A battery system for a vehicle, the system comprising: a battery assembly including at least one first battery module and at least one second battery module (121, 122, 123) that are connected to each other in series; wherein the first battery module serves as low voltage battery, wherein the second battery module (121, 122, 123) comprises at least one battery module, a first relay arranged to connect a first node and a first end of the first battery module or the first node and a first end of the second battery module (121, 122, 123), the first end of the second battery module (121, 122, 123) being connected to a second end of the first battery module; a second relay arranged to be closed or opened between a second node and a second end of the second battery module (121,122,123), wherein the first and second nodes (N1 and N2) are connected to an input end of an inverter arranged to convert direct current power of the battery assembly to alternating current power provided to a driving motor of the vehicle; a converter arranged to convert the voltage between the first node and the second node to a low voltage provided to power a low voltage electronic component; a third relay arranged to be closed or opened between an output end of the converter and the first end of the first battery module; wherein the other output end of the converter is connected to ground; a fourth relay arranged to be closed or opened between the second end of the first battery module and a ground; and a battery manager arranged to control the first to fourth relays (R1 to R4) based on a driving condition which includes whether or not a vehicle is started, is running, or the vehicle is charged and energy storage amounts of the first battery module and the second battery module (121,122,123), wherein a capacity of the first battery module is larger than a capacity of each of the at least one of the second battery modules (121, 122, 123), wherein when the vehicle is charged, charging power is supplied from outside through the first node and the second node by a high voltage charger that receives power from outside, the battery manager is arranged to control the first relay to connect the first node to the first end of the second battery module (121, 122, 123), control the second relay to connect the second node to the second end of the second battery module (121, 122, 123), and control the third and fourth relays (R3 and R4) to be closed, and the battery manager is arranged to operate the converter to charge the first battery module. a battery assembly including at least one first battery module and at least one second battery module (121, 122, 123) that are connected to each other in series; wherein the first battery module serves as low voltage battery, wherein the second battery module (121, 122, 123) comprises at least one battery module, a first relay arranged to connect a first node and a first end of the first battery module or the first node and a first end of the second battery module (121, 122, 123), the first end of the second battery module (121, 122, 123) being connected to a second end of the first battery module; a second relay arranged to be closed or opened between a second node and a second end of the second battery module (121,122,123), wherein the first and second nodes (N1 and N2) are connected to an input end of an inverter arranged to convert direct current power of the battery assembly to alternating current power provided to a driving motor of the vehicle; a converter arranged to convert the voltage between the first node and the second node to a low voltage provided to power a low voltage electronic component; a third relay arranged to be closed or opened between an output end of the converter and the first end of the first battery module; wherein the other output end of the converter is connected to ground; a fourth relay arranged to be closed or opened between the second end of the first battery module and a ground; and a battery manager arranged to control the first to fourth relays (R1 to R4) based on a driving condition which includes whether or not a vehicle is started, is running, or the vehicle is charged and energy storage amounts of the first battery module and the second battery module (121,122,123), wherein a capacity of the first battery module is larger than a capacity of each of the at least one of the second battery modules (121, 122, 123), wherein when the vehicle is charged, charging power is supplied from outside through the first node and the second node by a high voltage charger that receives power from outside, the battery manager is arranged to control the first relay to connect the first node to the first end of the second battery module (121, 122, 123), control the second relay to connect the second node to the second end of the second battery module (121, 122, 123), and control the third and fourth relays (R3 and R4) to be closed, and the battery manager is arranged to operate the converter to charge the first battery module.