What is claimed is:1. An electric power conversion system comprising:a first battery;a second battery;an electric power converter including a plurality of switching elements, the electric power converter being configured to bidirectionally step up or step down electric power between an output line and each the first and second batteries by turning on or off the plurality of switching elements in accordance with PWM signals; anda controller configured to control a first step-up and step-down circuit and a second step-up and step-down circuit by generating a first PWM signal and a second PWM signal, the first step-up and step-down circuit being established between the first battery and the output line, the first PWM signal being a signal for controlling step-up and step-down operation of the first step-up and step-down circuit, the second step-up and step-down circuit being established between the second battery and the output line, the second PWM signal being a signal for controlling step-up and step-down operation of the second step-up and step-down circuit, the controller being configured to, at the time of coupling on-duty periods of both the first and second PWM signals with each other by shifting a phase of at least one of the first and second PWM signals, execute an overlap phase shift that partially overlaps the on-duty periods of the first and second PWM signals by raising one of the PWM signals before the other one of the PWM signals falls,wherein a phase shift amount for the overlap phase shift is set in correspondence with fluctuation predictions of the on-duty periods, and the controller is configured to, when a total period of the on-duty periods of the first and second PWM signals based on the fluctuation predictions becomes short once every period, set the phase shift amount such that a fall of one of the first and second PWM signals in a predetermined period coincides with a rise of the other one of the first and second PWM signals in the predetermined period.2. The electric power conversion system according to claim 1, wherein the controller is configured to (i) carry out the fluctuation predictions of the on-duty periods from an execution period of the phase shift to the predetermined period, and (ii) when a first loss is smaller than a second loss as a result of comparison between the first loss and the second loss, execute the overlap phase shift, wherein the first loss includes a loss that arises in the switching elements in a period from the execution period to the predetermined period when the overlap phase shift is executed, and the second loss includes a loss that arises in the switching elements in the period from the execution period to the predetermined period when an edge-alignment phase shift is executed, the edge-alignment phase shift is a phase shift by which, when a fall of the one of the PWM signals and a rise of the other one of the PWM signals do not coincide with each other, the fall and the rise are brought into coincidence with each other.
