A resistance factor determining method of a secondary battery, the method comprising: (a) measuring, for a plurality of states of charge of the secondary battery, a plurality of temperatures of the secondary battery and for a plurality of values of a charging current, a voltage of the secondary battery at a first time during charging thus obtaining a plurality of charge initial voltage data and a voltage of the secondary battery at a second time during charging later than the first time thus obtaining a plurality of charge final voltage data, and storing the plurality of charge initial voltage data and the plurality of charge final voltage data in a memory; (b) for each combination of state of charge and temperature, determining a charge final current-voltage profile from the plurality of charge final voltage data by plotting the measured charge final voltage as a function of the charging current, and determining a point of intersection at which the charge final current-voltage profile meets either a boundary line corresponding to a charge upper limit current or a charge upper limit voltage pre-set to a charge upper limit condition; (c) for each combination of state of charge and temperature, determining a charge initial current-voltage profile from the plurality of charge initial voltage data by plotting the charge initial voltage as a function of the charging current, and determining a primary differential value for the charge initial current-voltage profile based on a current value of the point of intersection of the corresponding charge final current-voltage profile; and (d) for each combination of state of charge and temperature, establishing the determined primary differential value as the resistance factor for the considered temperature and state of charge of the secondary battery. (a) measuring, for a plurality of states of charge of the secondary battery, a plurality of temperatures of the secondary battery and for a plurality of values of a charging current, a voltage of the secondary battery at a first time during charging thus obtaining a plurality of charge initial voltage data and a voltage of the secondary battery at a second time during charging later than the first time thus obtaining a plurality of charge final voltage data, and storing the plurality of charge initial voltage data and the plurality of charge final voltage data in a memory; (b) for each combination of state of charge and temperature, determining a charge final current-voltage profile from the plurality of charge final voltage data by plotting the measured charge final voltage as a function of the charging current, and determining a point of intersection at which the charge final current-voltage profile meets either a boundary line corresponding to a charge upper limit current or a charge upper limit voltage pre-set to a charge upper limit condition; (c) for each combination of state of charge and temperature, determining a charge initial current-voltage profile from the plurality of charge initial voltage data by plotting the charge initial voltage as a function of the charging current, and determining a primary differential value for the charge initial current-voltage profile based on a current value of the point of intersection of the corresponding charge final current-voltage profile; and (d) for each combination of state of charge and temperature, establishing the determined primary differential value as the resistance factor for the considered temperature and state of charge of the secondary battery.