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.

--
Mike Lane
UK North Yorkshire
mike_lane at mac dot com

Mike Lane wrote:
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.

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

Bob Minchin wrote on Jan 15, 2012:

Mike Lane wrote:
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.

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

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.

--
Mike Lane
UK North Yorkshire
mike_lane at mac dot com

In article . com,
Mike Lane writes:
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.

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.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

In article . com,
Mike Lane wrote:
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?

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 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.

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.

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.

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.

--
*You're just jealous because the voices only talk to me *

Dave Plowman London SW
To e-mail, change noise into sound.

On Sun, 15 Jan 2012 10:41:05 +0000, Mike Lane wrote:

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.

You say new car, check what sort of battery technology it has in it.
Car batteries have never really liked deep discharge, sometimes one
deep discharge is enough to kill the battery... I wouldn't like to
say how some of these new silver(?) or calcium jobbies react to a
deep discharge.

10.6v does strike me as very, low 1.7v/cell not sure I'd like to go
below 2v/cell on a car battery. A battery specifically designed for
deep discharge (a "leisure" battery) might be able to take that
level.

Ideally you need to install a seperate battery to run your fridge and
split charging system to protect the engine battery from being deep
discharged and/or not having enough umph left to start the car. The
split charging may well mostly be there as it would be used to charge
the battery in a caravan via the 12S Auxillary towing connector.

--
Cheers
Dave.

On Jan 15, 10:41*am, Mike Lane
wrote:
Bob Minchin wrote on Jan 15, 2012:



Mike Lane wrote:
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.

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

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.

As Bob said, it isnt

I'm not after an exact
figure here, but a reasonable safety level.


NT

On Jan 15, 11:29*am, NT wrote:
On Jan 15, 10:41*am, Mike Lane
wrote:



Bob Minchin wrote on Jan 15, 2012:

Mike Lane wrote:
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.

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

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.

As Bob said, it isnt

I'm not after an exact
figure here, but a reasonable safety level.

NT

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

On Sun, 15 Jan 2012 10:18:26 +0000, Mike Lane
wrote:

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.

Run an ignition-controlled relay fed line to the socket.

In article o.uk,
Dave Liquorice wrote:
You say new car, check what sort of battery technology it has in it.
Car batteries have never really liked deep discharge, sometimes one
deep discharge is enough to kill the battery... I wouldn't like to
say how some of these new silver(?) or calcium jobbies react to a
deep discharge.

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...

--
*Just remember...if the world didn't suck, we'd all fall off*

Dave Plowman London SW
To e-mail, change noise into sound.

On Sun, 15 Jan 2012 10:18:26 +0000
Mike Lane wrote:

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.


Something like this, maybe?
http://www.maplin.co.uk/5-in-1-jumps...pressor-388125
--
Davey.

Mike Lane wrote:
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?
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.


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.
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/relay...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.


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.
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

Mike Lane wrote:

I agree, but I'd rather not tinker with the wiring of a new car if it can be
avoided.
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 reasonable indication of remaining capacity.
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
figure here, but a reasonable safety level.
To be sure the car will always start? 12V

Bill

Dave Plowman (News) wrote on Jan 15, 2012:

In article . com,
Mike Lane wrote:
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?

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 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.

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.

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.

Yes - the car won't start with a battery discharged to that level.

That's what I thought.

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.


Yes. So what I need to do is switch the thing off before that happens

--
Mike Lane
UK North Yorkshire
mike_lane at mac dot com

Andrew Gabriel wrote on Jan 15, 2012:

In article . com,
Mike Lane writes:
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.

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.


Agreed, but that would be way over the top for my purposes.

--
Mike Lane
UK North Yorkshire
mike_lane at mac dot com

I wrote on Jan 15, 2012:

I wonder if there's anyone who could tell me what is the minimum voltage to
which a car battery can be discharged...
[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 he
http://tinyurl.com/7z9b4xj
Since they specialise in mobile coolers it might work better.

--
Mike Lane
UK North Yorkshire
mike_lane at mac dot com

On Jan 15, 10:18*am, Mike Lane
wrote:
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.

--
Mike Lane
UK North Yorkshire
mike_lane at mac dot com

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.

In article
,
thirty-six 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.

Think 9 volts or so is the lower limit in practice.

--
*A bartender is just a pharmacist with a limited inventory *

Dave Plowman London SW
To e-mail, change noise into sound.

On Jan 15, 6:48*pm, "Dave Plowman (News)"
wrote:
In article
,
* *thirty-six 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.

Think 9 volts or so is the lower limit in practice.


As I see it, a batteryi s generally selected to work for at least 11
months of the year with an end point cranking voltage of 10V or above.

wrote:
Run an ignition-controlled relay fed line to the socket.

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

In message , Andrew Gabriel
writes
In article . com,
Mike Lane writes:
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.

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.

Voltage sensitive relays are used nowadays to simplify the wiring. The
op could put it in the boot to control the socket.
--
hugh

In message , "Dave Plowman (News)"
writes
In article
,
thirty-six 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.

Think 9 volts or so is the lower limit in practice.

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

Dave Plowman (News) wrote:
In article
,
thirty-six 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.

Think 9 volts or so is the lower limit in practice.

I suppose a lot depends on the internal resistance of the battery.

Bill

hugh wrote:
In message , "Dave Plowman (News)"
writes
In article
,
thirty-six 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.

Think 9 volts or so is the lower limit in practice.

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.
Oh yes I remember them. They used to fail occasionally.

In my old Bedford CA van I had a second battery and a big round red
switch that was off a yacht. The switch brought the second battery into
use when required. I could have battery A, battery B, or both at once.
This was so I could work at night with various lights on and yet still
be sure of the van starting. On a really cold morning I'd have both
batteries connected to start the van. It made a lot of difference.

I also remember a neighbour who started his 12VDC petrol engined lorry
by means of a second battery, but he connected it in series! It never
seemed to do any harm, and it certainly turned the engine over.

Also, who remembers

-- the business of building a fire under the diesel tank when it was
really cold?

-- winding the window down and using a Fairy liquid bottle full of water
to clean the windscreen, because screenwashers weren't fitted?

-- then, later, fitting a screenwasher kit so the MOT could be passed?

Leaving a paraffin lamp under the engine at night in the hope that it
would help the car to start? (it worked, but there was a risk...)

Hanging a paraffin road lamp on the driver's door handle when parked to
comply with the law? (as kids we used to steal them)

Manually changing the tyres round to even up the wear?

Running a mains lead across the pavement so the battery could be on
charge all night? (the short run to and from work with the lights on
wouldn't recharge the battery: the dynamo was rated at 22A and then only
when you were doing 60mph!)

Bill

In article ,
hugh ] wrote:
Think 9 volts or so is the lower limit in practice.

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.

Yup.

--
*If vegetable oil comes from vegetables, where does baby oil come from? *

Dave Plowman London SW
To e-mail, change noise into sound.

Bob Minchin formulated the question :
Mike Lane wrote:
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.

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

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.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

Bill Wright writes:

I also remember a neighbour who started his 12VDC petrol engined lorry
by means of a second battery, but he connected it in series! It never
seemed to do any harm, and it certainly turned the engine over.

It seems to be a piece of lore which is known to only a few
individuals, people who somehow have the skill to judge how far they
can push things without damage.

Also, who remembers

-- the business of building a fire under the diesel tank when it was
really cold?

-- winding the window down and using a Fairy liquid bottle full of water
to clean the windscreen, because screenwashers weren't fitted?

-- then, later, fitting a screenwasher kit so the MOT could be passed?

Leaving a paraffin lamp under the engine at night in the hope that it
would help the car to start? (it worked, but there was a risk...)

ISTR some lamps had a fine wire mesh around the flame to make it
unlikely that combustible vapour near the lamp could be ignited by the
lamp.

Hanging a paraffin road lamp on the driver's door handle when parked to
comply with the law? (as kids we used to steal them)

Manually changing the tyres round to even up the wear?

Running a mains lead across the pavement so the battery could be on
charge all night? (the short run to and from work with the lights on
wouldn't recharge the battery: the dynamo was rated at 22A and then only
when you were doing 60mph!)

During the last Edinburgh Festival, I saw a mains lead dangling from a
third-floor tenement window down to a Nissan Leaf parked (illegally) in
the street below.
Presumably he was far from home, and I suppose he had rented a flat
whose landlord would be in for a hefty electricity bill!

--
Windmill, Use t m i l l
J.R.R. Tolkien:- @ O n e t e l . c o m
All that is gold does not glister / Not all who wander are lost

In message , Bill Wright
writes
Dave Plowman (News) wrote:
In article
,
thirty-six 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.
Think 9 volts or so is the lower limit in practice.

I suppose a lot depends on the internal resistance of the battery.

Bill
State of battery, ambient temperature affecting battery performance,
also increasing cranking load due to thicker oil, etc. far too many
variables to give any meaningful estimate.
--
hugh

In article ,
hugh ] wrote:
I suppose a lot depends on the internal resistance of the battery.

Bill
State of battery, ambient temperature affecting battery performance,
also increasing cranking load due to thicker oil, etc. far too many
variables to give any meaningful estimate.

If the volts drop too far the ignition won't work. And probably much else
on a modern car.

--
*How do you tell when you run out of invisible ink? *

Dave Plowman London SW
To e-mail, change noise into sound.

On Jan 15, 6:57*pm, thirty-six wrote:
On Jan 15, 6:48*pm, "Dave Plowman (News)"
wrote:

In article
,
* *thirty-six 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.

Think 9 volts or so is the lower limit in practice.

As I see it, a *batteryi s generally selected to work for at least 11
months of the year with an end point cranking voltage of 10V or above.

Addendum: Yesterday was an exception. Car battery currently off
vehicle being charged.

In article ,
Windmill wrote:
I also remember a neighbour who started his 12VDC petrol engined lorry
by means of a second battery, but he connected it in series! It never
seemed to do any harm, and it certainly turned the engine over.

It seems to be a piece of lore which is known to only a few
individuals, people who somehow have the skill to judge how far they
can push things without damage.


Years ago, the RAC did just that to my Rover SD1 when the starter refused
to engage, even although the battery was fine. Sticking solenoid on the
pre-engage motor. I was horrified as it has a fair amount of electronics.
Including the ignition and injection. But it worked. And didn't appear to
do any damage. I'd not try it, though.

--
*My dog can lick anyone

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
Bill Wright wrote:
Running a mains lead across the pavement so the battery could be on
charge all night? (the short run to and from work with the lights on
wouldn't recharge the battery: the dynamo was rated at 22A and then only
when you were doing 60mph!)

I'm happy to do this at home if needed - no off street parking here. I put
a rubber matt over the cable. I have an outside socket by the front door
for just this. ;-)

--
*Half the people in the world are below average.

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:
In article ,
Bill Wright wrote:
Running a mains lead across the pavement so the battery could be on
charge all night? (the short run to and from work with the lights on
wouldn't recharge the battery: the dynamo was rated at 22A and then only
when you were doing 60mph!)

I'm happy to do this at home if needed - no off street parking here. I put
a rubber matt over the cable. I have an outside socket by the front door
for just this. ;-)

Round here any accessible source of mains power usually has a group of
teenagers around it charging up their devices.

Bill

On Jan 16, 11:46*pm, "Dave Plowman (News)"
wrote:
In article ,
* *hugh ] wrote:

I suppose a lot depends on the internal resistance of the battery.

Bill
State of *battery, ambient temperature affecting battery performance,
also increasing cranking load due to thicker oil, etc. far too many
variables to give any meaningful estimate.

If the volts drop too far the ignition won't work. And probably much else
on a modern car.


Usual problem is the solenoid dropping due to the starter motor
demand, the electronics should be working suuficiently to fire the
engine at any voltage that the starter will still turn the engine no
matter what the drag of the engine and gearbox. With 6V coils,
anything was possible, even an apparently dead battery. Used to just
pull the solenoid lead and push start while using the starting key
position and the engine would fire, usually after the first
compression.