Android - What is the difference between USB charging and AC charging?
In fact the charging time depends on two points:
the output current of the adapter or the USB. The output current of the PC is only 0.5a, it may charge slowly than most of the AC adapters (most of the AC Adapters are 1A or more).
the input current and the charging convert rate of the phone. Different phones have different input current due to Limited current protection for them. For example, the limites current for Nokia is 0.5a, the input current is only 0.5a even you use an ac adapter with 1.0a output. Beside, the charging convert rate is also different according to the models.
To charge at the maximum rate, the charger must identify itself as a charging device by shorting two middle USB pins. This is in the USB specification. Proper chargers come like this. Other chargers can be modified to short the pins together.
I have personally modded a Griffin USB car charger in this manner. Before the change, it would charge in "USB" mode. After soldering together the two pins, the phone charges in "AC" mode.
One caveat: I have not done any tests to see if this changes the charging rate, but it certainly charges quickly after the mod. Everything I have read about this phone says that it will only pull 500 mA in "USB" mode.
References: Wikipedia entry regarding USB charging spec. Look for the words "Dedicated charging port."
Amazon user review with instructions for modding a certain car charger.