member Pugwash, one red star [Online] Posted 09 April 2012 by Pugwash Quote | Report Am I right in thinking that you still a separate feed to the caravan that would be wired to circuit on the car which is controlled by the ignition switch? Or has that also been superseded by modern electronic circuitry? Without looking it up I can't remember what pins are which (13 pin or 12N/S dependant). One wire is fed via the split charge relay & powers the fridge when your car engine is running - split charge relay can tell via the voltage ie 14.4v that the engine is running. The permanant feed that you can power your caravan with as a back up becomes the feed for charging your caravan battery via the habitation relay inside the caravan - this gets a feed from the fridge supply. ;-) Write your comments here...The habitation relay cuts the 12V supply to lighting circuits etc when the car engine is running, this is a safety thing to reduce fire risk. The permant live powers the van when the plugged in with the engine stopped. Hence atc goes through the self check. When the engine is started, and car voltage is sufficient, the split charge relay closes and allows the fridge to run and via the split charge relay. It also allows the control voltage to the pdu to bring in battery charging via the permanent live. Post liked!