P.S. From Peter White -
"The Luxos U won't charge a car battery, or an iPad, though it will charge an iPhone or a Garmin Edge 810. The larger battery in the iPad needs too much current; more than the Luxos U can provide. If you need to charge an iPad on the road, you'll need to find another solution. Hub dynamos just don't have enough power output."
http://www.peterwhitecycles.com/b&m-hl.asp
It's not specific to the Luxos U either - any dynamo is just not going to provide that much power...