The patent application titled, "Power Adapter with Internal Battery" describes a wall charger with an integrated battery pack to provide some extra charge when an outlet isn't available.
Apple notes that current solutions are much more cumbersome.
"Such external batteries are generally cumbersome to use, at least because they must be unpacked for use and then repacked for storage," Apple's filing states of current options available on the market. "In addition, many users forget to bring the external battery in addition to the adapter while in transit.
"What is needed is a way to combine a power adapter and a battery so that a user does not have to carry an additional battery while traveling with a portable electronic device."
Apple's solution would intelligently determine how to distribute the charge between the integrated battery and the device. It would also offer a way to check the status of the integrated battery.
AppleInsider notes that the device could be augmented by a "trickle source" for power such as a solar panel. It could also provide a USB source to charge other devices and potentially even include a wireless adapter allowing devices to access the adapter even when its not physically connected.