The Intelligent Solenoid (IS) senses an incoming voltage of greater than 13.1V when you turn on the vehicle.
The IS starts an internal timer for 5 minutes, keeping the full charge to the primary battery to recoup for the discharge of starting the vehicle.
After 5 minutes, the IS automatically closes the circuit in the solenoid, allowing current to flow to the secondary battery.
This connection remains closed until the IS senses a voltage of less than 12.7V across the system. This is most commonly when the vehicle is shut off. Brand new batteries can often maintain a charge above 12.7V for a while. The IS will keep the connection closed until less than 12.7V is available, then it will open the solenoid circuit isolating the batteries from one another.
The IS will keep the batteries isolated until it once again senses a voltage greater than 13.1V. Then it starts the timer and the whole process begins again.
This process is fully automated. You do not have to manually manage the charging of the second battery.