This invention offers an battery system that reduces a conversion loss due to a charger to reduce power consumption. The battery system has a solar battery, a battery module, a bypass route to bypass a charger to charge the battery module with DC power generated by the solar battery, and a control unit to control so that the electric power generated by the solar battery is supplied to the battery module through the bypass route when an output voltage of the solar battery is equal to or higher than a first predetermined voltage, that is higher than a terminal voltage of the battery module, and equal to or lower than a second predetermined voltage.