Some iOS devices can be powered by a USB power source and an adapter cable. These include the smaller iPhones (not 6 Plus or 7 Plus), and iPod Touch models. Don't try this with an iPad.
The USB power is routed directly to the iOS device, so you must be certain to keep voltage levels within the normal IOS device range, and to observe correct polarity. We assume no liability for improperly built cables or using the wrong power source!
Follow the instructions on the attached drawing to make a USB power cable. Note that polarity is very important!