A battery system comprising: a first battery and a second battery connected in parallel, each of the batteries (10,20) being suitable to perform charge and discharge; a first relay (SMR-B1) that is connected in series with the first battery and is configured to be switched between an ON state in which the charge and discharge of the first battery are allowed and an OFF state in which the charge and discharge of the first battery are prohibited; a second relay (SMR-B2) that is connected in series with the second battery and is configured to be switched between an ON state in which the charge and discharge of the second battery are allowed and an OFF state in which the charge and discharge of the second battery are prohibited; and a controller for controlling the ON state and the OFF state of each of the first relay (SMR-B1) and the second relay (SMR-B2), the system is characterised in that a group of the first battery and the first relay (SMR-B1), and a group of the second battery and the second relay (SMR-82) are connected in parallel with respect to a motor generator that receives an electric energy from each of the first battery and the second battery to generate a kinetic energy for running of a vehicle, and the controller is configured to change a first case and a second case when the first relay (SMR-B1) and the second relay (SMR-B2) are switched to the ON state to connect both of the first battery and the second battery to the motor generator, the first case corresponding to where the second relay (SMR-B2) is switched to the ON state after switching the first relay (SMR-B1) to the ON state, and the second case corresponding to where the first relay (SMR-B1) is switched to the ON state after switching the second relay (SMR-B2) to the ON state. a first battery and a second battery connected in parallel, each of the batteries (10,20) being suitable to perform charge and discharge; a first relay (SMR-B1) that is connected in series with the first battery and is configured to be switched between an ON state in which the charge and discharge of the first battery are allowed and an OFF state in which the charge and discharge of the first battery are prohibited; a second relay (SMR-B2) that is connected in series with the second battery and is configured to be switched between an ON state in which the charge and discharge of the second battery are allowed and an OFF state in which the charge and discharge of the second battery are prohibited; and a controller for controlling the ON state and the OFF state of each of the first relay (SMR-B1) and the second relay (SMR-B2), the system is characterised in that a group of the first battery and the first relay (SMR-B1), and a group of the second battery and the second relay (SMR-82) are connected in parallel with respect to a motor generator that receives an electric energy from each of the first battery and the second battery to generate a kinetic energy for running of a vehicle, and the controller is configured to change a first case and a second case when the first relay (SMR-B1) and the second relay (SMR-B2) are switched to the ON state to connect both of the first battery and the second battery to the motor generator, the first case corresponding to where the second relay (SMR-B2) is switched to the ON state after switching the first relay (SMR-B1) to the ON state, and the second case corresponding to where the first relay (SMR-B1) is switched to the ON state after switching the second relay (SMR-B2) to the ON state.