A battery device includes a first battery and a second battery connected in series. An electronic device for determining a state of charge of the battery device includes a sense resistor connected in series to the second battery, and a fuel gauge. The fuel gauge is configured to determine the state of charge of the battery device based on a first terminal voltage of the first battery, a first calculation current of the first battery, a second terminal voltage of the second battery, and a measurement current of the second battery measured through the sense resistor. The fuel gauge calculates a second calculation current of the second battery using a battery parameter of an equivalent circuit model, corrects the battery parameter when a difference between the measurement current and the second calculation current is not less than a threshold value, and calculates the first calculation current using the battery parameter or a corrected battery parameter.