Your battery isolator is the clever gadget that protects your start battery from going flat... it does this by isolating it from the secondary battery when the engine is off. The clever battery isolator will pick up when the vehicle engine is running and charge is going to the starting battery via the alternator.
The battery isolator is affixed to your vehicle's alternator (the component responsible for giving the vehicle electrical power and charging the battery). The battery isolator uses sensors to determine when the cranking battery is charged and allows the alternator to charge the secondary battery until the motor is turned off.
Solenoid battery isolators are basically mechanical switches, using an electrical current to close the switch, transmitting charge to the auxiliary battery. In order to stop your starting battery draining, you should wire it into a circuit that's only active when the vehicle's engine is running.
If you're heading off road on a long trip or camping overnight, you will absolutely need auxiliary power. A dual battery system means you have two separate batteries working in unison with each other but performing different functions - so you'll have start-up power and auxiliary power.
