I tried charging my iPad Mini with the original Apple sync cable that came with the Mini via the USB port but could not get it to charge. Anyone else experience this?
Did you just upgrade to iOS7? If so, that could be the issue. Is the car adapter third-party? If so, Apple may be blocking it. Annoying to have to get a new charger...
Looks like using an adapter in the 12v cig plug might be the answer. Links from Prius v and PiP forums below; Why won't my iPad charge in my PiP? | PriusChat Entune vs USB for charging my iphone | PriusChat Enjoy ...
I have an iPad 3 and have charged it with the USB port. It charged much slower than with the Apple power cube but it did charge.
FWIW, this review/recommendation/article explains how not all USB chargers are capable of charging at full power and near AC speed: The Best USB Car Charger | The Wirecutter