For USB 2, widely-available battery packs compatible with whatever current negotiation standards your devices require work just as well, and have the added bene

For USB 2, widely-available battery packs compatible with whatever current negotiation standards your devices require work just as well, and have the added benefit of storing power. Less generally, these batteries can also break ground loops, which comes in handy when connecting a power supply to both a USB device and an attached analog audio device (e.g., iPhone and wired noise-canceling earbuds).