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?

 

Welcome to AAC!

How long can you leave your car headlights on before your battery dies?
One lamp draws about 60W. That's about 120W for both headlights.

 

In theory, a 44Ah battery supplies 4A for 11 hours, BUT..... the battery is completely dead after that . In practice it would be better to err on the side of caution and assume the box really takes 5A and the battery capacity is only 20Ah, so that would give a run time of 4 hours.

 

... at one time, the guideline for standard automotive battery repetitive discharge was 20% of the rated AH capacity. One would assume that battery design and construction has improved after 20 or 30 years. However, a functional pair of battery jumper cables may be a useful investment.
... Alternately, you might rig up one of the Deep Discharge type of batteries, which would provide additional battery discharge capacity for your cooler. A knife switch could be used to allow the alternator to recharge one battery or the other. An RV camper supply store usually carries hardware of this nature.

 

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?

Do not power the Cool Box from the car battery with the engine off ... not even once.
Your car battery is designed for a quick discharge into the starter motor only and then a quick recharge
You car battery is NOT designed for a Deep Discharge.
Purchase a Deep Cycle battery for your deep discharging application.

 

As mentioned above, it will shorten the service life of a starting battery to use it in a deep cycle (run it way down) type of situation. The other thing to consider is when you start the car and the battery is very low, the alternator will be dumping tons of current for an extended period to charge it, and some alternators don't hold up well in that use case. For 9 days of camping, I would consider a different solution. I'm not sure your budget, but a very small generator like this one (can you rent one?) would be a better solution. That plus 5 gallons of gas might last you the whole 9 days, and run a fan at night to keep you cool! They are super quiet, just keep it far from your tent to avoid CO. Or a quality big cooler packed with ice will last a couple of days, so if there's ice relatively near by then you may be able to get away with refilling the ice every couple of days.

 

Buy ice!

 

Lots of good suggestions already given above.
Another is get solar panels and use that to charge your RV (deep cycle) battery.

 

Do not power the Cool Box from the car battery with the engine off ... not even once.
Your car battery is designed for a quick discharge into the starter motor only and then a quick recharge
You car battery is NOT designed for a Deep Discharge.
Purchase a Deep Cycle battery for your deep discharging application.

Ditto. This is what boaters do. You have one battery for starting the motor and this is what gets you back to the dock at the end of the day. Your second battery is for lights, music, fish-finders, whatever. Things you might leave running for hours.

If you're not set up this way, every minute the cooler is on will be a minute of stress for you as you wonder if you'll be able to start the motor. You don't need the stress.

 

Also, with a 2nd battery, and jumper cables, you can charge it with your car if the 2nd battery dies.

 

Thanks guys, lots of useful information & suggestions!

 

The big thing is indeed that the cool box is a constant draw of the rated amps. So at the least you will need to have jumper cables along. But getting that deep discharge battery is the best advice of what I have read. And you may be able to charge the RV battery right in parallel with the cars battery. No knife switch in a current car. NOT a good idea.

 

Do not power the Cool Box from the car battery with the engine off ... not even once.
Your car battery is designed for a quick discharge into the starter motor only and then a quick recharge
You car battery is NOT designed for a Deep Discharge.
Purchase a Deep Cycle battery for your deep discharging application.

I think that's a bit of an overstatement ... a 44 AHr standard lead acid should deliver just that 44AHr ..."deep discharge" is misleading jargon. It just means the battery has modifications allowing regular complete discharge while still having a good life. Since you are not doing this regularly (for the whole life of the battery) a standard battery will be fine if completely discharged a few times.... But you don't want to do that anyway since you need to deliver 100's Amps to start the engine.

I would err on the safe side , you don't want to be stranded , unable to start the engine! ...monitor the voltage and find out the minimum required to start your engine.

I would run the cooler whenever the engine is running + a few hours ... then unplug the cooler and cover it with insulation ...duvets etc , this should keep the contents cool overnight ...

Again this a good reason to have a super capacitor across your battery , a good one of these should start the engine , and it can be charged from the smallest sized , almost dead 12V battery

 

I think that's a bit of an overstatement ... a 44 AHr standard lead acid should deliver just that 44AHr ..."deep discharge" is misleading jargon. It just means the battery has modifications allowing regular complete discharge , since you are not doing this regularly (for the whole life of the battery) a standard battery will be fine if completely discharged a few times.... But you don't want to do that anyway since you need to deliver 100's Amps to start the engine.

But I would err on the safe side , you don't want to be stranded , unable to start the engine! ...monitor the voltage and find out the minimum required to start your engine.

I would run the cooler whenever the engine is running + a few hours ... then unplug the cooler and cover it with insulation ...duvets etc , this should keep the contents cool overnight ...

Again this a good reason to have a super capacitor , a good one of these should start the engine , and it can be charged from the smallest sized , almost dead 12V battery

A "standard battery" is not designed for any deep cycle discharges.
"Standard Batteries" are made of sponge lead and will start shedding lead from the plates upon the first and each deep cycle discharge.
Once the sponge lead sheds from the plates, it falls to the bottom of the battery, that capacity is lost forever and the battery's life is shortened.
Telling someone to use their Starting Battery, as a Deep Cycle Battery, is bad advice.

"Deep Cycle" batteries are made of paste lead and they are designed for deep discharging.
Sponge Lead and Paste Lead are physically different, not just "jargon".
It is recommended that each battery be used for its intended application.

 

A "standard battery" is not designed for any deep cycle discharges.
.

You can deep cycle standard lead acid (use 100% capacity) ...but it will only give a life of 13 cycles .... using 50% capacity will give 110 cycles .... using 30% of capacity gives a life of 120 cycles .... https://batteryuniversity.com/learn/article/lead_based_batteries

So it would not be a calamity if TS did fully exhaust his battery once or twice a year when on holiday ... But I was not even suggesting that ... he will need to start his engine , which requires 100s of Amps , not easy if the battery went below about 10% capacity.

This thread has highlighted what rubbish tech lead acid is ... A battery listed as 45AHrs is nothing of the sort (unless you want to throw it away after 13 cycles)...It's really just 22AHr , but even then you only get 110 cycles ... Deep cycle batteries are better but much heavier and expensive and still give less than 200 cycles if deep cycled ...

Compare that to Lithium batteries which are 6 times lighter than deep cycle lead acid for same WHrs .....and give 400 cycles if "deep cycled" , (charged to 4.2V) ......... If cycled using 65% of capacity (charged to 3.93V/cell) ..4,000 cycles are achieved.

That heavy lead car battery consumes petrol every time you accelerate ...and more petrol to keep it moving , I'm sure it makes no economic sense to use them!

A small lithium battery ( charged only to 3,93V/cell) with a super capacitor must be best.

 

Those Deep Discharge Cycle numbers reported by Battery University came from the manufacturers.
Those manufacturers test their batteries under highly optimized Laboratory Conditions, in a temperature controlled water bath.
They quickly deep discharge the battery down to 50% and then they immediately recharge the battery to 100%, let it rest, and then repeat.
The manufacturers can reduce the "lead shedding" to an absolute minimum, we can not.
This "trick" significantly inflates the number of Deep Discharge Cycles that they can report.

Th number of Deep Cycles reported by every manufacturer is labelled "For Comparison Purposes Only".
In other words. "As tested by the mfr, their Battery #1, lasted twice as long as their Battery #2".
But the number of Deep Cycles reported in the Lab Tests is not the number of actual Deep Cycles that anyone will ever get in the Real World.
And you cannot compare "Number of Deep Cycle" between manufacturers, as each manufacturer's test is slightly different ( ie they all try to cheat )
Call the Battery Manufacturer and ask them, "How many deep discharge cycles can I get in the Real World".
It will not be the number reported on any chart.

Using "actual" Deep Discharge Cycles numbers from a generic chart is quite meaningless in the real world = a fantasy

A typical "turn-of-the-key" consumes only 2% of the Starting Battery capacity and then the battery is immediately recharged.
Just one (1) slow 50% Deep Cycle Discharge is much, much worse than twenty-five @ 2% Starting Cycles.
And twenty-five (25) Starting Cycles is about 2 1/2 weeks worth of battery wear-and-tear

After 9 days of camping at 50% Depth of Discharge = only 4 hours of run-time per day ...
You will permanently reduce your car battery's capacity and will shorten the battery life by, at least 5 1/2 months.

After 9 days of camping at 100% Depth of Discharge = only 8 hours of run-time per day ...
You will severely damage your car battery and the battery will need replaced before the first cold day in the fall.
Now, how do you restart your car in the morning? A Jump?

The voltages listed in the ( AGM ? ) Battery State-Of-Charge chart are ...
"The Voltage after resting the battery no-load for 12 hours", not the battery voltage when Under-Load.

A quote from the Battery University's web page
...
Starter and Deep-Cycle Batteries
The starter battery is designed to crank an engine with a momentary high-power load lasting a second, or so.
For its size, the battery is able to deliver high current, but it cannot be deep-cycled.
...

Actually, a Starter Battery can be Deep Discharged, ( you can do anything you want ) but you will pay a huge price, maybe even destroy the battery.

Portable Power ...
a) An SLA-AGM Deep Cycle Battery = cheapest per watt, but heavy
b) Lithium Battery / Jump Start = expensive per watt, but lightweight
c) Super Capacitor
d) 50 watt Solar Panel Charger + Small Battery pack for clouds
e) A small Inverter-Type "quiet" Gasoline Generator
use anything except your car's starting battery.

When running the car's engine, preferably above idle, you can use the car's alternator as a recharging station during the daytime.

 

That's very comprehensive and very informative - pretty startling too - thanks for taking the time to put this together!