A battery management system which manages a battery pack including at least two battery assemblies (10, 10-#1 - 10-#N) connected in series, the battery management system comprising: at least two battery management devices (20, 20-#1 - 20-#N) each having a first terminal outputting a variable voltage and a second terminal outputting a constant voltage, and provided for each battery assembly; an information transfer unit to transmit information from the battery management device to an external device, the information transfer unit being an opto-coupler which disconnects the information transfer unit from the external device while transmitting the information to the external device, characterized in that the battery management system further comprises: at least two slave switch units (30, 30-#1 - 30-#N) connected to each other in series, and provided for each battery management device and connected between the first terminal and the second terminal, the slave switch units configured to selectively turning on or turning off based on the variable voltage outputted from the first terminal; and wherein among the slave switch units, when a slave switch unit provided in a battery management device intended to transmit information to the external device turns on, the slave switch units turn on in series toward a last slave switch unit provided the last among the slave switch units, to form an information transfer path to allow the information transfer unit to transmit the information to the external device. at least two battery management devices (20, 20-#1 - 20-#N) each having a first terminal outputting a variable voltage and a second terminal outputting a constant voltage, and provided for each battery assembly; an information transfer unit to transmit information from the battery management device to an external device, the information transfer unit being an opto-coupler which disconnects the information transfer unit from the external device while transmitting the information to the external device, characterized in that the battery management system further comprises: at least two slave switch units (30, 30-#1 - 30-#N) connected to each other in series, and provided for each battery management device and connected between the first terminal and the second terminal, the slave switch units configured to selectively turning on or turning off based on the variable voltage outputted from the first terminal; and wherein among the slave switch units, when a slave switch unit provided in a battery management device intended to transmit information to the external device turns on, the slave switch units turn on in series toward a last slave switch unit provided the last among the slave switch units, to form an information transfer path to allow the information transfer unit to transmit the information to the external device. at least two slave switch units (30, 30-#1 - 30-#N) connected to each other in series, and provided for each battery management device and connected between the first terminal and the second terminal, the slave switch units configured to selectively turning on or turning off based on the variable voltage outputted from the first terminal; and wherein among the slave switch units, when a slave switch unit provided in a battery management device intended to transmit information to the external device turns on, the slave switch units turn on in series toward a last slave switch unit provided the last among the slave switch units, to form an information transfer path to allow the information transfer unit to transmit the information to the external device.