Here is a little bit of "knowledge" that I gained from a recent trip to Miami. This post is spurred by the one above on the amperage outputs.

I had to drive from South Florida to Virginia stopping at dozens of Publix grocery stores for business. I had all the addresses, and was relying on my phones GPS for the directions.

Well, the 12V car charger I had bought at Walmart would NOT keep up the charge on my Samsung S3 if I was using it for GPS, email, texting and talking. Drove me nuts till I did a little research.

Turns out that 12V car chargers come in two flavours. The 1A and 2.1A.

1A is NOT enough to maintain the battery while in use (under load) but the 2.1A is more then capable.

Moral of the story: Check the specs on the car charger to see what the output is and buy one with a 2.1A output in at least one of the ports