Disclosed are a multi-battery pack system, a method for controlling battery packs, and a battery pack, in which plural battery packs provided in the multi-battery pack system can be set as a master battery pack and slave battery packs by themselves. The method for controlling the battery packs constructing the multi-battery pack system includes: a first step of checking if a communication line ismaintained at a first level for a first desired amount of time when an activation signal or a message of setting a master battery pack is received; and a second step of outputting signals of a secondlevel to the communication line for a second desired amount of time when the communication line is maintained at the first level for the first desired amount of time, and setting a battery pack as a master battery pack.