A cap (8) for covering an open side of a battery case (15) containing a powe r generation element (51) includes a protective circuit with an electric circuit (11) formed on a circuit board (10). The circuit board (10) is mounted outside the cap (8). A positive terminal block (16) acting as positive terminals of rechargeable battery (A) and an S-pole terminal block (14) for controlling the operation of the electric circuit (11) by an external connection are provided on the outside of the circuit board (10). Since the protective circuit is provided in the cap so as to protect the battery from excessive discharge or charge, there i s no need for provision of a packed cell.
