It’s an inexpensive and small device that offers that extra bit of power, courtesy of a built-in rechargeable lithium ion battery and USB connectivity, both for charging and hooking up to your device. You can also charge via the mains. A full charge takes three hours, with overload and over charge protection built-in.
There’s also an on/off switch and an LED indicator for charging. Available online from Brando and in a choice of black or white, it retails for $20 (around £10).