to control signals developed by the control circuit 9 the gate 7 is switched when the power switch 5 and the gate 7 are switched to be conductive the constant power is transferred from the constant voltage circuit 4 into the load 6 the secondary battery 8 is charged by the constant output of the constant voltage circuit 4 the mode switch 10 is operated by the operator to select either a rapid charge mode or a trickle charge mode for charging the secondary battery 8 as will be described below the control circuit 9 is responsive to th conditions of the mode switch 10 for activating the gate 7 and the reference voltage switching circuit 11 when the mode switch 10 is place in the rapid charge mode the control circuit 9 controls the gate 7 and the reference voltage switching circuit 11 so that the gate 7 becomes unconductive and the reference voltage switching circuit 11 provides a low reference voltage into the constant voltage circuit 4 therefore regardless of the conditions of the power switch 5 any power to the load 6 is interrupted and the constant voltage circuit 4 provides a voltage higher than the normal condition the trickle charge mode thus all the power supplied by the constant voltage circuit 4 is entered into the secondary battery 8 to charge it so that it is fully charged in a short time when the mode switch 10 is placed in the normal mode the trickle charge mode rather than the rapid charge mode the control circuit 9 controls the gate 7 and the reference voltage switching circuit 11 such that the gate 7 becomes conductive and the reference voltage switching circuit 11 provides a normal reference voltage into the constant voltage circuit 4 therefore.