What battery should I get for Nikon D800 movie making, the EN-EL15 or the EN-EL18?
I could not find reliable data on how long the EN-EL15 battery will last while filming and Nikon does not specify his. I've tried it myself and found that recording 40 minutes of video, about 8GB, at the highest quality, uses about 75% of the battery. Further testing needs to be done, please let me know your experiences with live view and movie recording.
The EN-EL18 (that you can use with the optional MB-D12 grip) offers about 50% extra juice for photography according to the numbers in the manual, but I'm not sure it's safe to assume the same goes for video. It's almost twice as expensive and you need an extra battery lid, not to mention the MB-D12. The only advantage of the EN-EL18 compared to the EN-EL15 is the slightly increased maximum speed in photography, from 5fps in DX to 6fps in DX. For FX photography or video there is no added value except for the larger capacity.
My humble advice is to buy extra EN-EL15's. Their price-capacity ratio is a lot better than that of the EN-EL18's.
If you wonder why the D800 has a new battery type: The EN-EL3e used in the D300/D300s/D700 is banned by new battery regulations in Japan. Hopefully a set of EN-EL15 batteries will be a good investment for the future.
The D800 does not have the built-in AC adapter socket the D700 has. You can still use the same AC adapter (EH-5, EH-5a or EH-5b), but in addition you need to buy an EP-5B (not EP-5) Power Supply Connector which has the shape of an EN-EL15 and goes in the battery slot. It will work inside the MB-D12 too, the battery grip has an opening for the cable.