A battery management system (101-106) for managing a battery pack, comprising: N battery management units configured to manage secondary batteries included in the battery pack, in which N is a natural number greater than or equal to 2; a communication line electrically connected to each battery management unit to transmit data to an external device which controls the battery management system (101-106); an insulation element configured to transmit a signal while electrically separating the external device from the battery management system (101-106); and a signal mixer electrically connected to the communication line to mix signals received through the communication line , wherein each battery management unit are configured to transmit at least two pieces of data through time division to the external device; characterized in that the at least two pieces of data comprise a second protection signal (2 nd _PROT signal) representing that the secondary battery managed by the battery management unit is over-charged more than a preset voltage; and a diagnosis signal (Diag signal) representing whether an abnormality has occurred through self-diagnosis of the battery management unit; each battery management unit is configured to transmit the at least two pieces of data using a communication protocol, wherein one cycle of the communication protocol consists of a data transmission preparation section and a data transmission section with changing in logic level of a signal at the data transmission preparation section and a start point of the data transmission section, the at least two pieces of data being transmitted during the data transmission section. N battery management units configured to manage secondary batteries included in the battery pack, in which N is a natural number greater than or equal to 2; a communication line electrically connected to each battery management unit to transmit data to an external device which controls the battery management system (101-106); an insulation element configured to transmit a signal while electrically separating the external device from the battery management system (101-106); and a signal mixer electrically connected to the communication line to mix signals received through the communication line , wherein each battery management unit are configured to transmit at least two pieces of data through time division to the external device; characterized in that the at least two pieces of data comprise a second protection signal (2 nd _PROT signal) representing that the secondary battery managed by the battery management unit is over-charged more than a preset voltage; and a diagnosis signal (Diag signal) representing whether an abnormality has occurred through self-diagnosis of the battery management unit; each battery management unit is configured to transmit the at least two pieces of data using a communication protocol, wherein one cycle of the communication protocol consists of a data transmission preparation section and a data transmission section with changing in logic level of a signal at the data transmission preparation section and a start point of the data transmission section, the at least two pieces of data being transmitted during the data transmission section. <EOS>