A method for voltage balance, comprising: controlling charging or discharging of each battery unit in accordance with an operating state of each battery unit, after a first battery unit in a plurality of parallel battery units is replaced with a second battery unit, wherein each of the branches in which the battery units locate is connected with a charge switch and a discharge switch, and the battery units are connected to each other through a designated charging circuit; controlling charging or discharging of each battery unit when the operating state of each battery unit is in a resting state, comprising: acquiring an absolute value of a voltage difference between the second battery unit and the other battery units; enabling the designated charging circuit if the absolute value of the voltage difference is greater than or equal to a threshold value for the voltage difference, so as to achieve a voltage balance between the second battery unit and the other battery units by charging each other through the designated charging circuit; or turning on a charge switch of each of the branches in which the battery units locate and turning on a discharge switch of each of the branches in which the battery units locate if the absolute value of the voltage drop is smaller than the threshold value for the voltage difference, so as to achieve a voltage balance between the second battery unit and the other battery units through a connected parallel circuit. controlling charging or discharging of each battery unit in accordance with an operating state of each battery unit, after a first battery unit in a plurality of parallel battery units is replaced with a second battery unit, wherein each of the branches in which the battery units locate is connected with a charge switch and a discharge switch, and the battery units are connected to each other through a designated charging circuit; controlling charging or discharging of each battery unit when the operating state of each battery unit is in a resting state, comprising: acquiring an absolute value of a voltage difference between the second battery unit and the other battery units; enabling the designated charging circuit if the absolute value of the voltage difference is greater than or equal to a threshold value for the voltage difference, so as to achieve a voltage balance between the second battery unit and the other battery units by charging each other through the designated charging circuit; or turning on a charge switch of each of the branches in which the battery units locate and turning on a discharge switch of each of the branches in which the battery units locate if the absolute value of the voltage drop is smaller than the threshold value for the voltage difference, so as to achieve a voltage balance between the second battery unit and the other battery units through a connected parallel circuit. acquiring an absolute value of a voltage difference between the second battery unit and the other battery units; enabling the designated charging circuit if the absolute value of the voltage difference is greater than or equal to a threshold value for the voltage difference, so as to achieve a voltage balance between the second battery unit and the other battery units by charging each other through the designated charging circuit; or turning on a charge switch of each of the branches in which the battery units locate and turning on a discharge switch of each of the branches in which the battery units locate if the absolute value of the voltage drop is smaller than the threshold value for the voltage difference, so as to achieve a voltage balance between the second battery unit and the other battery units through a connected parallel circuit. <EOS>