A power unit including a plurality of series-connected battery modules and safety circuit wherein a service plug is inserted from the side of a terminal board of a battery pack, thereby establishing an electrical connection among battery modules. A connector is provided on a back of a terminal cover by way of a projecting section, and a second safety switch is activated by attachment of a terminal cover and insertion of the connector to the terminal board, thereby establishing an electrical connection among the battery modules. Even when the service plug is attached at the time of completion of maintenance without attachment of the terminal cover, the battery modules are still kept in an unconnected state by means of a second switch, and energization, which would otherwise arise with exposed terminals, is prevented.