The Battery BacPac essentially adds an additional battery to the HD HERO Original and HD HERO2 cameras increasing the recording time to four hours. 

You will need to charge the Battery BacPac and the camera separately as they each have their own respective USB port. Using one USB cable plugged into just the camera and not the Battery BacPac will not end up charging the Battery BacPac and vise versa. 

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