Has anyone used or know anything about the Case Relay Power System from Tether Tools. The system allows you to power your camera continually with an external battery pack, so you can swap out an external battery pack. Seems promising, but I would welcome any comments or thoughts.

Here is my DIY solution for external power supply to my Nikon D750. It's not pretty. Total weight of all 4 components about 9 lbs (4 Kg). Total cost $98 USD (Amazon).
From left to right we have a 12 volt motorcycle battery(YTX9-BS; $36)...CUZEC 6ft cord dual plug battery clamp clip on with cigarette lighter adapter ($13)...Foval 150 watt power inverter, 12 v DC to 110 v AC with dual USB ports ($17)...Gonine EH-5/EP-5B battery adapter/coupler for Nikon D750 ($32).

The Case Relay works pretty well and is very lightweight. Just note that you need the right Battery-Dummy for your camera.
I use the Case Relay and can recommend it.