The Canon Battery Grip BG-E6 is an optional battery grip designed specifically for the Canon EOS 5D Mark II. It allows users to load a choice of one or two LP-E6 battery packs. With two packs, the already-impressive battery life is doubled, compared to using a single LP-E6 pack in-camera. This is especially useful when taking advantage of the camera's movie mode or Live View mode - operating modes that use more power. In addition, with the included BGM-E6 Battery Magazine, you can alternatively also run the camera off six AA batteries.
Furthermore the BG-E6 provides full vertical shooting controls, which can be independently turned on or off as the photographer needs to enhance camera handling, especially for shooting vertical pictures.
Note: The battery grip BG-E4 for the previous EOS 5D is not compatible with the EOS 5D Mark II camera