Trailer charge system

I'm told that isolators are different now, or can be. You used to have two maybe three choice

1...(My opinion, the worst) a 3 terminal isolator
2...So called continuous duty solenoid, activated by the key
3....A manual A-B-Both battery switch

A....3 terminal isolator THEY CAN CAUSE ISSUES and I'm aware of at least two of them in boats. A 3 terminal isolator is nothing more that two big diodes. You BREAK the alternator output wire, and connect it to the common of the diodes. You connect the original cut charge wire to one of the two outside posts. This now runs the alternator output IN SERIES through one of the diodes to the main battery. You then connect a wire to the remaining post and run that to the aux battery. IF THE HEAVILY USED main diode pops, there is no SENSING to the alternator and it will charge the aux. battery WIDE OPEN along with NOT charging the main. THIS DOES HAPPEN

B...Continuous duty solenoid. This in my opinion is the best option. This is a solenoid which is DESIGNED to be powered for long periods, UNLIKE a starter solenoid such as Ford. You activate the coil with the switched igntiion "run" power, and run a new wire from the alternator (fused) back to the aux battery. Anytime the key is "on" the aux battery is connected in parallel. These are VERY popular and work well

C....A-B-Both manual switch. These are used in fire and other trucks, boats, etc and are perfectly fine IF YOU remember to switch them EVERY time, otherwise, you will discharge the main as well. In your case you would want to use only A and Both