I had a 12volt outlet wired to the battery. It worked fine. But the weather and humidity here, caused it to rust etc.
I found here somewhere, someone using a Levin Solar cell, USB charger. I ordered mine from Amazon. Now when my USB devices get low, I plug in to the solar cell. On a full charge, it has recharged my phone 4 times, before it needed recharging.
I have a Moto Centric tank bag with the map pocket. I place the solar cell there while riding, so that it is charged a ready.
Take a look here.