A rechargeable battery is coupled to a power delivery unit or an external load unit. In a charging mode, the power delivery unit converts an input power to a converted voltage and/or current. A charging circuit converts the converted voltage and/or current to a charging voltage and/or current for charging the rechargeable battery. Power data is communicated between the power delivery unit and the rechargeable battery by: 1) the power delivery unit adjusting the converted voltage, wherein the power data is expressed by plural voltage levels of the converted voltage; and/or 2) the rechargeable battery adjusting a battery input current, wherein the power data is expressed by plural current levels of the battery input current. At least one of the converted voltage, the converted current, the charging voltage, or the charging current is adjusted according to the power data.