Car battery question

I wonder if there's anyone who could tell me what is the minimum voltage to which a car battery can be discharged without any permanent damage, and still be capable of starting the engine?
The reason I'm asking is that I sometimes run a small portable fridge in my car which is fine while the engine is running but would obviously soon flatten the battery if it were not (it takes about 4A). This doesn't arise at the moment as I plug the fridge into the cigarette light socket on my present car, which is switched off with the ignition. However I'm thinking of changing the car to one which apparently has a 12V power socket in the boot which would obviously be much more convenient for the fridge, *but* it's permanently live.
I have a small device which is supposed to protect the battery against accidental discharge by switching off when the voltage drops below a "safe" level. I've checked it with a DVM though and it allows the voltage to drop to 10.6V before switching off. This seem to me much too low for safety - hence my question.
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Mike Lane
UK North Yorkshire
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Mike Lane wrote:

Measuring the terminal voltage of a lead acid battery is not a good way of assessing its remaining capacity. You would be far better off making a modification somewhere in the fusebox of the car to make the boot socket switch off with the ignition.
Bob
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Bob Minchin wrote on Jan 15, 2012:

I agree, but I'd rather not tinker with the wiring of a new car if it can be avoided. The voltage is being measured with a 4A load so I would have thought it's a reasonable indication of remaining capacity. I'm not after an exact figure here, but a reasonable safety level.
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Mike Lane
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wrote:

As Bob said, it isnt

NT
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4A at a vaguely guessed 50% duty cycle is 2A ave, which is 15hrs run time on a 30Ah battery, more on a bigger one. I'd be tempted to set a voltage cutoff of 13.6v, so the fridge would switch off whenever the ignition goes off, and use a couple of frozen tetrapaks to povide coolth storage.
NT
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The old Rover had a 'calcium' battery - a sort of no name one bought from the local car parts shop, but with a good guarantee. Not the cheapest one they had, though.
Due to some lengthy periods of no use and all the electrical toys in the car, the battery was too flat to start the car on a number of occasions.
I've just replaced it at 10 years old...
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Mike Lane wrote:

My suggestion in anotherpost would work entirely from the socket in the boot.
The voltage is being measured with a 4A load so I would have thought

It's a bit worrying if you have to start from cold, on a cold day. I wouldn't want to risk it.
I'm not after an exact

To be sure the car will always start? 12V
Bill
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Bob Minchin formulated the question :

Or get a voltage sensing relay, as used for a caravan towing socket which usually power a fridge in the caravan. They switch off when the voltage falls below the level where the engine/ alternator would normally be running.
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Regards,
Harry (M1BYT) (L)
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10.6V is quite common for battery protection. It's when you go below that voltage that you're in danger of reverse biasing the weakest cell, which will kill it. It's probably too low to expect the car to start (at least without the starting itself risking damaging the weakest cell).
One option would be to have a separate battery for the fridge, which is charged from the car, but when the engine isn't running, the fridge runs from the second battery only. There are controllers available for this which are sometimes used with caravans.
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Andrew Gabriel
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Andrew Gabriel wrote on Jan 15, 2012:

Agreed, but that would be way over the top for my purposes.
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Mike Lane
UK North Yorkshire
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Bit of a how long is a piece of string question. 10.8v is accepted as being the point to stop using a lead acid to prevent damage - but that could be for batteries designed for deep discharge which car ones ain't.

The capacity of the battery should be marked on it in amp.hr. This is measured at a 20 hour rate - so if an 80 amp.hr battery, would sustain a 4 amp load for 20 hours. Roughly. 80 amp hour is larger than most car batteries - smaller cars might have half that. And if you up the current beyond the 20 hour rate, it will have a lower than stated capacity.

Yes - the car won't start with a battery discharged to that level.
My guess is you don't want to take more than half the capacity out of the battery. So that might mean as little as 5 hours. And, of course, it won't re-charge fully on a short journey.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Dave Plowman (News) wrote on Jan 15, 2012:

That's what I thought.

Yes. So what I need to do is switch the thing off before that happens
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Mike Lane
UK North Yorkshire
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On Sun, 15 Jan 2012 10:18:26 +0000, Mike Lane

Run an ignition-controlled relay fed line to the socket.
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snipped-for-privacy@gmail.com wrote:

Is there some kind of wireless relay, that switches off if it doesn't see some sort of beacon after a while? Relay in the boot, transmitter in the dash lighter socket.
Theo
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On Sun, 15 Jan 2012 10:18:26 +0000

Something like this, maybe? http://www.maplin.co.uk/5-in-1-jumpstarter-with-digital-air-compressor-388125
--
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Mike Lane wrote:

Car batteries aren't really designed to be left even slightly discharged. The worse the discharge and the longer the time the more battery life is shortened. Car batteries are designed to provide a short duration very high current, then spend all the rest of their lives getting over it.

One option is to power the fridge from a small (20Ah) deep discharge* battery, charged from a voltage sensing relay. The latter connects the battery to the vehicle only when the alternator is working.
http://www.rawcomponents.co.uk/relays-holders/intelligent-voltage-sensing-relays/100-amp-12v-relay.html
The other option is to assume that the fridge will continue to maintain a safe temperature (this depends on what you use it for: cooked meat or beer) during periods during which the vehicle is stood. In that case just use the voltage sensing relay and forget the battery.

Shouldn't do that cruel thing to a car battery very often!
It's always very dodgy to use the battery that will have to start the vehicle for some purpose when the vehicle is stood. It always ends up with a call to the AA (or in the case of my daughters to me). My motorhome has a relay that totally isolates the starting battery when the ignition is off, so the starter battery can't be discharged even accidentally.

*designed to withstand frequent discharge cycles: used for golf buggies etc, Google: 20Ah + "deep discharge".
Bill
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I wrote on Jan 15, 2012:

[original post snipped]
Thanks everyone for all the replies and suggestions.
I agree there are various solutions involving additional stand-by batteries and so on, but what I possibly didn't make entirely clear, is that what I have at the moment is perfectly satisfactory under normal circumstances. All I need is to keep some perishables cool while on the move (usually on holiday). When I get to my destination there will either be a full size fridge I can use, or I can plug the portable one into the mains.
What I'm trying to guard against is the inevitable time when I accidentally leave the car parked for some hours (or even over-night!) forgetting that I've got the fridge running in the boot. The device I have (a cheapo Chinese-made gadget) should be ideal for this if it switched off at a higher voltage, but as most replies have said, 10.6V is just too low for a car battery.
I've since found that Waeco (the German firm who made my portable fridge) actually do a similar device described here: <http://tinyurl.com/7z9b4xj Since they specialise in mobile coolers it might work better.
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Mike Lane
UK North Yorkshire
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wrote:

When the starter is cranking, the battery should be at 10V (although may drop to 7 for peculiar conditions) and this usually means the battery is standing at 12.6V or over.
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In article

Think 9 volts or so is the lower limit in practice.
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*A bartender is just a pharmacist with a limited inventory *

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Remember the old ballast resistors on petrol car ignitions? They were introduced so that the ignition would temporarily work on 9v when the car was cranked, but 12v when running.
--
hugh

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