If you adhere to some or all of the basic rules you can increase the life of your freezer battery to last for days instead of a couple of hours without a recharge. You also will obviously need a dual charger that actually works. Most of the systems on the market are pretty useless when the chips are down.
The issue is to limit the absorption of heat into freezer. The term loosing the cold is technically incorrect.
It is all about managing the temperature around the freezer. The higher the temperature, the more heat absorption you will have. While you are driving this is OK as long as you do not exceed the capacity of your freezer cooling unit. When the engine is switched off this becomes critical since your battery must supply the power to remove the heat gained. Even a 100 Ah battery becomes small if you do not look after your freezer.
It will be impossible or impractical to follow all of these rules. The more of them you can adhere to the more efficient your freezer will be and the longer your battery will last.
Any or all of the following basics will help. Try and incorporate as many of them as practical.
Basics. Start with a fully frozen freezer. Do not expect the freezer to do the work en route. Especially if you do not use the baskets. If you do have baskets or spaces around your frozen stuff, install a small fan inside to stir the air. This will improve your freezing capacity tenfold. This should however switch off with the vehicle engine to conserve energy and not destroy the added air gap insulation when you are running on the battery alone.
Using the controls. Run the freezer at maximum while driving and turn it up to just colder than zero as soon as you stop for the night. This will build up a reserve while you are running on the alternator.
Insulation 1. Cover the freezer with blankets, towels or cushions. Anything that will shield it from the sun and surrounding heat.
Insulation 2. Put reflecting shield on your vehicle windows. Most of the darker types do not reflect heat. Install a mirror type. Ask the supplier to give you the specification on reflection and see the difference. Install blockout curtains on the inside of your vehicles windows. They help insulate and if they are a light colour will also reflect heat.
Insulation 3. Install insulation in the vehicles roof. Even a roof rack helps. If you have a glass sunroof, fit a thin plate on top of it with double-sided tape. This will create an air gap as well as stopping direct heat transfer between the plate and the glass.
Avoid the sun. Park your vehicle in the shade if at all possible. Move the vehicle to follow the shade during the day. Especially the side that houses the freezer. If there is no shade park the vehicle in it’s own shade. Turn the vehicle to face the sun, do not park the side of the vehicle to face the sun. If you are serious and you are not going anywhere you will change the direction your vehicle is facing two or three times during the day to follow the sun.
Ventilation 1. If possible open the doors and windows to ventilate the vehicle if you have to park in the sun. In full sun a vehicle that is cold inside by way of the aircon will heat up to 40 degrees within half an hour.
Ventilation 2. Vent the heat of the freezer cooling mechanism to the outside of the vehicle. This can go as far as installing a duct and a louvre to the outside.
Keep it cool. Run your aircon whenever practical.
Fridge and freezer. Install an insulated board one can diameter from the top and put your butter cheese etc there with your frosties. This will create an area that will be at a higher temperature. It will also insulate your all important frozen stuff from from any new loads you might be adding.
Adding an extra load. If you want to cool down your frosties in the same freezer, do it while you are driving. Do not overdo it. Three or four tins will be a heavy load for most off road freezers. If you overdo it you will steadily loose your freezer and the frozen food inside. One trick is to put the following days provisions, preferably in tins, on the vehicles roof to cool down overnight. The roof cools down overnight. It is higher and that will help with air circulation, it is also a good conductor of heat wich will also help to cool the cans down. It is too high for heyenas to reach. We tend to go to Botswana, Namibia etc in winter to escape some of the heat. In these desert areas the temperature drops to close to freezing every night in the winter months. This gets rid of a lot of heat that your freezer will not have to do. Drop it into the freezer first thing in the morning before it can start heating up again.
If you loose your freezer after a few days of over landing and your battery does not pick up again it is normally a matter of a dual charger that does not do the job properly.