A multi battery pack system is composed of a plurality of battery packs. The master battery pack receives a total voltage from each slave battery pack and calculates a target total voltage using its total voltage and total voltages of all slave battery pack whenever a predetermined time period passes, sends the calculated target total voltage to each slave battery pack, compares the target total voltage with its total voltage, and then connects or disconnects its cell group and output terminals according to the comparison result. The slave battery packs include at least one slave battery pack, which sends its total voltage according to a request of the master battery pack, receives a target total voltage from the master battery pack, compares the target total voltage with its total voltage, and then connects or disconnects its cell group and output terminals according to the comparison result.