Plugable Bluetooth keyboards can be charged using the USB cable included in the package.  A few minutes of charging provides hours of use. 

BT-KEY3XL:

1.  Connect the USB cable to a USB charging source, such as a USB charger, and the charging port of the keyboard. (Please be careful to orient the plug correctly.) 
2.   The power light will glow red while the keyboard is charging. The light will go out when charging is finished.
3.  You can check the keyboard's charge level by pressing Fn + Right Alt.  This LED indicates the battery level from 4 flashes (full battery) to 1 flash (needs charging).

BT-KEY3:

1.  Connect the USB cable to a USB charging source, such as a USB charger, and the charging port of the keyboard. (Please be careful to orient the plug correctly.) 
2.  The right LED light above the ‘7’ key will glow red while the keyboard is charging. The light will go out when charging is finished.

Note:  The included cable is for charging the keyboard only, and cannot be used to connect other devices for USB operations.