<p>Single batteries (1) are received in a metal case with their negative terminals and positive terminals connected by connection members. External positive and negative terminals are provided on the metal case, and the external terminals and the terminals of the batteries are connected by leads. A voltage monitoring connector is provided on the metal case, and the connector and voltage outlet wires extended from the positive and negative terminals of each battery are connected. The degree of charging of each battery can be known by the voltage monitoring connector, so that overcharging of the batteries can be prevented.</p>
