The Original Battery BacPac and Battery BacPac2 essentially adds 2x additional battery to the HD HERO Original,HD HERO2, and HERO3 cameras increasing the recording time to four hours. 

You can charge the Battery BacPac and the camera separately as they each have their own respective USB port, or you can do what is called pass-through charging and charge the Battery BacPac and camera at the same time by connecting the USB cable to the camera with the Battery BacPac connected.  Doing it this way will charge the camera first and then the Battery BacPac.

Note that the Battery BacPac (and all other Premium Accessories) are not compatible with Standard Definition HERO and HD HERO 960 cameras.