Abstract: Battery packs with a plurality of rechargeable batteries connected in series so as to obtain a voltage required by a load device are detachably accommodated in a case. The battery packs are connected in parallel and each output thereof is modified to a predetermined voltage by a discharge control section. The battery packs are connected to a power source line of the load device. Each battery pack is connected through an information transmission line to a power source control section where charge and discharge thereof are controlled, as well as the lifetime is judged. Thus, only the lifetime-judged battery pack is replaced.