If you've been on a long road trip with your portable USB power bank (battery pack) and discover halfway that Apple uses proprietary voltages to charge your 2nd-Gen iPod Shuffle, this instructable is for you.
The "Minty Boost" has a very good tutorial on how to wire up a USB plug to allow charging a range of different Apple devices. If you want to build something more generic to use with your power bank, such as a male-female USB cord which internally divides the right voltages into the D+ and D- USB lines, follow their tutorial or search for any Apple device battery charger instructable.
We will avoid the whole USB voltage problem by charging the shuffle straight through the audio jack, instead of through the dock (which contains circuitry of its own). This will result in a nice shuffle-only USB cable.
The wiring is based on this AllPinouts page. All we will be doing is soldering a 3.5mm 4-pole audio jack to a USB cable.
You will also need basic electronics equipment, namely