A battery management system of a vehicle is provided to prevent a lithium battery from being overcharged and over-discharged and to safely protect the lithium battery from various conditions degrading the lithium battery when the lithium battery is used as a low-voltage battery in the vehicle. The battery management system includes a relay that electrically connect and disconnects power supplied to a load from a battery and a reconnection switch that determines a connection state based on user manipulation and generates a signal for turning the relay on and off based on the connection state. Additionally, a controller turns the relay on and off based on the connection state of the reconnection switch.