Battery charging method and device using the same

A charging device using an unstable power source for charging a battery comprises:

• an unstable power source, supplying an Alternating Current power source;
• a charging unit, receiving the Alternating Current power source and transforming the Alternating Current power source into a Direct Current power source to charge the battery, wherein the charging unit defines first to third charging regions according to a first predetermined voltage and a charging saturation voltage of the battery and the charging saturation voltage is higher than the first predetermined voltage;
• and
• a voltage sampling unit, detecting the voltage of the battery and controlling the charging unit to charge the battery according to the detected voltage;
• wherein the voltage sampling unit determines, from the first to the third charging region, the charging region of the battery according to the initial voltage of the battery to set a threshold charging current and control the charging unit to charge the battery using the threshold charging current;
• wherein the voltage sampling unit continuously measures a plurality of voltages in the battery during a predetermined period, thereby determining the average voltage of the battery;
• wherein, according to the average voltage of the battery, if the voltage sampling unit determines the previously determined charging region has changed and the average voltage of the battery is less than the charging saturation voltage, then the voltage sampling unit controls the charging unit to lower the threshold charging current.