will this work!

power lead from starting batter to aux battery with a isolation switch, and a ground for aux battery.

all my current aux devices run off starting battery, so if i turn my iso switch to on once my starting battery has gone flat the aux battery if full charge, it should provide enough power to start the car...

only reason i see a problem is the connections from the two batteries wont allow enough current to start.

The whole point of the secondary battery is to run all of your auxiliary devices off it, and allow it to die, meaning your cranking battery is unaffected.

The issue with doing it that way is that your cranking battery may absorb power from the aux battery, meaning that the car may not receive all the power from the aux battery. If the cranking battery is big enough, it may not allow for a jump start from another source. I noticed this with the optima on my EL. When it died, I had to remove the positive terminal from the battery (and leave the jump lead on the terminal), use the booster battery to start the car, then re-connect the terminal to the optima.

i would have my gear running off the aux batt althou i do not fancey rerouting power for everything i run, going to wing it. should work ok!