I had an el cheapo 12V adapter for USB charging in the vehicle, but it was only charging the phone 1/4 of the time; usually the phone would say 'invalid charging device' or some such. This week I ordered (on Amazon) and received a new one by Anker. They say they include 'smart' electronics that senses the device being charged and tailors the output to that device's need. It seems to be working perfectly... no more 'invalid' messages from the phone.
While I was at it, I also bought one of the Anker Astro E5 portable charging units with 16 amp-hour battery, for about $30. It seems useful for those times when I have set up camp and I don't go anywhere for several days; it can recharge my phone a half dozen times if necessary. I looked at some charge units that have a little solar panel built in, but reviewers all say the panels are useless (too small to accomplish anything significant), so I passed on those.
edit: Mine is an iPhone 3GS.
