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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.
Voltage-sensing can be added to devices that lack it by adding a a relay-delay.2). Note that the relay-delay can sense voltage on any circuit, not just the IGN circuit.