A battery system comprising: a plurality of batteries (2a-2c); a plurality of switching units (4a-4c), each switching unit being configured to switch between a connected state in which a corresponding one of the plurality of batteries (2a-2c) is connected in series with other batteries of the plurality of batteries (2a-2c) and a non-connected state in which the battery is bypassed and disconnected from the other batteries; a charger configured to charge the batteries of the plurality of batteries connected in series, the charger being switchable between a constant current charging mode and a constant voltage charging mode; a controller configured to control the plurality of switching units (4a-4c) and the charger ; wherein the controller is configured to cause the charger to charge the plurality of series connected batteries with a constant current, to determine whether one of the plurality of batteries has reached a charge changing voltage during the constant current charging, to cause the respective switching unit to bypass and disconnect the battery determined to have reached the charge changing voltage, to cause the plurality of switching units to connect all of the plurality of batteries in series when all of the plurality of batteries have reached the charge changing voltage, and to switch the charger from the constant current charging mode to the constant voltage charging mode so as to charge the plurality of series connected batteries at a constant voltage, characterized in that the controller is further configured to perform operations comprising determining whether one of the plurality of series connected batteries has reached a charge termination voltage during constant voltage charging, and causing the respective switching unit to bypass and disconnect the battery determined to have reached the charge termination voltage, the operations being repeated until all of the plurality of batteries are determined to have reached the charge termination voltage. a plurality of batteries (2a-2c); a plurality of switching units (4a-4c), each switching unit being configured to switch between a connected state in which a corresponding one of the plurality of batteries (2a-2c) is connected in series with other batteries of the plurality of batteries (2a-2c) and a non-connected state in which the battery is bypassed and disconnected from the other batteries; a charger configured to charge the batteries of the plurality of batteries connected in series, the charger being switchable between a constant current charging mode and a constant voltage charging mode; a controller configured to control the plurality of switching units (4a-4c) and the charger ; wherein the controller is configured to cause the charger to charge the plurality of series connected batteries with a constant current, to determine whether one of the plurality of batteries has reached a charge changing voltage during the constant current charging, to cause the respective switching unit to bypass and disconnect the battery determined to have reached the charge changing voltage, to cause the plurality of switching units to connect all of the plurality of batteries in series when all of the plurality of batteries have reached the charge changing voltage, and to switch the charger from the constant current charging mode to the constant voltage charging mode so as to charge the plurality of series connected batteries at a constant voltage, characterized in that the controller is further configured to perform operations comprising determining whether one of the plurality of series connected batteries has reached a charge termination voltage during constant voltage charging, and causing the respective switching unit to bypass and disconnect the battery determined to have reached the charge termination voltage, the operations being repeated until all of the plurality of batteries are determined to have reached the charge termination voltage. <EOS>