What USB 3.1 ports are you using on your bike to charge smart phones ?

[mtread]

I was wondering about this. USB 3 may be faster than USB 2, but isn't that to do with data transfer speeds, but that's not really what is important for charging? I've got a USB car charger which puts out about 2.4 amps but hardly seems to charge the phone at all. I think my home charger puts out about 5 amps which does the job quickly.
So is it amps we're after?