I use a bosch headlight relay, with direct #8 (or next thicker size) wire to
the starter battery.  I decided for no logical reason to fuse the line from the
main battery to the aux. battery (optima) ...and it has blown a 30 amp fuse a
couple of times after deep drain-down of the aux battery... 30 amp. was the
maximum amp. fuse i had on hand at the time.  It has not blown the 40 amp. fuse
now in it.
steve