Split Charge Relay (VSR)
An automatic relay that connects and disconnects the starter and service batteries based on a voltage threshold. When the engine runs and the voltage rises above typically 13.3 V it parallels the two batteries; when the voltage falls to around 12.8 V it separates them and prevents the starter battery from being drained.
The logic of the relay is simple: if the alternator is running, line voltage rises, which is the sign that charging is under way; the relay closes and the surplus current flows to the service battery. When the engine stops and the voltage drops the relay opens, the living-area loads are then supplied from the service battery alone and the starter battery is kept in reserve.
Relays are typically available in 80–200 A current classes and are protected with separate fuses on both the input and the output side. The connecting cable must be sized so that the voltage drop over the distance between the two batteries stays acceptable; a thin cable largely cancels out the benefit the relay provides.
The limitation of the split charge relay is this: the voltage it delivers to the service battery is what remains of the alternator voltage after the cable losses. That value often fails to reach the absorption threshold, so the battery struggles to take the last 20%. On vehicles with smart alternators the relay may also open and close continuously. In these two cases moving to a DC-DC charger is the right step.
Context in which this term is used: RV automatic levelling guide