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Running cool box from my car battery

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I’ll be spending 9 days on a French campsite soon (Normandy - 75th anniversary D-Day celebrations) and have bought a good quality Cool Box (12V/220-240V) to run off my car battery. The only information I can glean is that at 12V it runs at 48W, so presumably it is 4 amps? The car battery (Exide, 44Ah) is about 6 months old and in excellent condition, starting the engine (Ford Fiesta) instantly.

I’ll run it on my domestic supply for 24 hours before departure, then drive perhaps 100 miles from the UK to the campsite. The beaches stretch over about 65 miles of coastline and there’ll be no problem whilst driving, but I expect to need to ration usage when stationary - any ideas how much? I have a basic multimeter - what readings would tell me I’m overstretching the car battery?
Normal car batteries are not designed to be significantly discharged in normal use. Running them down drastically shortens the battery life.

You want to restrict discharge possibly 20% at most from full, if you do not want to damage the battery. That only gives you a couple of hours use.

Ideally you would have a separate "leisure battery", which are designed for deep discharge cyclic use. That connects to the car via a split charge relay, so it charges when the car is running but does not allow any drain from the normal car battery.

**broken link removed**
You should also consider that it won't be running all the time (hopefully only a small percentage of the time), which makes it difficult to guess the battery life. However, as already suggested, it's not a good idea to run it off your car battery, with the hope it might still start afterwards.
Running your beer fridge off the car's starting battery is a really, really bad idea.
A small car starting battery has a capacity of about 50 to 80 Ah (that's 4A for 12.5h to 20h to completely kill the battery). You wouldn't be able to start the engine with more than 1/3 to 1/2 of that capacity gone.

Running the engine (say a couple of times a day) to recharge the battery is not realistic, either. At best, the car's charging system charges the battery at about 35A, provided that the engine runs at ~1500 rpm, so you would have to start the car and run it for an hour about twice a day just to replenish the charge and still be able to start the car the next time.

A large deep-cycle storage battery is expensive, and heavy. One of those, fully-charged, still only has a capacity of ~120Ah, so that will not do it.

I have gone on backcountry river trips (including running the Grand Canyon) that lasted 8days. The only way we could keep meat frozen until the six/seventh day was a very large ice chest (cooler) solidly packed with frozen food surrounded by dry-ice. You had to minimize the number of times you open the lid to less than twice per day, so this was not used to cool drinks. This ice chest was huge. It would take up the entire back seat of your Fiesta and it took two men to carry it...

There are two energy sources that are routinely used in the Recreational Vehicle Industry for this purpose, and duration. One is gasoline (to run a generator), and the other is propane or LPG (to run a gas fridge). Batteries do not even figure into this equation.
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I would return the cool box and either...
A) buy a high quality cooler that you can fill with ice.
B) learn to drink room-temp (warm?) beer
C) learn to drink wine while in France.

If you plan on idling your car in a French campsite to recharge your battery so your beer stays cold, you will be run off as they throw empty wine bottles at you and your noisy, stinky car disturbs the peace and serenity of those hallowed beaches.
I have a Peltier effect cooler that I keep in my truck in the summer, for transporting food back and forth to my cabin in the country. It has excellent insulation, so it keeps contents cool for a long time even without any power. Mine is at least 15 years old. So I expect that the newer ones work even better than mine. The important thing is to get everything cooled down properly to begin with. Add some freeze packs etc. Once everything is cold, it should stay cold unless you're constantly opening and closing it.

As the others have mentioned, I wouldn't recommend running it off the same battery that starts your vehicle, unless you only power it when the engine is running.
I found with mine there was no thermostat on 12v and it ran too cold, so I plugged it in for an hour and unplugged it for an hour during the trip, you have to remember but it works.
A car battery in considered full when around 12.7v and empty at around 12.2v.
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