A battery management system 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; an insulation element configured to transmit a signal while electrically separating the external device from the battery management system; 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; an insulation element configured to transmit a signal while electrically separating the external device from the battery management system; 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.