A voltage sensitive relay (VSR) is a relay. It is used for automotive, truck and marine applications.
A voltage sensitive relay senses the input voltage generated and automatically connects/disconnects the appliance or circuit at pre-set voltages for a variety of applications. It is typically used to prevent over-discharging of car battery and is very common in a dual battery system.
A VSR is usually classified into 12V / 24V range with varying amperage. A single sense relay only detects voltage at one pole while a dual sense relay detects voltage at both poles.
An ignition-protected voltage sensitive relay can withstand repetitive engine cranking transients without significant effect.
VSRs are inexpensive and offer little or no voltage drop. However, the relay may chatter (repeatedly open/close at very short intervals) if the voltage is not stable.