For example you can connect it with your car battery or a 9V PP3 battery for emergency charging.

The circuit is simple to build and using only three components. The heart of the circuit is an LM2940CT-5.0 IC which provides accurate 5V at the output. For emergency charging this IC is more efficient as compare to the LM7805, because it will provide 5V with only 0.5V difference. So a 9V PP3 battery will charge your devices until it reaches to 5.5V or less. The maximum output of the circuit is 1 ampere, that is enough for charging most USB charging devices. The IC become warm during operaion therefore use a suitable heatsink with it.

Note: It is adviced to check and confirm the connections and 5 volt output voltage of the circuit with multimeter before connecting any USB device for charging to make sure the circuit is working fine without any soldering or wiring error and providing 5 volt DC output.
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The figure below shows a very useful project of simple emergency cell phone or mobile charger. The circuit can also be used for charging other devices, which require 5V input for charging for example MP3 players, iPods, iPads etc. The input of the circuit can be from any 12V, 9V or 6V battery.
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Emergency Cell Phone / Mobile Charger Circuit Diagram