Combined fridge power supply relay and charging relay which enables the caravan refrigerator to function and an auxiliary battery to be charged whilst towing. Supplied with cables and full instructions plus all connectors necessary for installation. 'E' Mark approved : E11/020053. Difference between the Combi-Master and the Self Switching Combi Master is explained below. The self-switching combi master monitors you main vehicle battery level and turns the charging circuit on only if your main battery is full enough. Should your main batter be low the charging circuit is turned off so as to save the main vehicle battery. With the standard combi (This product) , once the vehicle ignition is turned on the charging starts irrespective of the state of the main battery.