Well, do you think it's safe to say that most trips fall into the 48hr range for the average person? If so, how many hours is the rig off?
I'm saying I can sit for 48 hours and the vehicle will start on its primary and that isn't utilizing the low voltage protection which can be set on the ARB fridges at various levels based on your needs, as you know. Set those to an conservative level and obviously the fridge is going to revert into a cooler.
I think the majority of trips fall into the 48-72 hour range with a travel time of 2-12hrs round trip.
Likely as different as the users themselves. The majority of my trips include less than 15 hours of non-running time per day as we are generally moving to a new place each night thus driving all day. If I were to 'estimate' the general uses of my customers I would say parked for no more then 40 hours would be the standard? Again its totally dependent on the trip, the user, etc. In my opinion a dual battery should follow other more obvious upgrades, i.e. choosing gear that has a low draw. You'll notice a pretty wild amp draw variance between accessories and generally speaking the higher prices are associated with lower amp draws. For example that Engel fridge you've used has a amp draw of roughly half that of the budget fridges available under different badging from K-Mart, Compact Appliance, Home Depot, etc. While it might not be an issue for some, it is a deal breaker for others considering that Engel will run twice as long as the others on a given charge. Lights, radios, etc are all similar... choose the accessories that match your 12V needs. I'm now using an LED camp light as it has a much lower amp draw than even the halogen I was using before, when you consider a batter that has ~60 amp hours of reserve even a light running for several hours can make a difference.
Speaking of reserve, its always a good practice to add up your amp-hour needs when not only selecting your accessories but also the battery that will power them, particularly if your integrating a deep cycle second battery. In the case of the fridges I sell, I conservatively tell customers to plan on the fridge running 50% of the time in hot conditions (worst case scenario) and pulling 3 amps (Engel). So with a 60 amp-hour batter under those conditions you have 40 hours of fridge run time. Add lights, and other accessories and that number drops.
What if you could push 'er over the edge?
Another 'what if'
I don't think you'll find anyone that has a problem with 'bonus' amp-hours, dual batteries or not.
