Abstract: The invention relates to an electric power supply system for a by-wire vehicle system. There is a need for a power supply system which allows the integrity of the battery and alternator to be monitored. A vehicle electric power supply system includes a battery, an alternator, a key operated switch and at least two electronic control units (ECUs). A battery line is connected to the battery, to the second ECU, and to the key switch. An alternator line connects the alternator to the first ECU and to the key switch. A relay includes a relay switch which opens and closes a connection between the battery line and the alternator line. A relay coil is energizable to cause the relay switch to connect the battery line to the alternator line. Energization of the relay coil is controlled by one of the ECUs. This ECU causes the relay coil to move the switch element disconnect the alternator line from the battery line so that voltage from the alternator and battery can be independently tested.