At Arts suggestion here are a few photos of my 'after market' battery grip which I originally purchased for my D800E... but now is for my 810 (fits just the same)
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There are two battery boxes supplied one takes a spare EN-EL15.. the other 8 AA cells.. in this case 8 Camelion 2300mAH Ni-MH cells.
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As you can see the camera recognises the grip...
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In this last photo you can see a mini USB socket normally covered by a rubber cover... I don't know what this is for and I can't find any reference to it in the (small)manual.
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And a look into the inside...
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The grip is covered with a pretty much matching rubberised surface like the 810, the command dials feel very similar to the 810 in action... I don't know how weather sealed it is. The rocker switch on the back is reasonably positive in operation but feels a bit flimsy. I think this is a well made unit and matches the 810 body well, fits it well and feels quite solid.