A battery system, comprising: a plurality of battery modules including a first battery module (BMa) and a second battery module (BMb), the first battery module and the second battery module being connected between a first system terminal and a second system terminal in parallel; and a controller configured to control the plurality of battery modules, wherein: the first battery module (BMa) includes a first battery (1 10a) and a first main switch, which are serially connected between the first system terminal and the second system terminal, and a first balancing switch and a first balancing resistor, which are connected to the first main switch in parallel and are connected to each other in series, the second battery module (BMb) includes a second battery and a secondary main switch, which are serially connected between the first system terminal and the second system terminal, and a second balancing switch and a second balancing resistor, which are connected to the second main switch in parallel and are connected to each other in series, and the controller is configured to detect a first battery voltage of the first battery and a second battery voltage of the second battery and, when an absolute value of a difference between the first battery voltage and the second battery voltage is greater than a first reference value (Vb), open the first main switch and the second main switch and close the first balancing switch and the second balancing switch; and, in a case where the absolute value of the difference between the first battery voltage and the second battery voltage is greater than a second reference value (Va) and is less than or equal to the first reference value (Vb), close the first main switch and the second balancing switch and open the first balancing switch and the second main switch, or configured to close the second main switch and the first balancing switch and to open the second balancing switch and the first main switch. a plurality of battery modules including a first battery module (BMa) and a second battery module (BMb), the first battery module and the second battery module being connected between a first system terminal and a second system terminal in parallel; and a controller configured to control the plurality of battery modules, wherein: the first battery module (BMa) includes a first battery (1 10a) and a first main switch, which are serially connected between the first system terminal and the second system terminal, and a first balancing switch and a first balancing resistor, which are connected to the first main switch in parallel and are connected to each other in series, the second battery module (BMb) includes a second battery and a secondary main switch, which are serially connected between the first system terminal and the second system terminal, and a second balancing switch and a second balancing resistor, which are connected to the second main switch in parallel and are connected to each other in series, and the controller is configured to detect a first battery voltage of the first battery and a second battery voltage of the second battery and, when an absolute value of a difference between the first battery voltage and the second battery voltage is greater than a first reference value (Vb), open the first main switch and the second main switch and close the first balancing switch and the second balancing switch; and, in a case where the absolute value of the difference between the first battery voltage and the second battery voltage is greater than a second reference value (Va) and is less than or equal to the first reference value (Vb), close the first main switch and the second balancing switch and open the first balancing switch and the second main switch, or configured to close the second main switch and the first balancing switch and to open the second balancing switch and the first main switch. the first battery module (BMa) includes a first battery (1 10a) and a first main switch, which are serially connected between the first system terminal and the second system terminal, and a first balancing switch and a first balancing resistor, which are connected to the first main switch in parallel and are connected to each other in series, the second battery module (BMb) includes a second battery and a secondary main switch, which are serially connected between the first system terminal and the second system terminal, and a second balancing switch and a second balancing resistor, which are connected to the second main switch in parallel and are connected to each other in series, and the controller is configured to detect a first battery voltage of the first battery and a second battery voltage of the second battery and, when an absolute value of a difference between the first battery voltage and the second battery voltage is greater than a first reference value (Vb), open the first main switch and the second main switch and close the first balancing switch and the second balancing switch; and, in a case where the absolute value of the difference between the first battery voltage and the second battery voltage is greater than a second reference value (Va) and is less than or equal to the first reference value (Vb), close the first main switch and the second balancing switch and open the first balancing switch and the second main switch, or configured to close the second main switch and the first balancing switch and to open the second balancing switch and the first main switch.