An electric device, comprising a plurality of detachable battery units 2, 3, 4, each comprising a battery pack 2B, 3B, 4B and a storage mechanism 11, 12, 13 for storing information about the charging and discharging status of the battery. A connection mechanism is provided to electrically connect and disconnect the electric device 1 in response to each battery cell being attached and detached. The device includes a driving mechanism 9 for driving a load, a charging mechanism 5 for charging the accumulators 2B, 3B, 4B with reference to information stored in the storage mechanisms 11, 12, 13 of the installed battery units 2, 3, 4, and The control means 6 controls the electric power supplied from each battery unit 2, 3, 4 to the drive means 9 with reference to the above-mentioned information.