A battery charging device suitable for a portable electronic device includes a rechargeable battery, a controlling chip, an adjusting transistor, a feedback circuit and a charger with adjustable output. The charger has an external alternating-current power source and provides a changeable voltage to the rechargeable battery of the portable electronic device. The feedback circuit captures a signal of electric quantities of the rechargeable battery and feeds back to the charger to adjust its output voltage and then the charger is controlled to charge the rechargeable battery. The adjustable transistor is used to control the charging of the rechargeable battery to reduce the number of parts of the battery charging device and waste of circuit traces.