This is a genuine Joyetech eGo Pass-Through rated at 1000mAh capacity.

Passthrough battery compatible with eGo/eGo-T/eGo-C parts

The particular passthrough can be used both while plugged into the USB or on the go, making it perfect for someone that is very mobile.  Simply unscrew the end cap and plug it in using the included cable via the Mini-USB port located directly on the end of the battery, you can still use it while it charges, and then unplug it and take it with you when you leave.  When not connected to the charger it performs just like any other eGo battery and provides a long life between charges.