Twin battery isolation.

[Green Bob]

Just mulling things over and I know it has been done to death, but I can't find an answer as to whether it would work.

My aim, in the above, would be, with the engine not running, to have the starter battery connected only to things necessary for starting, while the auxilliary is connected to everything else.

Everything I've seen on split charge and the like seems to have the relay between the batteries so, as far as I can make out, the radio, lights etc would still drain the starter battery as it's connected to all the vehicle's standard electrics via the back of the alternator.

Have I completely misunderstood the principle/am I just being stupid? Also, have I managed to make some sort of monumental cock-up in the above diagram?

[Tal]

I'm no expert but it looks like you might need the alternator on the glowplugs and starter motor side of the circuit to me, you need a small current going through the alternator to produce an electromagnetic effect before the engine turns over for the alternator to start charging and if I have my head on right, your engine  would have to start turning before any current gets to the alternator (magic relay) to make it produce more electric. I think it should work as you have it but puts an extra load on the battery every time you start that could be avoided,

Or I have no idea what I am talking about LOL    

[Peaklander]

What you've drawn would work. The alt still has a battery connected to it and the "everything else" load includes the charge lamp which is from where the alt gets the startup volts that Tai mentioned.

Don't think you will improve anything though as now everything is loaded onto one battery except two things that are infrequently used. 

[vulcan bomber]

you only need to run 2 wires to the radio from the 2nd battery to stop it draining the starter battery, I'll do it on my 110 one day.