[p2k]

A standard USB has a max rating of 500mA, so if your phone is using more than that, it will slowly discharge.

One solution although bulky, is to use a powered USB hub inline. They usually use 2A rated adapters.

The other solution is buying a USB cable that has 2 male usb connectors on it. One carries the data + power and the 2nd adds more power giving you (500mA+500mA)