FIG. 5 shows a view at the time when the first step-up/step-down circuit CNV1 is in the charge mode and the second step-up/step-down circuit CNV2 is in the discharge mode. The upper arm off and the lower arm on (S1 off S2 off S3 on, S4 on) are set in the first step-up/step-down circuit CNV1, and the upper arm on and the lower arm off (S1 on, S2 off, S3 off, S4 on) are set in the second step-up/step-down circuit CNV2. According to the principle of superposition, by calculating the logical addition of both circuits, the on/off states of the switching elements are S1 on, S2 off, S3 on, S4 on.
Current components respectively flowing through the switching elements S1 to S4 in the drawing are shown to the top left of FIG. 5. As shown in the circuit diagram of FIG. 5, almost no current flows through the switching element S1. The switching element S2 is in the off state. The current I1 from the first battery B1 flows through the switching elements S3, S4.
