a is low the transistor q 37 is turned on so as to cause the light emitting diode pca of the photo coupler pc 1 to generate a light ray received by the light sensitive transistor pcb which is turned on to control the power switch 16 on the contrary if the output signal of the comparator u 32 a is high the transistor q 37 is turned off and so the light emitting diode pca so that the light sensitive transistor pcb is also turned off this operation of turning on and off is rapidly repeated to maintain the charging voltage as 4 2v on the other hand if the microprocessor 46 generates a high signal through ports u2018 s u2019 and u2018 t u2019 for the voltage type of 4 1v in step 305 the divided voltage produced by the registers r 40 and r 41 and variable resistor vr 1 is applied to the inverting input of the comparator u 32 a used as the voltage for setting the charging voltage of the battery of pocket u2018 a u2019 to 4 1v then the comparator u 32 a compares the set voltage with the reference voltage applied to the non inverting input in order to generate a logically low or high signal depending on whether the set voltage is higher or lower than the reference voltage if the output signal of the comparator u 32 a is low the transistor q 37 is turned on so as to cause the light emitting diode pca of the photo coupler pc 1 to generate a.