An SOC estimation method for a battery system includes mapping an upper limit and a lower limit of the state of charge of the second-system battery to a state of charge interval of the battery system, to establish a mapping relationship between the upper limit and the lower limit of the state of charge of the second-system battery and the state of charge interval of the battery system. The method includes calculating the state of charge of the second-system battery and estimating a state of charge of the battery system according to the mapping relationship.