The design brief of any system should have included: “Store as much A-hour in as short a period as practically possible”
If you can do the above you will be in the best possible position in any situation. The power that is available should be used and abused as much as possible because when you switch off the engine, so does the supply. That should sound familiar, when you stop paying, they stop supplying. Your biggest priority must be to have the most Ah available at any given time.
Most modern vehicles have pretty big alternators. Most of them have capacities of 80A or more. (20 Years ago we managed pretty well with a 37 A alternator) You should easily have 60 Ah available for you auxiliary batteries if you can remove all limitations. During your 4 hour drive this ads up to 240 Ah !! You will most probably not be able to remove all limitations, but let us start with a couple of the big easy ones.
Charging rate. The elephant in the room that nobody seems to notice. Any system is limiting the charging rate for some or other technical reason. Ideally you should go for a system that does the least limiting. At the very least you should calculate what your worst case would be and get a system that will be able to charge enough for your expected requirement in the time limit you will determine. Then keep in mind how often your best planning can go wrong. You will easily land up in an unplanned situation where you will need more Ah.
Battery capacity. This is the next big limitation that we all run into. It does not help to have the best charging rate if you cannot store it. Here we are the culprits ourselves because these things are big and one must find space to pack them. They are also expensive and we only need them once a year on our annual pilgrimage. The range of types, capacities and prices is confusing to say the least. All batteries have a limitation on how much you can discharge them. Realise and calculate that in. Do not go beyond this point, you can destroy a good battery by discharging it completely in 3 or 4 instances. This could be the first 3 days of your holiday and you will have a cooler box in the back in stead of a fridge. The industry standard 105 Ah battery can be discharged with a little bit of stretch to 50 %. (12,1 V) Any battery takes a long time to charge fully. Up to 14 hours and we seldom drive that long. The 105 Ah standard batteries can be safely charged to 90 % in less than 2 hours. That gives you 40 Ah useable capacity. (If you run at the required charging voltage a battery will regulate its own charging rate. A 105 Ah battery will easily and safely accept 35A at 50 % state of charge)
Installation. Do the best installation you can find. Stress the point of thick wires (Cables). If the guy is at all vague about this specify what you expect beforehand. (Get technical advice if needed.) This can become expensive as copper is expensive. Some system types are more critical of wire thickness than others. Be sure to stress that cabling must be installed in such a manner that it would not chafe and has play around bends and ‘live’ areas, and check the guy. You could light up a huge bon fire in the middle of nowhere and that is one thing none of us can afford.
What do the different systems offer ?
Here you must screen out the sales talk and misconceptions to get to what you really need. There are two basic systems available and the primary objective is to isolate the auxiliary batteries when the vehicle is switched off and charge the battery when the engine is running.
The invertor type. This is arguably the most popular as the installation is easy and it performs reasonably well for the weekend warriors. They seem pricey but quality does not come cheap. They seem to have a problem with high temperatures so do not install them in the engine bay. The principle is that it picks up the charging voltage to increase the charging rate. This compensates for bad wiring and hot alternators that cut back on voltage. It’s main disadvantage is the charging rate limitation. The most popular make is limited to 20 A. From this one must subtract the above determined 4 A required per fridge which leaves us with a 16 A maximum charging rate. This figure of 16 A drops as the battery charges up. This is a law of physics and a graph that most suppliers cannot or will not supply or simply do not know about. If you run a second fridge another 4 A should be subtracted. If you run more than one auxiliary battery, as most well designed over landing systems do, the charging rate stays the same. This is a serious drawback. They claim to give your battery the ultimate charge and conditioning. This however never happens as we do not drive for 14 to 24 hours continuously. Beware of this statement “Suitable for any type of battery, flooded lead acid, AGM or lead crystal.” It may only be true if there is an option to pick the different types. This is crytical as the performance and life of your expensive battery may be seriously compromised. Most also offers the control of solar panels. People add panels to compensate for the low charging rate at huge cost and then it is rather inefficient as the controller is rather outdated compared to new controllers available. Victron seem to have a decent MPPT controller for solar.
The other type is the solenoid systems. These have a bad name in the industry mostly because of bad installations. It comes at less than a quarter of the price of the invertor types. Remember that one does not get cheap quality. Then the installation should be rather expensive if done properly. Despite this it will easily outperform the invertor types if you have two or more batteries. The only provision is that the cheap system must not be hampered by a cheap installation. Spend the money on enough copper and quality workmanship. Then this type of system will serve you remarkably well. If you work it hard you may have to replace the solenoid regularly as it is not quite up to the hard work we expect to throw at it. Then again it is so cheap and easy one can take a spare solenoid with to replace when necessary. There is a very limited range available and in my opinion none that will perform any better than the cheap well known local offering. Batteries at a 50 % discharge will accept more than 35 A charge rate per battery safely. This system will start charging 3 batteries at a combined rate of 105 A. That is called performance. It is quite capable of charging 3 x 105 Ah batteries from 50 % to 90 % in less than 2 hours. This means that you can drive for 2 hours a day and have 3 x 40 Ah stored = 120 Ah. If your fridge then runs at 4 Ah as determined above you have 30 hours of running time available. Add another fridge and the whole problem starts from the beginning.
If you go for an industrial solenoid system you can easily run out of alternator capacity (Easy to upgrade), you will be able to keep a fridge and freezer happy in extreme conditions wihout the hassles and cost of a solar system with an hour game drive in the morning and another in the afternoon. You will not have to rely on sunny wheather either. No other system can do that.
Beware of electronic solenoids as they charge at much reduced rates. There is a local offering but I am not convinced the voltage drop problem can be soved technically and it is a killer..
What can we learn from this basic reasoning ?
Do not underestimate what you expect from the system, it will bite you.
Do not overestimate what you can get from a specific system.
Do not overlook what the car’s system has to offer. You may require a larger alternator but this is highly unlikely.
If your local expert cannot explain the above, find a better expert.