There is provided a battery system made up by connecting multiple battery units in parallel with each other, the battery units having multiple battery banks, respectively, and multiple switching circuits connected in series to the battery banks, respectively. A switching circuit is made up by connecting a first circuit connected in series to a precharge resistor having a known resistance value, in parallel with a second circuit having a second switch. Timing for turning the second switch into the on state under an equal-current condition when the connection-target battery unit is connected to the battery system is controlled by a controller for controlling the respective outputs of the plural battery units.