Keeping the 12 Volt battery fully charged when you’re off the grid is a requirement to be considered before heading off. Whether you’re heading away for the weekend or preparing for an expedition a good knowledge of your fridges power requirements should be understood. From experience, you should probably try and get some real data on what your fridge needs. In the real world, you can bet that your fridge may use a bit more power than the manufacturer specifies.
Being in the tropics we get to see a variety of fridges operating in our high ambient. On average for most common brands of 12-volt fridges in the 40 to 50-liter size operated as a fridge will require between 45 and 65 amps per day. With this being the case if you have a fully charged 120Amp Hour battery you would expect it to run the fridge for approximately 2 days without any inputs. The problem now is replenishing the battery before its empty. If you’re relying on driving to recharge you need to know how many amps your alternator is actually being allowed to put back into the battery. If it’s the main start battery it may be a healthy dose of amps up to the capacity of the alternator. This is great but it would still require up to an hour driving each day to top up what the fridge has taken. If you have a dual battery system then you may be limited what is going into the 2nd battery by your specific controller. I’ve seen some that only feed 8 amps and hour and other that feed 25 amps. But you can see where heading at 8 amps you will need to drive from 6 to 8 hours a day to replenish the fridges requirements.
As a service, we can calculate the power consumption of your fridge data logged over a 24 hour period. We can do an in-vehicle test to see what your volt drop is and also measure the charging current going back into the battery. Armed with this advice you can make confident decisions on your battery/fridge management.