It should be no different than streaming Pandora.
And pandora sucks up battery hard on my Pre hard. Using the 3G/4G connection over an hour and a half drive would destroy my battery.
And I would use a car charger, but the only slot in my car is powering my radar detector (still legal here in NY...saved my butt more times that I can count). Plus, I've already bought a 32gb class 10 card for it, so I don't mind having gigs of music on my phone