A vehicle battery controller comprising: a sensor configured to acquire information related to a current, a voltage, and a temperature of a subordinate battery configured to back up a main battery during autonomous driving; a DC-DC converter provided between the main battery and the subordinate battery; a switching unit configured to switch a connection state of the subordinate battery between a connection state for manual driving and a connection state for the autonomous driving; and a control unit configured to: control charging and discharging of the subordinate battery by controlling the DC-DC converter and the switching unit based on the information acquired by the sensor; tentatively determine, by executing a first battery control during a period of the manual driving after an ignition is turned ON, whether the subordinate battery is able to output backup power necessary during a limp home mode of the autonomous driving; permit the autonomous driving when determination is made, through the first battery control, that the subordinate battery is able to output the backup power; execute the autonomous driving in response to a request after the autonomous driving is permitted; and determine, by executing a second battery control after the autonomous driving is permitted, whether the subordinate battery is able to output the backup power, the second battery control having higher accuracy than accuracy of the first battery control. a sensor configured to acquire information related to a current, a voltage, and a temperature of a subordinate battery configured to back up a main battery during autonomous driving; a DC-DC converter provided between the main battery and the subordinate battery; a switching unit configured to switch a connection state of the subordinate battery between a connection state for manual driving and a connection state for the autonomous driving; and a control unit configured to: control charging and discharging of the subordinate battery by controlling the DC-DC converter and the switching unit based on the information acquired by the sensor; tentatively determine, by executing a first battery control during a period of the manual driving after an ignition is turned ON, whether the subordinate battery is able to output backup power necessary during a limp home mode of the autonomous driving; permit the autonomous driving when determination is made, through the first battery control, that the subordinate battery is able to output the backup power; execute the autonomous driving in response to a request after the autonomous driving is permitted; and determine, by executing a second battery control after the autonomous driving is permitted, whether the subordinate battery is able to output the backup power, the second battery control having higher accuracy than accuracy of the first battery control. control charging and discharging of the subordinate battery by controlling the DC-DC converter and the switching unit based on the information acquired by the sensor; tentatively determine, by executing a first battery control during a period of the manual driving after an ignition is turned ON, whether the subordinate battery is able to output backup power necessary during a limp home mode of the autonomous driving; permit the autonomous driving when determination is made, through the first battery control, that the subordinate battery is able to output the backup power; execute the autonomous driving in response to a request after the autonomous driving is permitted; and determine, by executing a second battery control after the autonomous driving is permitted, whether the subordinate battery is able to output the backup power, the second battery control having higher accuracy than accuracy of the first battery control.