1. A method operative in a processing apparatus including a battery and a system, the method comprising: receiving a supply of power from an external power adapter to simultaneously charge the battery and supply power to the system; acquiring system specification information of at least one system component that has been changed; determining a constant power level to be provided to the system based on the at least one system component that has been changed, corresponding to a load level of the at least one system component, wherein the constant power level is a power level at which the power supplied to the battery is decreased; and controlling the supply of power to the battery and to the system in accordance with the determined constant power level, wherein the system specification information includes information regarding a processing speed of at least one of a Central Processing Unit (CPU), a graphic chipset, and a memory, the load level being determined as a function of the processing speed.