This controller is connected to your main battery and senses when the voltage reaches 13.5v by your alternator charging, Connect the trigger wire from the controller to activate your Dual battery Relay or solenoid and away you go.
Wired as the above way you can then have both your main and auxiliary batteries charging automatically when your vehicle is running and then when your main battery drops below 13.5v eg moments after you turn off your vehicle, the voltage sensing controller will detect this and open the circuit and break the connection between both batteries, ensuring to keep your main cars battery fully charged.
