In the present invention: the remaining capacity of a battery is detected (S10); the power generation output of an engine power generation unit is detected (S14); a load requirement output required from a load is detected (S14); when the detected remaining capacity of the battery has reached a first threshold value or lower, and if the detected power generation output of the engine power generation unit is not less than the detected load requirement output and not more than a predetermined low load value a, then low-load charging of the battery is executed (S18, S20, S24 to S30); and when the detected remaining capacity of the battery has reached the first threshold value or higher, then the low-load charging of the battery is ended (S22).