AUX battery - how is it possible?

[davepl]

I have been ruminating about this off and on for quite some time, and cannot understand it.

When you have the camper battery, as you can see from the schematic, as soon as "IGN UNFUSED" has power the two batteries are connected in parallel. When you turn the key off, they are separated (aux battery positive side isolated from rest of truck).

That's great, but it's a small wire from the aux battery to the junction. So there are two scenarios I can't figure out:

1) When the state of charge is very different. If the AUX battery is depleted and you start the truck, it connects the two batteries together. The fully charged MAIN battery plus the help of the alternator should be pushing a ton of amps to the discharged AUX battery. Yet it's like a 14-gauge connection, so how is that possible?

2) Similarly, if IGN UNFUSED at the fusebox is hot during cranking, then both batteries are again in parallel. I don't know why the AUX battery does not then participate in the cranking and completely fry the 14 gauge wire.

Since a number of people have upgraded to HEI and connected it to IGN UNFUSED I think that -has- to be live during cranking.

Can someone explain it?