Engel Smart Battery box

Hi Rohan,

There would be little advantage in wiring it up direct from starter battery with an isolator in the circuit.

With its own built-in dc-dc 6 amp charger, you basically have a dual battery system anyway. You would not drain the starter battery by running accessories off the auxiliary battery inside the box. The dc-dc charger is to all intents and purposes, the isolator.
It will boost the voltage to an optimum level, overcoming any voltage drop in the long cable run, but you will be limited to a 6 amp maximum charging rate.
Yes you could wire it up directly to the starter battery, including an in-line fuse and run a twin cable back to the remote location where you choose to mount the battery box, but I think it would be impractical to try and bypass the internal dc-dc charger which looks good enough anyway to manage the charging of an AGM style battery.
I would terminate the cable run to a Merit/Hella style socket, or even better, to an Engel socket, then use a short "patch lead" to connect the battery box to.

Any alternative solution would require the disconnection of the dc-dc charger and use the alternator smarts to manage the charging of the remote battery. In this case the cable run would be the same (using appropriate size dual core cable) and include an isolator in the circuit as you first mentioned.

Rohan,
What sort of battery is in the box?
Not all batteries can accept high amps when charging, so bypassing your inbuilt dc-dc charger may not be the best thing to do.
Eg, I have a Supercharge Allrounder, 105 AH wet cell. Max charging current (from the manufacturer's blurb) is 10-11amps.
OTOH, AGM type batteries, and others, can typically take much higher currents.
Be careful.

Cheers