I believe I may be the owner of the new Hella aux bat wiring kit. Let me see
if I can answer your question.

First and foremost, when you put in the aux battery, you will need to
manually wire accessories to the new battery. With the ignition off, the aux
battery is isolated from the rest of the "original" electrical system. Ex.;
You wire a TV&VCR to the aux battery and go camping. You then watch videos
for the entire time you are camping (not practical because you went camping
to get away from things in the first place, but it works for our example.),
thus draining the aux battery. When you turn the ignition on (and hopefully
start the engine soon after), the 2 batteries are connected in parallel (+ to
+, - to -), and the alternator charges both batteries.

As soon as the ignition is turned off, the aux battery is isolated again.

Hope this helps a little,
Sean B.
'90 V'gon, "Happy Bus"