PURPOSE: To permit the reduction of cost as well as the decrease of the volume of a charging device by a method wherein the charging device is provided with an A/D converting means, a voltage comparing means, a timer means and a charging control means.

CONSTITUTION: A battery 1 is charged by the output of an electric power supply 8 through terminals 2. A microcomputer 9 effects the A/D conversion of a charging voltage at regular time intervals and compares it with the stored value of a voltage comparing means 4. When the A/D converting value has exceeded the stored value, the converting value at that time is specified as a new stored value. When an A/D conversion value becomes newer than the stored value by a predetermined value or more, a timer means 5 is started. Upon time-up, a timer means 5 outputs to shift charging into trickle charging through a charging control means 6 while the indication on an indicating means is changed from 'charging' into 'charging is finished'. According to this method, a smoothing capacitor, coping with the serious fluctuation of a voltage at the terminal period of charging, can be omitted whereby the increase of cost as well as the increase of volume can be prevented.