After doing some research as to why my some of my 1A usb ac adapters take twice as long as the stock adapter, I've finally found an answer.
All USB adapters have 4 pins: pin 1 supplies the 5V, pins 2 and 3 are data lines, and pin 4 is the gnd. According to the latest charging standard, for a portable device to see 1A, pins 2 and 3 have to be shorted (standard specifies 200 ohms) for the device to allow 1A. If these data lines are not shorted, the phone will just see 500mA, the same as a computer's usb port. This is probably why it will show usb charging as opposed to ac on the phone. Because older devices do not rely on this standard (some ipods for example), chargers oriented for these devices will not work properly.
So, beware the next time you buy a usb charger because there's a chance it won't be any better than charging straight from your computer.
USB Standards via usb.org
All USB adapters have 4 pins: pin 1 supplies the 5V, pins 2 and 3 are data lines, and pin 4 is the gnd. According to the latest charging standard, for a portable device to see 1A, pins 2 and 3 have to be shorted (standard specifies 200 ohms) for the device to allow 1A. If these data lines are not shorted, the phone will just see 500mA, the same as a computer's usb port. This is probably why it will show usb charging as opposed to ac on the phone. Because older devices do not rely on this standard (some ipods for example), chargers oriented for these devices will not work properly.
So, beware the next time you buy a usb charger because there's a chance it won't be any better than charging straight from your computer.
USB Standards via usb.org