That model Ranger does, indeed, have what’s called a variable-voltage or `smart’ alternator. The idea is that when the car’s main battery is fully charged, the alternator reduces its workload to save fuel. But the reality is that the system doesn’t always recognise that accessories (like a second battery) have been fitted to the car. If that happens, the alternator doesn’t always make enough power to keep accessories like that second battery system fully charged. Which means that suddenly, your car-fridge won’t run overnight. The good news is that this smart function is purely electronically controlled, and a Ford dealer can disable it for you, returning the alternator to conventional operation.