A method for determining a State of Charge of a battery module with a first battery having a first rated capacity and a second battery having a second rated capacity including determining an adjusted rated capacity according to at least one of property data of the first battery, property data of the second battery and a system requirement, wherein the adjusted rated capacity is smaller than a summation of the first rated capacity and the second rated capacity; and determining a State of Charge (SoC) which indicates a level of charge of the battery module relative to a total capacity of the battery module according to the level of charge of the battery module and the adjusted rated capacity.