PURPOSE: To prevent data loss by monitoring a cell voltage constantly and warning a host side if the residual capacity voltage of the cell is below a tolerable and a terminating voltage.

CONSTITUTION: Power supply circuits (A), (B) at a power system voltage VA and a logic system voltage VB consist of a dropper system and a switching system power supply circuit respectively. If the residual capacity voltage of a cell 1 is below a tolerable value(e.g.10%) in a voltage detection circuit (A)2, alarm circuits (A)17,(A2)8 are activated to sound as buzzer so that a host computer is warned. In this case, the host side can be notified of using a flashing display device instead of a buzzer or both methods can be employed. Next, if a terminating voltage of the call 1 is detected using a voltage detection circuit (B)3, a switching circuit 4 is activated by the detection voltage to shut off a power system circuit to the power supply circuit (A)5 and also stop an overelectric discharge to the cell 1. At the same time, the alarm circuit (B)9 is turned to H level and the host side is notified of the alarm state.