Your accessory battery runs in two states.  Charging when the alternator is
providing power and discharging when the motor is off.

You want to isolate the batteries when you start the motor so as avert
trying to draw  starting current through the charging wiring for the
accessory battery.  You can draw >250A when you start the car from the
starting battery.

A diode isolator with a ~0.7VDC drop across it will effectively isolate the
accessory battery when starting but will reduce the charging voltage by
0.7VDC.

A relay isolator will isolate the battery with virtually no drop in charging
voltage.

A switch will do the same but you have to remember to turn it on and off.

The "standard" westie wiring has a "special" relay that has a built in diode
that helps isolate the battery when the alternator isn't supplying power or
the starter is engaged AND disconnects the electric fridge when the the key
is off.

Works just great for me.

Optima Yellow Top dual post 750 is my auxiliary.  It charges correctly from
the alternator power when the motor is running (>13.6VDC), is isolated from
the starting battery when the motor is starting, and provides auxiliary
power with key on or key off when the motor is not running.

They have a website.  www.optimabatteries.com if IIRC.

cheers,

pensioner