Slave BMS for Diagnosing an Error of a Battery Module and Battery Pack Comprising Same Slave BMS

A battery pack comprising a slave BMS which diagnoses an error in a battery module. The battery pack includes a master BMS and a first slave BMS. The first slave BMS, when an error has occurred in a first battery module, determines whether or not the error is critical. The first slave BMS communicates with the master BMS in a time segment corresponding to the Identification (ID) of the first slave BMS, changes the ID based on whether or not the error is critical, then outputs information on the error to the master BMS in a time segment corresponding to the changed ID.