The battery charging method includes: defining first to third charging regions according to a first predetermined voltage and the charging saturation voltage of a battery; determining a charging region of the battery according to the initial voltage of the battery, and determining a threshold charging current according to the charging regions of the battery; charging the battery according to the threshold charging current; continuously measuring a plurality of voltages in the battery during a predetermined period and determining an average voltage of the battery; determining if the previously determined charging region of the battery has changed according to the average voltage of the battery; and when the charging region of the battery has changed and the average voltage of the battery is lower than the charging saturation voltage, then lowering the threshold charging current and repeating the above steps.