Solenoids and some DC-DC chargers use a circuit powered when the ignition is on (IGN) to start their operation. Voltage-Sensing Relays (VSR) do this by reading voltage to know when alternator charging1) is occurring.
it operates by sensing the voltage on the main 12 volt system. When this voltage goes above 13.3 volts for approximately 12 seconds… it will close the isolator relay providing charging current to the auxiliary battery.4)
Note that these can sense voltage on any circuit, not just the IGN circuit.