To start with one needs to determine what the amp/hour draw will be when looking at the size of a suitable auxiliary battery. The most widely used battery in the Leisure market has been the standard Deep Cycle 105A/h lead acid which gives you about 50 A/h to operate with taking into account that the battery will get hurt if drawn below 50% on a regular basis. The voltage cut out level on most fridges is below 12v which ensures that the battery takes a pounding on a camping two day week end.
A standard 40l fridge draws around 3-4 amps per hour on average in an environment up to 25 degrees. Doing the maths this gives you 12.5 amps per hour before the battery starts to take strain.
Luckily the fridge will switch off when it reaches the right temperature so this gives you some extra hours to play with. On average with 50 amp hours you can stand a day and night running the fridge at setting of zero to two degrees.
Should the temperature go over 30 degrees or the fridge setting is at -10 degrees you easily double the amps drawn from the battery. That leaves you with around 6 hours. Adding an inverter will again increase the load when in use.
A good option is to invest in a dry cell battery like the latest AGM Discover distributed by ENERTEC Batteries. For a variety of additional features the 100A/h AGM will give you double the run time as a standard lead acid battery as you can bring the capacity down to 20% before you start hurting the battery.
To give your dual battery system a sporting chance here are some points to consider:
When reaching your destination the Auxiliary battery must be fully charged. This can be achieved with the latest intelligent DC to DC chargers. (a solenoid unfortunately don’t qualify here)
Should you make use of an inverter or have a bigger fridge than a 40l and the setting is around -5 to -10 degrees you need to supplement the battery with the likes of a solar panel. This must be done as soon as you arrive at your camp site.
Make use of a device to give you proper feedback on how the battery is doing. Guessing wont get you anywhere. The CTEK Battery Sense gives you a graph telling you how the battery is doing. By having the info on your smartphone you can now make an informed decision.