The Ryder (TF1170-4) voltage-sensitive caravan relay (12-volt, 22 amp) switches power to a caravan's battery-charging and fridge circuits when the vehicle alternator is charging. It requires no trigger and protects the vehicle battery from drain when the engine is off. 

Fitting instructions may be found at:

Voltage Sensitive Caravan Relay, 12 Volt, 12 Amp

VAT Included