secondary battery.
