Preventing warm beer & flat batteries

The problem of warm beer plagues us all with this towbar. BMW did not equip the 13-pin socket on the towbar with pins 10, 11 and 12. Pin 10 is the “ignition on” supply which powers the caravan fridge while towing and pin 11 is its earth. Pin 12 is not normally used in a 13-pin system, but in my conversion, I utilised this to provide a dedicated circuit to charge the car 12V battery via the 13-pin socket. The ability to charge a car’s battery via modification of the towbar wiring can be fitted to any car that has a 13-pin socket using the spare pin 12 and any earth pin.

To provide an “ignition on” supply to pin 10 you need a relay, known as a caravan, smart, combi or split charge relay. This is a voltage sensing device that detects the higher voltage from the alternator over the cars battery voltage and switches on to provide power to pin 10 only when