A method includes recording a branch current and a terminal voltage of each of two parallel connected cells of a battery pack during the battery pack being switched from a charging or discharging state through a rest state to a balance state, to obtain an internal resistance of each cell, and an open-circuit voltage (OCV) and a state of charge (SOC) of the battery pack in the balance state, to obtain an OCV and a SOC of each cell just before the battery pack is switched to the rest state; and obtaining an aging differential index according to the SOC of each cell just before the battery pack is switched to the rest state, the branch current of each cell just after the battery pack is switched to the rest state, and a rated capacity of the battery pack, to determine a relative aging level of the two cells.