Nikon's battery grips are notoriously overpriced. If you want to add one to the Nikon D810 or D800, for example, you're looking at about $369.
But Nikon is not the only one who makes them. There are now some some aftermarket options as well.
For the past few months I've been shooting a Vello BG-N7 battery grip on a Nikon D810. It retails for $89.95, so it's only a fraction of the price of the Nikon original.
How it Works
This model, BG-N7, is compatible with the Nikon D810, D800, and D800E. (It doesn't specifically mention compatibility with the D810A, but I know of no reason it wouldn't work with that one too.)
The D810 and D800 have a port on the bottom of the camera that's used to communicate data from the grip to the camera. By default, it's covered with a rubber contact cover. (Once removed, they're easy to lose. If you need a replacement, it's Nikon part number 1K467-348; you can find them here.)
The main structural support holding the grip in place is the screw into the tripod hole on the bottom of the camera. There's a small metal rod on one end that goes into a small hole in the bottom of the camera that you've probably never noticed before (near the serial number) and a smaller, plastic one near the data connection, that provide more support and prevent the grip from twisting.
One thing you'll notice when you're attaching the grip is that it covers over the battery compartment on the camera. So make sure the battery in the camera itself is fully charged first.
Once attached, the grip adds controls for shooting vertically. There's an AF-On button, a settings dial next to it, a multi-selector joystick for navigating the menu and controlling focus points, and a shutter button on top. On the front, with easy access for your index finger, is another dial, much like the one on the camera itself in front of the shutter.
Around the shutter button is an on/off switch. This isn't for turning the main camera on and off--it just disables the buttons on the grip so you can prevent them from interfering if you accidentally bump them.
The finish on the grip is almost identical to the finish on the camera, with the same kind of semi-rough texture that feels natural to hold and grip.
On the very bottom is a standard 1/4-20 tripod attachment.
The grip comes with two battery cartridges. One is for a single EN-EL15 battery. The other is for using AA batteries; it takes eight of them. If you're on an intensive shoot, there's no reason you can't swap one out for the other; there aren't any extra settings you need to change when swapping them.
When the camera is drawing power from the battery grip a small BP icon displays on the top screen next to the battery status indicator.
There's no installation necessary. Just connect it and start shooting--the camera recognizes it right away. But there are couple of things you can set in the camera settings.
For one, you can choose whether the camera uses its internal battery first or the one in the grip. You can find this setting under the Custom Setting Menu of the D810/D800 settings. Scroll down to item d13: Battery Order.
Go into that setting by hitting the OK button (or going right). You can now set whether to use the internal battery first or the grip's battery.
If you're using AA batteries in the grip, you can also set the type of AA batteries. This matters if you're using store-bought single-use batteries or rechargeable batteries. This setting is immediately above in d12: Battery Order. You have a choice of AA alkaline, AA Ni-MH, or AA lithium.
And there's another trick on the camera. The battery life indicator on the small screen on top of the camera and in the viewfinder only shows the status of one battery. But you can see the status of each of them by hitting the "i" button to the right of the main screen. (Exit out of live view, playback, or the menu before you do it--it only works if the screen is otherwise off as though to shoot with the viewfinder.) You'll see a bunch of settings displayed. At the far right is the charge status for each of the batteries.
How it Performs
I've been using this grip on my D810 for a few months now. While I was originally planning on only using it occasionally, I've taken to leaving it on. The extra bulk is a bit of a hassle when putting in and out of a camera bag like the Think Tank Retrospective 30, which is my go-to travel camera bag, but I like everything else about having the grip attached.
What's in the Box?
In addition to the main grip, it comes with two battery cartridges. One is for Nikon EN-EL15 batteries; the other is for AA batteries.
It doesn't come with any batteries--you'll need to get those separately.
There's also a small fabric pouch. It's big enough to hold the unused battery cartridge but not big enough for the grip itself.
The biggest pro is the price. If the price was the same, you'd go with the Nikon MB-D12. But the price difference is quite a lot--$370 vs $90.
You have to replace the battery pack to replace the battery in the camera. That's a design issue with the camera itself, and you run into the same thing with the Nikon-branded battery grip. That leads to more taking on and off than is ideal.
Some of the custom L-brackets and moulded covers might not fit with this grip because it's not exactly the same shape as the Nikon MB-D12 original.
The Vello BG-N7 is not a direct clone of the Nikon original, and there are downsides to it. If your gear budget stretches to it, the Nikon MB-D12 is better--more refined and has the peace of mind of having Nikon's quality control and guarantee that all functions will work with the camera.
But for a fraction of the cost, the Vello BG-N7 performs very well. Functionally, it does the same thing. And for many users, it's going to be well and truly good enough.
If you're shooting something where a vertical battery grip is pretty much essential, like professional sports, there's a reasonable chance you're shooting with something like the Nikon D5 or D4, which already has an integrated grip. But if you're looking to use it on a D810 or D800 and are willing to live with the ways in which the Vello grip isn't quite as refined as the Nikon one but love the idea of a battery grip that doesn't cost an arm and a leg, then the BG-N7 is an excellent alternative.
Buy It At
I got mine at B&H Photo.