That's what I had to do with my iPAD seems the USB port for whatever reason just don't out put enough juice to provide a charge. I bought the car adapter and it charges like mad.USB isn't always the same for charging capabilities. I suspect that the Volt's USB port just doesn't supply a lot of extra power. I've noticed that some computers/laptops can charge my phone faster than others. And that charging from the wall adapter is almost always the fastest.
Edit: If charging speed is your only concern, you could try one of those USB adapters that go into your power port. I am willing to bet that'll charge faster for you.