Battery Grips (also known as Battery Packs) are an accessory designed for SLR/DSLR cameras and used to extend the life of the battery life of the camera by allowing the interchange of batteries for photographers on the move. They also add an extra shutter release along with a vertical grip for ease of management.
There's a huge range of Battery Grips at Leederville Cameras of various sizes designed to help photographers make the most of their photographic equipment.
Panasonic DMW-BGGH5E Battery Grip for GH5
Olympus ECG-3 Grip for E-M10 Mark II
Fujifilm MHG-XT2 Metal Hand Grip
Fujifilm EF-BP1 Battery Pack
Phottix Battery Grip Canon 7D Mark II
Phottix BG-6D Battery Grip for Canon 6D
Pixel Nikon MB-D16 Battery Grip for D750
Sony AG-R2 Attachment Grip