luthj
Engineer In Residence
The easiest method in my view is to get a second battery (true deep cycle) for the fridge, and just wire a split-charge relay. If you have a bit of extra cash, a 50-100AH lithium battery pack will charge just fine from a 13.8+ volt alternator. It will be smaller, lighter, and won't suffer from partial charge issues like lead will. Lithium batteries do have issues with low temps (below freezing), so if you need to operate in winter temps, lead is likely your best bet.
Part of the reason I suggested a cheaper flooded battery, is that true deep cycle units will last a lot longer, but they are quite expensive in the group sizes, and selection is limited. In many cases, where the usage is not frequent, its more cost effective to use a cheaper "dual purpose" flooded battery. Put a good wall charger on it occasionally, and replace every 2-3 years. As opposed to a battery thats 2-3x the cost, and will only last 3-4 years tops.
Again, get a decent volt meter, and start poking around to get some data. You may not need to rewire everything, so confirm for yourself the voltage drops and alternator voltage.
Part of the reason I suggested a cheaper flooded battery, is that true deep cycle units will last a lot longer, but they are quite expensive in the group sizes, and selection is limited. In many cases, where the usage is not frequent, its more cost effective to use a cheaper "dual purpose" flooded battery. Put a good wall charger on it occasionally, and replace every 2-3 years. As opposed to a battery thats 2-3x the cost, and will only last 3-4 years tops.
Again, get a decent volt meter, and start poking around to get some data. You may not need to rewire everything, so confirm for yourself the voltage drops and alternator voltage.