1. Field of the Invention The present invention relates to an apparatus and method for managing power, and more particularly to an apparatus and method for managing power that controls power transmission by using a power transmission switch. 2. Description of the Related Art Recently, various portable electronic devices such as portable computers, digital cameras, and smart phones have been developed. The portable electronic devices are generally driven by a rechargeable battery due to portability. In order to charge the rechargeable battery, various different charging methods may be applied to the portable electronic devices. The methods include a method of charging by using a wall power supply, a method of wirelessly charging the rechargeable battery by using electromagnetic induction, a method of wirelessly charging the rechargeable battery by using a magnetic field resonance, a method of receiving and converting commercial electricity into voltage necessary for charging the rechargeable battery, etc. As mentioned above, various methods for charging a battery of a portable electronic device are available. However, when the portable electronic device receives a charging operation, an amount of consumed electric power is increased. In particular, a standby power mode is maintained while the portable electronic device is turned off (or put into a sleep mode). In this case, even if the portable electronic device receives the charging operation, the portable electronic device is supplied with power only when the rechargeable battery needs to be charged. As a result, the standby power mode of the portable electronic device may be maintained. Accordingly, there is a need for a power management method for charging a battery in a portable electronic device.