I have the older CFX45w and it runs off group34 battery, set to 2ºc(36ºf) it runs about 48-54 hour before hitting the high level cut off in normal ambient temps. I have set it to -4ºc(25ºf) with external to the car ambient temps in the mid 30sºc(94-97ºf) car parked in the sun and it hit the high level cut off in less than 24h. If it's on your start battery. set it to the high level cut off and your battery is in good condition it should have enough juice to start the car.
A Dometic Rep posted this on another forum.
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"There are a lot of factors involved in determining this: type of battery or batteries, condition of the charging system, wiring to the fridge, how often you open the fridge, actual electrical consumption that the fridge requires, etc. While we may not be able to determine the condition of your wiring or charging system, we can help with info on what the CFX3 line of fridges consumes power-wise.
In the above graph, we've measured each of our CFX3 models running at typical fridge (39 degrees F) and freezer temperatures (5 degrees F) and how external ambient temp impacts energy consumption. Given that the hottest temps will be in the day, you can rest assured that power consumption at night will be much lower regardless of what temp you set."