Fix USB charging Problem on Galaxy S2
Battery charges slow or not at all from USB port, depending on the quality of the USB cable. Can be as little as 100 ma. Two ways to solve this problem are (1) get a heavier cable and (2) most important short out the two middle wires going to the phone, NOT THE USB PORT. When the two middle pins are NOT shorted it thinks it is connected to a computer USB port (the S2 reports that when you connect it to any charging unit that is not configured as a charger ONLY ) and reduces the charge current, probably to protect the computer. I dissected the USB cable and charger that charged fastest and found they had the two middle pins shorted on the charger. I emulated this and the charge rate went up immediately to over 500 ma. I have a USB charger for my car I purchased at a dollar store and found it could not keep up with the phone discharge when I was running GPS and a few other apps. I modified the Car charger now it charges with those apps running. DO NOT SHORT the computer end of the cable. Hope this helps.
PS: I am an Electronic Technologist with 40+ years Experience