Perfect for my situation as a piano student practicing with my very portable Xkey 37 (usb) when away from my piano or even practicing in bed or whatever

It's self powered by a rechargeable internal battery, that will power a USB keyboard (or charge your phone - heck, it's as cheap as a standalone phone battery and has a class compliant USB interface AND a straight MIDI DIN connector to a 128 preset 64 voice synthesizer as a bonus)

And the price is a no-brainer, about the cost of a really good USB battery back-up/charger (with a 64 voice polyphonic 128 instrument MIDI synth).

MKII 25-Key Ultra-Portable USB MIDI Drum Pad and Keyboard Controller with Joystick

This is so comfortable that I can play my EWI-USB without a clumsy USB cable, while the EWI transmits its data via Yamaha's UD-BT01 battery-powered