Referring back to FIG. 1, the controller 22, as will be described later, executes various operation controls over the vehicle, including voltage conversion and switching of power supply connection over the electric power converter 11 or the inverter 18. The controller 22 may be a computer called electronic control unit (ECU). The controller 22 includes, for example, a CPU that is an arithmetic circuit, a storage unit, such as a memory, and a device and sensor interface, which are connected with each other via an internal bus.

The storage unit of the controller 22 stores a control program for the switching elements S1 to S4, a program for a phase shift (described later), a loss map of the switching elements S1 to S4, and a shift amount map.

The controller 22 receives signals from various sensors via the device and sensor interface. Specifically, the controller 22 receives detected values from battery voltage sensors 46A, 46B and battery current sensors 48A, 48B as the signals associated with the first battery B1 and the second battery B2. The battery voltage sensors 46A, 46B respectively measure battery voltage values VB1,VB2. The battery current sensors 48A, 48B respectively measure battery current values 11, 12. The controller 22 receives a detected value from an output voltage sensor 50 as the signal associated with the output voltage of the electric power converter 11. The output voltage sensor 50 is connected in parallel with a smoothing capacitor CH, and measures a potential difference VH (output voltage) between the high-voltage line 26 and the reference line 28.