A vehicle update system for updating a vehicle's ROM data is provided. More specifically, a telematics unit is configured to receive new ROM data transmitted from a telematics server. An electronic control unit (ECU) is configured first check to determine whether certain update conditions are satisfied and when they are satisfied, update the old ROM data to the new ROM data using at least one of a main battery and an auxiliary battery. More specifically, the update conditions are that the charged state and the voltage level for at least one of the main battery and the auxiliary battery is greater than a certain level, the vehicle is stopped or parked, and the controller area network (CAN) communication state of the vehicle is normal.