Be careful when using a diode type battery isolator. These are the "atomatic solid state" devices. I prefer the relay type.

The diode type have a small voltage drop (.6v) which does not sound like much, but the difference between a charged and discharged battery is not much more. Therefore you must increase the charging voltage when using the diode type. To do this, you move the voltage sensing circut from you regulator to the battery side of the isolator. Now the alternator puts out a bit of a higher voltage to compensate for the voltage drop.

If the diode fails open, the battery sense circuit will see no voltage and begin to increase the charge voltage. The result will be an explosion. Been there, done that.

The relay type is much cheaper, easier to install and gives you control over the charging conditions (with a switch if you want to),. Otherwise you wire it so your auxiliary battery is connected when ever your key switch is powered. Turn off the key and the relay opens. No chance of discharging the main battery.

Plus, should you somehow discharge the main battery, you can jump the relay and use the aux to charge the main. (or if there is enough voltage in the main to activate the relay).

I have been using aux batteries for 20 or more years and the relay system has been a consistant winner for me. I have seen (and experienced) many problems with the diode method. YMMV.