An emulation system for determining an arbitrary charging protocol in USB charging ports and for optimally charging portable devices. The emulation system comprises a power switch for powering on the emulation system, a high-speed data switch for transferring data to and from the portable device, a USB receptacle port including data pins (DP and DM), VBUS, and GND. The emulation system further comprises a profile database that stores one or more charging profiles including one or more stimulus-response pairs for each charging profile. The emulation circuit further includes emulation circuitry for detecting stimulus generated by the portable device and for generating responses according to the charging profiles.