isn't a solenoid wired at a main power supply ( high amperage), and 
controlled by a accesory current?  That would make it fully automatic.  

    Also it is of interest that the EuroVan with a factory aux battery 
is charged and isolated with a 30 amp relay.  Since I thought this was 
too wimpy to handle the deep cycle I was installing in my EV I looked 
into isolation options.  
    When the relay frys I will get a solenoid, unless the solid state 
isolators actually can prevent alternator overload by switching off.
    I was told that a completely discharged batery can draw enormous 
amperage and fry an alternator.
    My EV has a 120 amp alternator and since I installed the deep 
cyclye I noticed that a plastic covering on the main Alternator 
terminal actually melted, makes me wonder if Iam overworking my 
alternator, and if I can some how lower the amperage potential draw of 
the battery. 

    Elliot