Fridge power in transit

Anyone know how much the fridge eats the leisure batteries whilst on the move?

Recent trip away and what usually happens, I arrived with my batteries just above 12volt, the fridge was on batteries whilst I was driving.

On the way home I left the fridge on gas and arrived home with the leisure batteries at 13volt.

Is this the norm?

Mine doesn't have an Auto, I switch it manually. That's a good point, you would think it would use the Alternator, but does it keep using the alternator once the engine battery is charged?

Going by the Battery to Battery chargers that seem very popular, the Alternator stops charging once the engine battery is full. It tricks the alternator into thinking the engine battery is not full.

But I am not sure either... anyone?

There is an article on running fridges on 12 volt in this month's MMM. From memory the stat doesn't work on the fridge on 12 volt and the fridge runs full tilt, it gives the amp hour running requirements but I haven't brought it away with me. Will read it again when I get back home.

I know what you are saying but the difference between 12 and 13 volt is huge. Some would say 12volt is a flat battery.

I had a wee look at the article and it suggests that the 3 way fridge will run continuously on 12 volt with out thermostatic control. It will draw between 8 - 12 amps per hour. The reason for this is to avail of the free 12v supply from the engine, when driving as the fridges absorption principle will not be the same as it is when static and level.
This can lead to very cold fridges after a few hours of journey, freezing your lettuce and carlsberg.