Originally Posted by Fountainhead

Hi BAF,
Never use a cheap Wal-Mart or Pep Boys charger with a phone either in home or auto. When I buy a 12V to USB 5V convertor I always load it with the rated load (1A rated then I put 5 ohms across) and then measure the voltage under load. I also measure the voltage no load, which is important too. I wouldn't use an adapter that has a no load voltage over 5.25 V, and a loaded voltage below 4.75V. Those voltages are upper and lower limit of USB 2.0 standard, which governs the design of all USB chargers which output 5V.

I use car chargers all the time but I make sure they are name brand and tested by me. If they fail the tests above then I fix them, but if they are cheap Chinese knock offs I don't even bother.
I'm an electrical engineer and I design things to do with audio, car stereo, power supplies, etc.