A battery system includes: first and second battery modules connected between first and second system terminals in parallel; and a controller controlling the first and second battery modules. The first battery module includes a first battery and a first main switch, and a first balancing switch and a first balancing resistor, which are connected to the first main switch in parallel. The second battery module includes a second battery and a second main switch, and a second balancing switch and a second balancing resistor, which are connected to the second main switch in parallel. The controller is configured to detect a first battery voltage and a second battery voltage, and when an absolute value of a difference between the first and second battery voltages is greater than a first reference value, to open the first and second main switches and to close the first and second balancing switches.