I mounted a camera on my bike and wanted to power it from the bike (via USB). Wanting to do somthing a little more sophisticated than putting a 12V plug under the cowling and plugging a USB adapter into it, I decided to do this:
Purchased a Castle Creations BEC (Battery Eliminator Circuit). This is used in RC vehicles to remove the need for a second battery, but it's basically just a voltage converter that takes 12V in and puts out a configurable voltage that is set to 5.1V out of the box, perfect!
The bare wires are connected to the bike's power (battery or whatever you use, I have a PDM60), the other end goes to my USB cable. I shorted the wires, added some heatshrink to improve water resistance, and put connectors on both ends. The yellow wire on the output side is a signal wire used for changing the output voltage, and I just cut it off.
Next I created a splitter so I could plug two USB cables into it, I used the turn-signal type connectors to hopefully make it easier to crimp terminals onto the tiny USB wires (24 gauge if you get a good cable).
Next I chopped off the "A" end of the USB cable and exposed the wires.
The white and green wires are the data lines, I just cut them off, but you could short them together to ensure that whatever (non Apple) device you use it with sees it as a "charge only" cable.
Then I attached terminals to the power lines, even these small terminals are too big to properly crimp, but I made it work by putting the insulation crimp in the uninsulated crimping slot of the tool and using pliers on the uninsulated crimp to crush it down smaller.
Finally, added some heatshrink and popped on the connector.
Plugged them all together and ran the USB cable up to the camera in front, works like a charm!
Next project will be to build my own USB cable from scratch. Regular USB cables have far thicker PVC sheathing than needed for this use as well as metal sheathing for noise reduction, which isn't need for simple power supply. I'm going to build a custom USB cable with a mini-USB connector on one end, 20 gauge wires, thin PVC sheath, and turn-signal type connector on the other end. Also I'll short the data ports together for "charge only" functionality.