Assuming that a transformer has a primary to secondary winding turn ratio 1:N, VR denotes a voltage of regenerated power, VCd denotes a discharge final voltage of a first battery connected with a transformer-primary-side center tap, VCc denotes a charge final voltage of the first battery, VBd denotes a discharge final voltage of a second battery connected with a transformer-secondary-side full bridge circuit, VBc denotes a charge final voltage of the second battery and Drain denotes a lower limit of a duty ratio D of a transformer-primary-side full bridge circuit, an apparatus determines upon (VBd/N)≦VR≦(VBc/N) that the first and second batteries may be able to be charged with regenerated power and determines upon VCd≦VR≦(VCc/Dmin) that the first battery may be able to be charged with the regenerated power.