Good day all.
I am posting these comments to see if anyone else has experienced D850 problems similar to mine. I would be very interested to hear how the issue was resolved for other Nikon users.
About 6 weeks ago, at the beginning of a shoot, my D850 suddenly and completely lost power. The camera was completely dead, as evidenced by the blank top LCD screen. I was using a new 24-70mm f/2.8, a freshly charged Nikon EN/EL-15a in the camera and a freshly charged EN/EL-18b in the MB-D18 battery grip. Nothing would revive the camera. After returning home I removed and tested the batteries, which were all fully charged, and removed and replaced both the XQD and SD (UHS-II) cards. The camera came back on when I replaced the camera battery (without the MB-D18) and remained on for several test shots before suddenly losing power entirely. I cleaned all battery terminals and battery contacts in the camera and tried to get the camera to turn on with 6 different, fully charged, OME Nikon EN/EL-15a batteries. I sent the camera back to Nikon for repair.
Two weeks later the camera was returned with a note saying that the camera had been cleaned and tested and was working properly.
After about 10 minutes of use the camera died again. I once again went through the battery/card replacement process. Occasionally the camera would power up for a short while, but it would inevitably lose all power within seconds or minutes. When the camera did power up, I could now hear a distinct, soft popping sound coming from the top of the camera. This popping sound would coincide with the top LCD screen flickering, right before all power was lost. So, within 4 hours of getting my D850 back, it was repackaged and on its way back to Nikon. So far I have spent $220 on shipping and insurance.
It has been a couple of weeks since Nikon took receipt of the camera but, other than an email to tell me they had the camera, I haven't heard from them.
The D850 has been a real letdown so far, and I am very disappointed with Nikon's service department. Part of my frustration stems from this not being my first rodeo - last year I had another brand new Nikon DSLR that required multiple round trips to the repair shop before they were able to diagnose a defective shutter and replace it.
If anyone else has experienced this apparent "dead battery syndrome" with the Nikon D850, please let me know what was found and what was done. Most importantly, please also let me know if your D850 has had any additional electrical problems after repair.
Any feedback would be most appreciated. Thanks, Allan

I share your concern, Doug. I was thinking this was a contact issue in the battery bay, or possible the contacts in the battery grip, but the popping sound really bothers me. I have been shooting with Nikon DSLRs from day-1 and I have never heard such an ominous sound from any of my cameras. It does sound like a muted spark. I even started sniffing the camera when it was making this noise, to see if I could smell burning (I couldn't). I guess the weather sealing would keep any smoke inside, like it keeps water vapor out.
One of the reasons my confidence in Nikon is at an all-time low is because they just don't communicate with the people who buy their cameras. I have written a couple of letters to the company to get more information on what the technician found and get details about the repair, but I've never received a reply. Nikon really needs to think about upping their customer service game. I would not be surprised to find out that Sony's mirrorless business is booming, in part, because long-term Nikon users have given up on the company's product support.
I really appreciate all the helpful and supportive comments from this awesome community. I will certainly keep everyone posted on the outcome of this frustrating episode.
Allan