common words when shopping for this sort of stuff..
solenoid-simple, dumb, same as a starter solenoid(not really, but to keep it simple)
separator-smart solenoid, moving parts
isolator-smart, no moving parts
i run both a separator and a solenoid(40 amp relay) in two different vehicles.
the truck with the separator is using a surepower 1315-100. It is smart and just reads the battery voltage and determines when to connect. it monitors both batteries for charging voltage.
Example: main battery is being charged by the alt, when the voltage rises past 13.8 it will connect the aux battery. if the voltage drops below, then it disconnects(excessive load, no charging voltage). Now if the truck is off and there is a charger on the aux battery and the voltage rises past 13.8 it will connect the main battery and charge both. when the voltage drops below that number, it will disconnect. I can energize the separator using the voltage from the aux battery if the main runs low with a momentary switch to the left of the steering wheel.
in my poor mans camper, fullsize van, i have a much simpler setup. i use an aux battery to run my cabin accessories(fridge, dvd, inverter). I have a 40 amp relay that is wired to connect the two batteries when the key is on and disconnect them when the key is off. So the aux battery is charging when i am driving. With this setup, i cannot jump start, and if my main battery goes dead and i try to start the truck, it will blow the fuse in my relay circuit.
both system work for what i intended them for.
now with all the fancy talk about this stuff, there are also contactors and solenoids that are magnetic and only use voltage to switch positions. they are usually expensive.
if you are gonna use a simple solenoid, look for one that is continuous or constant duty. usually you can find those online for about $40.
i'm sure there are things wrong with my reply, and i'm sure some people here won't hesitate to correct me.....