Power Sony's A7S MK3 from an external power source for extended run-times with the LR-16Z dummy cable. This allows the use of more powerful batteries to be used with the camera.
GR-100 or something like VL-70 or if you wish to keep weight to a minimum our BPU range of batteries BP-75UX can also be used.
No further regulation is required as the LR-16Z is regulated to the correct voltage the A7S mk3 requires.