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Battery Drain From Using 12V Socket.

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jackthehat | 14:19 Sat 18th Jan 2020 | Motoring
15 Answers
Mrs JtH and I are trying to get all our ducks in a row for our camping holidays this year.
We are considering buying an 'electric' coolbox.
The prices between one operating at 240V mains and one operating from the 12V 'cigarette-lighter' in the car are quite substantial.

One I have seen tells me
"Wattage is 40W
Draw in amps is approximately 3.5A from a 12V supply".

What sort of impact will this have on our car battery given that the car will be stationary and quiet overnight?

Thanks in advance. :o)

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I think I'm right in saying, Jack, that the average car battery has a capacity of around 50 AmpHours (Ah) Roughly, it means you could draw 3.5A for around 14 hours. Then, you have a dead battery. I doubt if that is nowhere near enough for camping. I know they're a whole lot more expensive, but possibly a better choice would be a 3-way power option (12v, 240v, and...
14:41 Sat 18th Jan 2020
No actual experience of running these things from a car but, are you leaving the car engine running overnight ? 3.5 Amp is a fair old amount. I think maybe you'd be unplugging and relying on the box's good thermal insulation.

I had a cooler once, worked on mains and battery. Was never up to chilling stuff but did manage to be a few degrees lower than ambient for items cooled down elsewhere.
Consider leaving the car interior lights on overnight. How is the car at starting next morning ? How many watts is a lamp ?
Question Author
Not leaving car running overnight, hence my explaining that it 'will be stationary and quiet'.

We would be more likely to unplug during the day when we are out and about and rely on the box keeping things cool whilst we are away.

As to your other questions.....I don't know, hence my asking here. :o)
I think I'm right in saying, Jack, that the average car battery has a capacity of around 50 AmpHours (Ah)

Roughly, it means you could draw 3.5A for around 14 hours. Then, you have a dead battery.

I doubt if that is nowhere near enough for camping.

I know they're a whole lot more expensive, but possibly a better choice would be a 3-way power option (12v, 240v, and gas.)
That way, you could use the 240v on a proper campsite, and gas for everywhere else. Maybe 12v for short stays.
And there was me thinking that your car had a quiet engine ;-)
^^^^ doubt if that anywhere near enough ^^
From memory, I think you'll find that the instructions will tell you to use it plugged in to the 12v. socket ONLY when the engine is running....the thermal insulation of the box keeps things cold, say, overnight.
Whilst your battery might have a capacity of 50AH, due to the very high current required to start the engine (600amps+) you can be struggling to start if the battery is more than about half discharged. There is no way I would run a cold box overnight.
Question Author
Thankyou, both.
That's rather what I thought TB. :o)

We usually try to get a pitch with a 240V hook-up so powering it oughtn't to be a problem, it was just the purchase price which caused us to pause.

Question Author
Thanks, bhg481 and gingejbee.

We usually rely on freezer packs and an insulated coolbox.
However, despite this, last year the milk and butter seemed to swap properties; we were able to pour the butter onto the bread and get the milk out of the container with a spoon... :o(
We used to do a lot of tent-camping down is South Carolina (where it's obviously quite hot), and all we used was a sturdy Coleman cooler and a bag of ice which lasted all day and overnight. We always left the ice in the bags so the food would not be in touch with any melted ice.
We actually had two coolers: one for the food-stuff, and one for my stock of Colt 45 Malt Liquor (a type of beer)...No way was I going to let that stuff get warm:)
Question Author
ooooo.......Colt 45!
It used to be my most favouritist-beer-in-a-can but I haven't seen it in the shops for many a year. :o(
If you are thinking of camping in fairly hot places (South of France, Spain etc) the only coldbox I would recommend is this one:

Amazon.co.uk User Recommendation

It's expensive but can be used with 12V or mains and will keep cool even in 30deg temperatures.
The tri-fuel ones are convenient but also expensive and do not cool as well as the Tropicool.
If you consider a Tropicool look carefully at the model details as there is at least two sizes and they also make a freezer version which, obviously, is more expensive but will actually freeze your food, which is not what you want.
A car's 12-volt battery stores electricity used to briefly turn the car's engine until it starts and runs. Then, the engine's power turns the alternator, which generates electricity to recharge the battery so it can start the car again next time. The battery and alternator are part of the car's electrical system, working together to power the headlights, radio, horn and other electrical devices.

Most cars require 12-volt batteries that last from three to five years. Therefore, you should know how old the battery is so you can replace it before it fails. Hopefully, you kept the receipt or noted the date when you bought your current battery. Read More about car jump starter: https://anewwayforward.org/best-portable-car-jump-starter/

You might notice that when you turn the ignition key, the engine cranks slowly. This could be a sign that your battery is about to die, Mazor says.

check first to see if the battery was drained because you left the lights on, or some other electrical device. Once the car is jump started, it could start, run and seem reliable.

When performing maintenance on a battery, work in a well-ventilated area and wear gloves and eye protection. Never lean over a battery when charging, testing or jump-starting an engine.

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