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'[OT] jumpstarting takes a few minutes of "charging'
2012\05\15@145438 by Eric Kort

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So, this morning I jump started our van (kids wanted to finish
listening to a story on the CD player when we got home last night, and
a person who shall rename nameless forgot to go back out later and
take the keys out of the ignition which was then left in "Acc"
position all night).  This brought to mind a question I have long had
about jump starting.  The battery in my car starts my car immediately.
But when I connect that same battery to another car (the one I am
jump starting), that car will not start immediately...I need to wait
several minutes with the cables connected until it will start.

My question: why is that?  I will share some hypotheses I have, mostly
for their entertainment value:

1. The resistance of the cables is too high to deliver the needed
current to start the dead vehicle.  (It seems if that were true,
someone would be out there selling uber-cables that don't have this
problem, but I don't recall seeing such cables.)

2. The dead battery is a low impedance current sink while it is
charging, sucking away the available current from the higher impedance
starter until it is charged enough to start the car on its own.

And while we are on it, why does the owner's manual suggest connecting
the negative end of the cable to the engine block of the dead vehicle
(doesn't the engine have continuity with the negative terminal on the
battery--making it simpler from a procedure point of view to just
connect the battery terminals on both vehicles)?

Thanks,
Eri

2012\05\15@150919 by Herbert Graf

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On Tue, 2012-05-15 at 14:54 -0400, Eric Kort wrote:
> So, this morning I jump started our van (kids wanted to finish
> listening to a story on the CD player when we got home last night, and
> a person who shall rename nameless forgot to go back out later and
> take the keys out of the ignition which was then left in "Acc"
> position all night).  This brought to mind a question I have long had
> about jump starting.  The battery in my car starts my car immediately.
>  But when I connect that same battery to another car (the one I am
> jump starting), that car will not start immediately...I need to wait
> several minutes with the cables connected until it will start.
>
> My question: why is that?  I will share some hypotheses I have, mostly
> for their entertainment value:

FWIW I came back from a 3 week trip to a car with a completely dead
battery (nothing worked).

Driving stick, I've always assumed I could push start a dead car. Turns
out, nope. If the battery is completely dead, the computer has no juice
to fire injectors or spark plugs.

I wonder if that may have something to do with it? Was the other car
completely dead, or just dead enough that the starter wouldn't start the
engine?


> 1. The resistance of the cables is too high to deliver the needed
> current to start the dead vehicle.  (It seems if that were true,
> someone would be out there selling uber-cables that don't have this
> problem, but I don't recall seeing such cables.)
>
> 2. The dead battery is a low impedance current sink while it is
> charging, sucking away the available current from the higher impedance
> starter until it is charged enough to start the car on its own.

I'm guessing this might be a cause. Perhaps for the first short while
the dead battery is sucking up enough current that when you add the
starter current it isn't enough to start the car.

Common thick I've used to get around this issue is to rev the good cars
engine. That raises the voltage of the good car a bit and is often
enough to get the starter in the other car moving.

> And while we are on it, why does the owner's manual suggest connecting
> the negative end of the cable to the engine block of the dead vehicle
> (doesn't the engine have continuity with the negative terminal on the
> battery--making it simpler from a procedure point of view to just
> connect the battery terminals on both vehicles)?

The starter is generally bolted to the engine/transmission, and gets
it's ground from there. Being a huge chunk of metal, it's probably going
to give you a little more margin when starting the dead car vs. the
small run of thick cable coming from the battery.

TTYL

I think it's simply a "prevent people from doing stupid things" thing.

2012\05\15@152116 by Chris Pearson

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"And while we are on it, why does the owner's manual suggest connecting the
negative end of the cable to the engine block of the dead vehicle...?"

I've always been told that is done to avoid a spark occurring near the
battery and it's potentially explosive charge/discharge gasses.

Excerpt from the wiki: "The amount of overcharging is usually very small
and generates little hydrogen, which dissipates quickly. However, when
'jumping' a car battery, the high current can cause the rapid release of
large volumes of hydrogen, which can be ignited explosively by a nearby
spark, for example, when disconnecting a jumper cable."

- ChrisP


On Tue, May 15, 2012 at 11:54 AM, Eric Kort <spam_OUTeric.kortTakeThisOuTspamexsilico.com> wrote:

{Quote hidden}

>

2012\05\15@160535 by Denny Esterline

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On Tue, May 15, 2012 at 11:54 AM, Eric Kort <.....eric.kortKILLspamspam@spam@exsilico.com> wrote:

{Quote hidden}

Yes, yes and yes.
Common consumer grade jumper cables are not capable of delivering starting
current (in the 100's of amps)
The "dead" battery is already drawing significant current trying to charge.
Commercial/industrial/agricultural grade cables are available - but not
at Pep boys/autozone.
Often they will use cables size 0000 (I've seen larger), they can be
significantly more expensive than a new battery.

"Make the last connection to the engine block" is intended to keep sparks
away from the battery. Batteries can
(and do) produce hydrogen gas and sparks can (and do) make for a very
"interesting" day.

-Denn

2012\05\15@161749 by Richard Prosser

picon face
In my experience, jumper cables and their connections often have too
high resistance to actually start a car. (Professional models excepted
- I'm talking about the cheap ones I always end up using.) Provided
the dead battery has some capacity to hold charge, then connecting the
cables and revving the engine of the good car will pump some charge
back into the dead battery. This is normally enough to get things
turning over. A few months ago I tried to start a car with a
completely dead (sulphated) battery that wouldn't accept charge and my
cables/connectors just weren't good enough.

Connection to the engine block rather than the battery is often
easier, reduces resistance very slightly and move the sparks further
away from the battery - although why a flat battery would be gassing
is questionable. I guess it's more important at the "good" car end of
things

RP

On 16 May 2012 07:21, Chris Pearson <christopherpatrickpearsonspamKILLspamgmail.com> wrote:
{Quote hidden}

>> -

2012\05\15@170752 by Robert Rolf

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"Interesting" is an understatement.

http://www.vinland.com/car-battery.html

https://www.google.ca/search?q=car+battery+explosion&hl=en<https://www.google.ca/search?q=car+battery+explosion&hl=en&safe=off&prmd=imvns&tbm=isch&tbo=u&source=univ&sa=X&ei=GsWyT-S2EqqdiQKGvLS0BA&sqi=2&ved=0CHcQsAQ&biw=1458&bih=911

2012\05\15@175247 by Bob Blick

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It happened to me once back in the late 1970's. For a few days leading
up to the "rapid self-disassembly", in the mornings the starter wouldn't
engage for a second or two. I'd hold the key in the start position and
there would be nothing, then it would start normally. I checked the
connections to the battery and they were good. I thought the starter or
the keyswitch were going bad. On the last day it went kaboom and looked
like the pictures at the link, except it was a sizzling and steamy mess.
The battery might have had an internal bad connection, who knows. I
washed out the engine compartment with water and put in a new battery.
BTW, very similar looking battery, VW OEM in my situation.

Friendly regards,

Bob

On Tue, May 15, 2012, at 03:07 PM, Robert Rolf wrote:
> "Interesting" is an understatement.
>
> http://www.vinland.com/car-battery.html

-- http://www.fastmail.fm - A fast, anti-spam email service.

2012\05\15@201518 by Dwayne Reid

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At 12:54 PM 5/15/2012, Eric Kort wrote:

>1. The resistance of the cables is too high to deliver the needed
>current to start the dead vehicle.  (It seems if that were true,
>someone would be out there selling uber-cables that don't have this
>problem, but I don't recall seeing such cables.)

Yep.  The starter needs significant current to operate.


>2. The dead battery is a low impedance current sink while it is
>charging, sucking away the available current from the higher impedance
>starter until it is charged enough to start the car on its own.

Also Yep.  A discharged Lead-Acid battery that is otherwise in good shape (not sulfated) has an extremely low impedance and will consume just about as much current as you can give it.  That causes significant voltage drop in the jumper cables, which contributes to (1) above.


>And while we are on it, why does the owner's manual suggest connecting
>the negative end of the cable to the engine block of the dead vehicle
>(doesn't the engine have continuity with the negative terminal on the
>battery--making it simpler from a procedure point of view to just
>connect the battery terminals on both vehicles)?

A battery that is being charged at a high rate gives off Hydrogen gas.  You want to keep sparks as far away as possible from that gas.


I had an unfortunate incident during High School where I was in close proximity to a rather large car battery that was being charged.  The charging lead became dislodged somehow and the battery promptly exploded.  Luckily, the lab was in fairly close proximity to the Gym showers and I was quickly dragged to where I could be rinsed off.  No damage done, other than to clothing.  Large holes did appear <grin>.

dwayne

-- Dwayne Reid   <EraseMEdwaynerspam_OUTspamTakeThisOuTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2012\05\15@203231 by Lyle Hazelwood

picon face
Many "consumer grade" jumper cables have reduced copper and added
extra insulation to make the cables look as thick as better cables.
Long ago I invested in a long, very heavy set of cables, and a decent
zipper bag to store them in. Never regretted it, and the bag has done
it's job by protecting the cables between uses. The only other item in
the bag is a pair of safety glasses, just in case.
They have never failed to start a car immediately after connecting.
And I do insist that anyone without eye protection takes a few steps
back. ;)


On 5/16/12, Dwayne Reid <dwaynerspamspam_OUTplanet.eon.net> wrote:
{Quote hidden}

>

2012\05\16@075422 by Lee Jones

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> this morning I jump started our van [...] This brought to mind a
> question I have long had about jump starting. The battery in my
> car starts my car immediately. But when I connect that same battery
> to another car (the one I am jump starting), that car will not start
> immediately... I need to wait several minutes
>
> 1. The resistance of the cables is too high to deliver the needed
> current to start the dead vehicle.

It's the resistance of the cables, the resistance of the clamps,
the cleanliness of the battery posts, and the comparative size
of the batteries & supply vehicle charger & taret vehicle starter.

I carry cables made with size 0(?, same size as my welder's ground
wire) copper wire & top-notch clamps.  Clamps have strong springs.
When I put the clamps on -- both ends -- I wiggle them around to
ensure that the surface oxidation is broken and there is good
contact (to battery terminal or to heavy frame member).

With my old V8 Surburan supplying current, I can immediately crank
any new politically-correct car. If target vehicle won't crank, it
is a poor connection at the clamp ends.

> (It seems if that were true, someone would be out there selling
> uber-cables that don't have this problem, but I don't recall
> seeing such cables.)

They are.  Look at car parts stores that cater to professional
vehical repair stations.  That means not Autozone, not Pep Boys,
not O'Reilly/Kragen.  However, you may be able to special order
decent jumper cables from them.  Remember, if you want quality,
you have to pay for it.

You can also get raw cable & clamps for jumpe rsets from welding
stores, aviation supplies, industrial supplies, etc.

> And while we are on it, why does the owner's manual suggest connecting
> the negative end of the cable to the engine block of the dead vehicle
> (doesn't the engine have continuity with the negative terminal on the
> battery -- making it simpler from a procedure point of view to just
> connect the battery terminals on both vehicles)?

As others have mentioned, there's the issue of vented hydrogen.

There's also the issue of the cable resistance from the target
vehicle's battery negative terminal to the engine block (and
thence to the starter).  When trying to jump start, it helps
to minimize losses everywhere you can.

                                               Lee Jone

2012\05\16@094332 by Spehro Pefhany

picon face
At 02:54 PM 15/05/2012, you wrote:
>So, this morning I jump started our van (kids wanted to finish
>listening to a story on the CD player when we got home last night, and
>a person who shall rename nameless forgot to go back out later and
>take the keys out of the ignition which was then left in "Acc"
>position all night).  This brought to mind a question I have long had
>about jump starting.  The battery in my car starts my car immediately.
>  But when I connect that same battery to another car (the one I am
>jump starting), that car will not start immediately...I need to wait
>several minutes with the cables connected until it will start.
>
>My question: why is that?  I will share some hypotheses I have, mostly
>for their entertainment value:
>
>1. The resistance of the cables is too high to deliver the needed
>current to start the dead vehicle.  (It seems if that were true,
>someone would be out there selling uber-cables that don't have this
>problem, but I don't recall seeing such cables.)

Maybe you didn't look? Cheap cables have the characteristic you
mention because they are too light to deliver the starting current
without a large voltage drop.

I've seen ones as light as AWG8 or even AWG10!.

Good ones are AWG4, 2 or even AWG 1. Naturally, the price tends to
go up with the gauge, because there is more copper (and also the
clips tend to be better made). If you want them long, of course
the gauge has to be larger again for the resistance.

There's almost an order of magnitude difference in milliohms per
foot between the worst and the best. That's WAY more than the
difference between "starts instantly" and "not a chance".

Here's one:
http://www.amazon.com/Coleman-Cable-08862-25-Foot-Ultra-Heavy-Duty/dp/B000PJD7HY/ref=sr_1_4?ie=UTF8&qid=1337175552&sr=8-4

Something like this is a more reasonable trade-off to keep in all
the cars:
http://www.amazon.com/True-Power-Booster-Jumper-Cable/dp/B002RJK3Q8/ref=pd_sbs_misc_6


BTW, in the really old days, we used to move the cars so that
the bumpers contacted and it would bypass half the resistance
of the jumper cable (you could then double up on the cables on
the + terminal if you wanted). Back when bumpers were universally
exposed chrome-plated metal.


>2. The dead battery is a low impedance current sink while it is
>charging, sucking away the available current from the higher impedance
>starter until it is charged enough to start the car on its own.
>
>And while we are on it, why does the owner's manual suggest connecting
>the negative end of the cable to the engine block of the dead vehicle
>(doesn't the engine have continuity with the negative terminal on the
>battery--making it simpler from a procedure point of view to just
>connect the battery terminals on both vehicles)?

You're adding the chunk of wire between the battery and chassis.
Better to go around that. The starter motor has one wire going
to it, and the ground return is the chassis.

2012\05\16@184538 by Mark Hanchey

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On 5/15/2012 2:54 PM, Eric Kort wrote:
> So, this morning I jump started our van (kids wanted to finish
> listening to a story on the CD player when we got home last night, and
> a person who shall rename nameless forgot to go back out later and
> take the keys out of the ignition which was then left in "Acc"
> position all night).  This brought to mind a question I have long had
> about jump starting.  The battery in my car starts my car immediately.
>   But when I connect that same battery to another car (the one I am
> jump starting), that car will not start immediately...I need to wait
> several minutes with the cables connected until it will start.
>
> My question: why is that?  I will share some hypotheses I have, mostly
> for their entertainment value:
>
> 1. The resistance of the cables is too high to deliver the needed
> current to start the dead vehicle.  (It seems if that were true,
> someone would be out there selling uber-cables that don't have this
> problem, but I don't recall seeing such cables.)
It isn't so much the resistance of the cables as it is the poor connections at each end. Measure the surface area contacting both ends and compare it to the size of the cable, if it isn't at least the same amount of surface area then it doesn't matter how large the cable in between , it is like connecting a 100 ohm resistor to a 2 inch thick copper cable, the limit will still be the resistor and increasing cable size isn't going to change that.

> And while we are on it, why does the owner's manual suggest connecting
> the negative end of the cable to the engine block of the dead vehicle
> (doesn't the engine have continuity with the negative terminal on the
> battery--making it simpler from a procedure point of view to just
> connect the battery terminals on both vehicles)?
>
> Thanks,
> Eric
This is all about safety.  Hydrogen gas can be an issue but the real reason for not placing both cables on the battery is because the  clamps easily come loose from the terminals. If the two ends touch you now have an  arc  of current equal to that of an arc welder  with the clamps bouncing around from the arcs and the user trying to stop the shorting out,  plastics, wires, and anything near by can easily catch fire from the high temps the arc creates. I have seen what happened when a mechanic dropped a wrench and it fell across battery terminals, not a pretty sight. Most other surfaces are painted and don't pose as much a threat from the positive contacting the frame.

Mark

2012\05\16@193146 by Bob Ammerman

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----- Original Message ----- From: "Mark Hanchey" <KILLspammarkKILLspamspampixeltrickery.com>
To: <RemoveMEpiclistTakeThisOuTspammit.edu>
Sent: Wednesday, May 16, 2012 6:45 PM
Subject: Re: [OT] jumpstarting takes a few minutes of "charging"?


{Quote hidden}

This is not true. The resistance of the cable is in series with that of the connections and thus it is the total of the two that controls the current flow.

-- Bob Ammerman
RAm Systems

2012\05\16@205526 by Sean Breheny

face picon face
I agree, Bob. The resistance of good clamps on fairly clean metal is
going to be maybe 1 or 2 milliohms per connection. This would be 8
milliohms max for the entire circuit. 4 AWG wire is about 0.25
milliohms per foot so if the set of cables is 10 feet long, the entire
path length is 20 feet or 5 milliohms for the wire. The total path
resistance is then 13 milliohms. If you went down to 0 AWG wire, the
resistance of the wire would be 0.1 milliohms per foot so the total
contribution from the wire would be 2 milliohms making the entire path
10 milliohms which is significantly less. Going to even larger wire
would let you approach 8 milliohms total. Actually, 8 milliohms for
the clamps is probably higher than the real value anyway, which makes
the cable even more important.

I think that people are not giving enough consideration to the current
it takes to charge the dead battery. Assuming that it is merely low
and not damaged permanently, it will probably draw 200 or 300 Amps
when connected to the other car. This is higher than the starter
current so it would likely be the more dominant factor in producing a
large voltage drop across the cable and preventing you from starting
the car right away. I wouldn't think that revving the engine on the
"donor" car would make all that much difference since the alternator
is probably not capable of supplying more than 100 Amps.

Sean


On Wed, May 16, 2012 at 7:31 PM, Bob Ammerman <spamBeGonepicramspamBeGonespamroadrunner.com> wrote:
>
> {Original Message removed}

2012\05\17@060224 by Jake Anderson

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I often wonder about the clamps on jumper cables,
They all seem to just have a small number of contact surfaces, I often wonder if something like a half inch wide copper braid under the clamp would do the job better than just the jaws.
sure they are good at biting through the oxide but still your only going to have a few square mm at best.

Most modern cars have teensy wires from the engine to the battery, just big enough for the starter to turn over, I wouldn't be surprised if there was 1-2 volts of drop in the loom on a modern car.
(older cars are generally better but no guarantee, I've improved the starting of many cars by running new ground and a dedicated power wire, from alternator to battery and starter)
So going to the block can be quite helpful.

Another tip I have used more than once is when somebody has toy cables just rip the wire out of the clamp (its generally not hard, you can do it by hand) then wrap the wires around the terminals/connections
much better connection.

Revving the engine will let the alternator supply more current (up to its limit) as a rule. Many cars at idle only just make enough power to run the car itself not even charge the battery.
(partly because sucking a 12KW from an engine at idle will often make it stall as its 2x the load of the AC (12v @100A))
High reving engines are worse for it, at idle many of them can't make enough power to keep the engine running so a dead battery means the car won't idle.

Leaving them connected for a while will help charge the flat battery somewhat which will also help the car run if the battery is really flat.
One thing i have done on older (no computer) cars and motor bikes was to disconnect the dead battery
connect the jumper to the battery terminal on the dead target, start the thing using only the donor vehicle, then connect it to the battery on the target car.
leave the donor connected at a fast idle (highway RPM) for a few minutes and make sure the target battery hasn't gone boom or got any hot spots and the client car is able to run at idle.
(I use the donor car at a fast idle because I don't want its battery dying, and the target engine is often having trouble making a spark in the first place at whatever pathetic voltage its got to run the coil off)

Not that I've jumped many dodgy old cars ;->

I'm thinking about putting some decent terminals on my car to make life easier in the future

On 17/05/12 10:55, Sean Breheny wrote:
{Quote hidden}

>> {Original Message removed}

2012\05\17@073058 by Mark Hanchey

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On 5/16/2012 8:55 PM, Sean Breheny wrote:
> I agree, Bob. The resistance of good clamps on fairly clean metal is
> going to be maybe 1 or 2 milliohms per connection. This would be 8
> milliohms max for the entire circuit.

The resistance is not the problem, it is the small surface area making the connection on both ends. The clamps on most cables have very small contact area with the terminals. It would not surprise me if the contact area was equal to that of a 10AWG wire. The resistance may read as 0 but that will change when the metal begins to heat up from a lack of being able to handle the high currents. It is like trying to power a home using a needle at the end of the service wire and expecting all 200 amps to flow across the needles contact point.

Mark

2012\05\17@073530 by Mark Hanchey

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On 5/17/2012 6:02 AM, Jake Anderson wrote:
{Quote hidden}

I have seen that done too.  On older cars I have seen people loosen the terminals on the battery and put the wire inside  and re-tighten .    I think that since jumper cables don't get used often most people just buy whatever the cheapest ones are and just put up with the problems they may cause .

Mark

2012\05\17@082015 by Mark Hanchey

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I was curious if someone had designed a better clamp. Looks like someone has, seems a bit overkill for most , but I have to admit, it is one hell of a design.
http://www.freepatentsonline.com/5030106.pdf

Mark

2012\05\17@084604 by Sean Breheny

face picon face
Hi Mark,

What you are talking about is all part of what goes into the final
effective resistance. The 1 to 2 milliohm figure is an educated guess
but it is roughly what I think you will see even after the heating
effect that you are talking about.

What goes on at the interface between two pieces of metal in an
electrical contact is very complex and to some degree, at the
microscopic level, heat can actually help make the connection better
by microscopic melting. I'm sure that the total contact area is
actually much less than the cross-section of a 10 AWG wire but because
the path length is so short, the total contribution to resistance (or
voltage drop) is only moderate.

Sean


On Thu, May 17, 2012 at 7:31 AM, Mark Hanchey <RemoveMEmarkspamTakeThisOuTpixeltrickery.com> wrote:
{Quote hidden}

>

2012\05\17@142125 by Spehro Pefhany
picon face
At 08:55 PM 16/05/2012, you wrote:
{Quote hidden}

The current taken by the charging battery isn't important, because
the net flow is OUT of the 'dead' battery when the engine is being
cranked. Unless the 'dead' battery is internally knackered (shorted
cell etc), it will have an OPEN CIRCUIT voltage that is more than 10V.
It's more the internal resistance that goes up as the battery discharges.

I suppose a _really_ cr*ppy cable will get significantly hot from the
charging current and its resistance will thus increase. A 25°C rise would
result in about 10% less current being available.

Experience says that revving the engine does help (a bit). It supplies a
bit of extra current and raises the voltage. It's also hard on the
alternator rectifiers.

I have a portable battery that has relatively short heavy cables on it
(similar to this one, but with LEDs in the alligators)
www.amazon.com/Clore-Automotive-JNC660-12-Volt-Starter/dp/B000JFJLP6/ref=acc_glance_e_ai_ps_t_2
It works amazingly well (short AWG 4 cables).
(downside is that it takes overnight to charge, so if you are caught
with it discharged and a dead battery, and have to leave _now_, you're
scuppered.


Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speffEraseMEspam.....interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Please help out: http://dailynews.yahoo.com/fc/US/Emergency_Information/

2012\05\18@135240 by alan.b.pearce

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> I've seen ones as light as AWG8 or even AWG10!.

I am aware of a case where my mate reckoned he got a bulldozer started on a building site using "figure 8" flex. It was never clear to me if he ran 2 lengths of flex, or used the two wire as supply and return. I suspect he actually ended up getting enough charge into the dead battery to start the machine, rather than supply enough current through the wire !!

> BTW, in the really old days, we used to move the cars so that the
> bumpers contacted and it would bypass half the resistance of the jumper
> cable (you could then double up on the cables on the + terminal if you
> wanted). Back when bumpers were universally exposed chrome-plated
> metal.

How many times did you leave marks on the chrome when the bumpers sparked ??


-- Scanned by iCritical.

2012\05\18@184332 by Sean Breheny

face picon face
On Thu, May 17, 2012 at 2:21 PM, Spehro Pefhany <EraseMEspeffspaminterlog.com> wrote:
> The current taken by the charging battery isn't important, because
> the net flow is OUT of the 'dead' battery when the engine is being
> cranked. Unless the 'dead' battery is internally knackered (shorted
> cell etc), it will have an OPEN CIRCUIT voltage that is more than 10V.
> It's more the internal resistance that goes up as the battery discharges.
>

I don't think this is always the case. You are assuming that the
cranking current will be much greater than the battery charge current.
A healthy but deeply-discharged lead acid battery (at, say, 10V open
circuit as you say), may well draw 300 Amps when connected to a stiff
13V voltage source. This is more than the starter is likely to draw. I
think that the dead battery may significantly increase the voltage
drop across the jumper cables compared to just the starter itself.

Sea

2012\05\18@212415 by John Gardner

picon face
....the dead battery may significantly increase the voltage
drop across the jumper cables compared to just the starter itself.

My VAT-40 left me with the same impression.

Jac

2012\05\18@215641 by Dave Tweed

face
flavicon
face
Sean Breheny wrote:
> On Thu, May 17, 2012 at 2:21 PM, Spehro Pefhany <RemoveMEspeffEraseMEspamEraseMEinterlog.com> wrote:
> > The current taken by the charging battery isn't important, because
> > the net flow is OUT of the 'dead' battery when the engine is being
> > cranked. Unless the 'dead' battery is internally knackered (shorted
> > cell etc), it will have an OPEN CIRCUIT voltage that is more than 10V.
> > It's more the internal resistance that goes up as the battery discharges.
>
> I don't think this is always the case. You are assuming that the
> cranking current will be much greater than the battery charge current.

No, you've missed Spehro's point: The drop in the cables due to the cranking
current will cause the voltage at the dead battery to drop well below its
open-circuit voltage, so it will not be charging at all. In other words,
the combination of the rescue vehicle plus the jumper cables is anything
BUT "a stiff 13V voltage source", as you put it.

-- Dave Twee

2012\05\19@012620 by Sean Breheny

face picon face
I think we are really all saying the same thing. All I'm saying is
this: what Spehro says MAY be true, but it also might not be true,
depending on the circuit parameters.

Have a look at this image to see what I am thinking:

http://i.imgur.com/QmQZE.png

There are four cases in this photo: lower right-hand one is a normal
start (not a jump-start). The starter sees 11.7 Volts in this case.

Upper right-hand is a jump-start with a battery which is so dead that
it has a very high internal resistance. In this case, the starter sees
10.8V.

The two left-hand ones are where there is a dead battery with either
10 times the "fresh" battery internal resistance, or 2 times. In both
cases, the voltage delivered to the starter is from 100 to almost
400mV less. Not much less, but not the same or more than without the
dead battery connected. Note that in both of these cases, the dead
battery is still charging while the cranking is happening. It does not
begin sourcing current (in this example with these circuit
parameters).

My only point is that it is possible for the dead battery to be a
liability (rather than a neutral player or a helper) when trying to
jump-start a car. It would, in some cases, be better if the dead
battery were not connected at all.

Sean


On Fri, May 18, 2012 at 9:56 PM, Dave Tweed <RemoveMEpicspam_OUTspamKILLspamdtweed.com> wrote:
{Quote hidden}

>

2012\05\19@093241 by Jake Anderson

flavicon
face
when cranking my battery voltage drops to around 8V or so
When my alternator died my battery read 5 volts and the car was still (just) running, with the headlights off (at night time of course)

On 19/05/12 11:56, Dave Tweed wrote:
{Quote hidden}

> -- Dave Tweed

2012\05\19@101929 by Spehro Pefhany

picon face
At 01:26 AM 5/19/2012, you wrote:
{Quote hidden}

Your approach is conceptually correct, however I think it would be
better if you put more realistic numbers in for the starter current
etc.

A typical car battery is rated to supply 875A with output voltage
equal or better than 7.2V, or 700A at 0°C. So, the internal resistance
of the fully charged battery won't exceed 5.5milliohms. Similarly, typical
voltage drops under normal cranking should not exceed around a volt
total. If we assume typical cranking current is 300A, then we have
3.3 milliohms for the wiring.

If, again, we assume a 300A cranking current then we have a
starter + wiring resistance of around 24 milliohms.

However, just by inspection we can see that the conjecture cannot be
true... if the starter is just barely cranking over, then the
voltage at the recipient vehicle battery must be less than the rated
7.2V (assuming the battery is the only problem!), so the "dead"
battery will be supplying some net current if it's internal voltage
is >10V.

Best regards,

2012\05\19@125738 by Electron

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face
At 16.19 2012.05.19, you wrote:
>A typical car battery is rated to supply 875A with output voltage
>equal or better than 7.2V, or 700A at 0°C. So, the internal resistance
>of the fully charged battery won't exceed 5.5milliohms. Similarly, typical
>voltage drops under normal cranking should not exceed around a volt
>total. If we assume typical cranking current is 300A, then we have
>3.3 milliohms for the wiring.

A 100nF bypass ceramic cap will do the trick. ;)

2012\05\19@222537 by Spehro Pefhany

picon face
At 12:57 PM 5/19/2012, you wrote:
>At 16.19 2012.05.19, you wrote:
> >A typical car battery is rated to supply 875A with output voltage
> >equal or better than 7.2V, or 700A at 0°C. So, the internal resistance
> >of the fully charged battery won't exceed 5.5milliohms. Similarly, typical
> >voltage drops under normal cranking should not exceed around a volt
> >total. If we assume typical cranking current is 300A, then we have
> >3.3 milliohms for the wiring.
>
>A 100nF bypass ceramic cap will do the trick. ;)

LOL

But with a _somewhat_ larger capacitor (500F in this case), it's possible
to give things a bit of a boost- especially useful in very cold conditions.

http://www.youtube.com/watch?v=BE-v9KiAaVo

Getting a large cold motor moving requires a very high current initially, and
the ultracaps can help a lot (or even do the whole job).

--sp

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
RemoveMEspeffKILLspamspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com


2012\05\19@231042 by John Gardner

picon face
....Getting a large cold motor moving requires a very high current initially

Or a GI & a bit of firewood...   :

2012\05\21@020516 by RussellMc

face picon face
> ...Getting a large cold motor moving requires a very high current initially
>
> Or a GI & a bit of firewood...   :)

I've heard that Cat's, and German tanks in Russia, respond to blow torches.


             R

2012\05\21@034659 by Rodney Pont

flavicon
face
On Mon, 21 May 2012 18:04:34 +1200, RussellMc wrote:

>I've heard that Cat's, and German tanks in Russia, respond to blow torches..

My cat says people respond to claws :-)

-- Regards - Rodney Pont
The from address exists but is mostly dumped,
please send any emails to the address below
e-mail        rpont (at) gmail (dot) com

2012\05\21@135242 by David VanHorn

picon face
I've been meaning to chime in on this.

I had a battery blow in my face one afternoon, when attaching the
negative cable.
It was the FIRST cable I was attaching, and the only thing I can think
of would be ESD between the two vehicles.
It was an exciting experience. Fortunately I favor oversize glasses,
so I didn't take any in the eyes.

These days, my jumper cables are a normal pair of gator clips for the
"victim" and a powerpole which connects both leads at the same time to
a matching powerpole in my car, several feet from the battery,
avoiding the sparky-boom problem on both ends, and making it less
likely for someone to swipe my jumper cables.

The PPs I use are rated at something like 100A, not as large as what
you see on tow trucks, but then I don't use them all that often
either

2012\05\21@160851 by Dwayne Reid

flavicon
face
At 11:52 AM 5/21/2012, David VanHorn wrote:
>I've been meaning to chime in on this.
>
>These days, my jumper cables are a normal pair of gator clips for the
>"victim" and a powerpole which connects both leads at the same time to
>a matching powerpole in my car, several feet from the battery,
>avoiding the sparky-boom problem on both ends, and making it less
>likely for someone to swipe my jumper cables.

I do something similar for one setup I have: its a largish 12V AGM battery (something on the order of 75 AH) in a carrying bag, along with AGM charger, a 400W power inverter and a set of jumper cables.

The Anderson Power Pole connectors I am using are rated at 60A and they have been reliable for at least 10 years now.  The wire is 6 AWG from the battery but only 8 AWG to the jumper clips.  However, it works well.

Gets used for jump-starting vehicles only a few times per year and has seen a LOT of use over the past couple of years for testing a couple of pipeline Injection Pump controllers that we manufacture.

dwayne

-- Dwayne Reid   <dwaynerSTOPspamspamspam_OUTplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2012\05\26@154253 by Jonathan Hallameyer

picon face
On Mon, May 21, 2012 at 4:08 PM, Dwayne Reid <spamBeGonedwaynerSTOPspamspamEraseMEplanet.eon.net> wrote:

{Quote hidden}

Since I think at least some of the people on here could appreciate the
excess, I'll post it here.   I built myself a 12v jumper pack using 16X
(4s4p) Olevin power 26650 LiFePO4 cells. Olevin power is Chinese made A123
cells (I.e. A123 had manufacturing moved to china, not just whatever
LiFePO4 cell a chinese factory could come up with) if the RC car websites I
browed had their facts straight.   Some 120A 4AWG Power poles, a 150A
breaker (200A on the way, both good for 4x rated for 10 s before popping),
a distribution block  and a pelican case, and I have something that can
start a car on its own that I can hold with a pinky :o)

http://i.imgur.com/9v5lu.jpg
http://i.imgur.com/hamV3.jpg

I installed balance leads on the cells when I built the packs, for a
balancing circuit in the future, but 4S LiFePO4 matches up really well with
a 6S Pb battery system.  (14.6V max charge voltage, ~12.0V open circuit is
dead, a bulk of the discharge at 12.8V.) So its safe enough for storage and
jumpstarting stuff now I just wont let it sit too long on a vehicle with an
unknown quality charging system.  Maintenance charging is done via the
balance leads with a hobby charger.

I hooked it up to my buddies Grand Cherokee V8 with 6foot of 4 AWG jumper
cables, and it successfully started the cold engine on a ~45F day with the
original starting battery disconnected.   It was definitely a lot more
labored sounding starting up then the normal FLA battery, but if it starts
it, it will most certainly put some charge into a dead battery :)

-Jon H

2012\05\27@215643 by Joe Wronski

flavicon
face
On 5/26/2012 3:42 PM, Jonathan Hallameyer wrote:
{Quote hidden}

How would it compare in price and performance with this battery on ebay?
It's a Tenergy LiFePO4 12V 10Ah 128Wh Rechargeable Battery for 160 USD.
http://r.ebay.com/CqfXtv
I've been looking around to build a 28V 288 Wh battery for the Ford thinkbike, but can't really justify the cost until I try the thing out.  Almost ready to fire it up with the 18 lb VRLA battery.

Joe W

2012\05\28@174525 by Jonathan Hallameyer

picon face
On Sun, May 27, 2012 at 10:00 PM, Joe Wronski <spamBeGonejwronskispamKILLspamstillwatereng.net>wrote:

{Quote hidden}

>

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