A car power source apparatus is provided with a battery having a plurality of battery modules connected in series on positive and negative sides of a midpoint reference node, and a voltage detection circuit that detects the voltage of one or a plurality of battery modules with respect to the midpoint reference node of the battery. The battery midpoint reference node of the power source apparatus is connected to the voltage detection circuit via a plurality of reference connection lines. Further, one or a plurality of conduction detection circuits connect to the reference connection lines, supply a voltage to each reference connection line, and detect current. The connection status of each reference connection line to the midpoint reference node is determined by this conduction detection circuit.