RE: Switch with 1315
Mark did not discuss one feature of the 1315, the ability to add a switch
(and indicator light if so desired) that will connect the aux and starting
battery together for starting, if needed.  The normal function of the 1315
depends on the battery receiving current from a charging source (like the
alternator) to rise to 13.3 volts and then the other battery is connected
and is thus being charged also.  But in that normal configuration the two
batteries are NOT connected together if neither one is being charged. The at
rest voltage of the batteries will be below the 13.3 volt trigger threshold.
That's how the separation is achieved. But the 1315 has the above mentioned
feature as an option.  By adding a switch and some additional wiring you can
join the two batteries together if needed, like starting if the normal
starting battery has been run down.