Battery Relocation Question

The problem with "a main fuse" is that it must absorb the full impact of the alternator

That is, let's say you run the battery down "a fair amount." Either you let the lights on for hours, or flooded, whatever. "it's down."

Maybe it's night. Maybe it's night AND COLD. The alternator is REALLY cranking out the suds. If it's a 100A alternator, you need at least a 100A fuse or link, and "charging wire" sized to absorb it.