I have a Samsung Galaxy Ace 2 x. It only charges when plugged in to the AC adapter and my laptop, and doesn't charge using battery packs or in the car. I use the same cord for everything. Why is this?
Open your car charger and at the back of the USB socket solder together the two middle contacts. These are the D- and D+ signals. Most Galaxy phones don't like an open circuit there, so now you have made a nice loop. Info: http://en.wikipedia.org/wiki/Universal_Serial_Bus