The battery device includes an energy storage unit, a temperature sensing unit, a storage unit, and a processing unit. The processing unit calculates the internal resistance of the energy storage unit to obtain a first increment corresponding to the internal resistance, and detects the charging voltage that is charging the battery device to obtain a second increment that corresponds to the charging voltage. The processing unit detects the discharging current of the energy storage unit to obtain a third increment corresponding to the discharging current. The processing unit further reads the cycle count from the storage unit to obtain a fourth increment that corresponds to the cycle count, reads the temperature from the temperature sensing unit to obtain a fifth increment that corresponds to the temperature, and determines the swelling rate of the battery device according to the product value of the first, second, third, fourth, and fifth increments.