A battery system and method for connecting a battery to the battery system are disclosed. In one aspect, the battery system includes at least one first battery, a second battery, a first switch configured to connect the second battery in parallel to the first battery and a second switch and a resistor connected in series and configured to connect the second battery in parallel to the first battery. The first switch and the second switch are connected in parallel. The battery system further includes a battery management system (BMS) configured to turn on the second switch so as to connect the second battery to the first battery. The BMS is further configured to turn off the second switch and turn on the first switch when the difference in voltage between the second battery and the first battery reaches a predetermined value.