A battery pack apparatus includes a battery pack having multiple rechargeable batteries arranged in parallel and heat medium passages formed therebetween. The battery pack apparatus also includes a heat insulation cover formed of a heat insulation material that covers a circumferential surface of the battery pack substantially entirely. A supply passage and a discharge passage are provided between the heat insulation cover and the battery pack. The supply passage supplies a heat medium to the heat medium passages while the discharge passage discharges the heat medium from the heat medium passages. The battery pack apparatus also includes a supply device that supplies the heat medium to the supply passage.