I would not power the D800 directly from a car battery. It could output as much as 16V when fully charged unless you add some voltage regulation. When the D800 is powered with a larger battery in the grip, the connections are different from those in the battery compartment, so there is no guarantee that the ones in the battery compartment will take more than 9V, which is the rating of the original Nikon power supply.
The reason you have problems with too much power draw from your battery bank setup is likely the power spike during the faster mirror movement. If your 5V battery bank can output 2A and output from the 5-9V DC converter is good for 1A or so, this can be remedied by applying a supercapacitor circuit between the DC converter and the D800. Here are the data from my D500; I would expect D800 to be pretty similar:
The dark blue line shows power draw by the D500, while the red line shows the leveled power draw from the DC converter. The voltage drop (ice blue line) is a little higher than in a real situation as I am measuring current drop over a 0.1ohm resistor. The two spikes are mirror up and mirror down movements. I believe the last lower plateau is the writing to the SD-card.
Here is the schematics. C1 is a 1F 9V supercapacitor. The diodes and the resistor are needed to limit the charging current of the supercapacitor and it also brings voltage to the supercapaitor down to a safe level since 9V is just at the rating. The 10 Ohm resistor is large to not burn out at the start of the supercapacitor charging. I ordered the components from Digi-Key..
I built this circuit into my SkyTracker that had some space beneath the circuit board. It is powered by a USB based power bank and a 5V-9V DC converter. I might get another set and make a standalone version in a box or embedded in potting material for occasions when I am not bringing my SkyTracker (i.e. meteor showers).
(It looks a bit messy due to the hot glue used to fix it in place).
Edited by otoien, 19 September 2020 - 05:43 PM.