Rethinking Jumper Cables

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Northern has some very heavy duty cables for $65 on sale. I got a set in my pickup.

I also have a jump-pak in my house where it stays warm.

If you need a jump, your in luck if I am around due to dual batteries in my pickup.

If my pickup needs a jump, I am in trouble, few cars will have enough juice.

Still looking at newer technology jump-pak, such as LiIon and capacitor.
 
IMHO there's a difference between a "dead" battery which is discharged but still good and a failed battery which isn't capable of absorbing charge any longer. The failed battery isn't going to provide the damping effect, much like a capacitor, which would damp a voltage spike. What we need is an opinion from someone who does automotive electrical system design.
 
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Originally Posted By: Leo99
I don't futz around with older marginal batteries. Once they get marginal, they get replaced.

I'm going to buy a li ion jumper when I see one on sale.


I think marginal batteries cause lots of problems
 
A vehicle's owner's manual will specify if jump starting is safe, and what precautions must be taken when doing so.

As far back as the early 90's, BMW noted that high-load accessories like the headlights and blower fan were to be switched on to absorb any voltage spikes, before disconnecting the jumper cables. And they had what would be considered a modest amount of electronics compared to vehicles of today.
 
Originally Posted By: wrcsixeight
Jhzr2

Think Jumper cables jumping modern vehicles could benefit from ferrite chokes and twisted leads, or are such practices only for RFI?


Dunno...

Twisting drops inductance from 0.4 to 0.08uH/ft according to this:

http://www.nationalwire.com/pdf/cat07_design_guideV10.pdf

Which is good reading btw, including for conductor configurations, etc.

I think thar cheap power electronics will show up (like the michelin cables) before twisting heavy 100% copper becomes the norm.
 
Originally Posted By: eljefino
I have a couple sets of cheezy 10 ga copper cables. The alligator clip connection is the current limiting factor, IMO.

They force the user to hook up and charge up for five minutes or so, so the victim's battery gets a surface charge and helps everything out.

I rev the donor car to the moon before trying to start, so the inrush/outrush of current during the *click* of the solenoid is spread across as many of my alternator's diodes as possible.

Still lucky. Or is it technique?
50.gif


That's pretty much what I have/do, no problems so far.
 
Originally Posted By: HerrStig
IMHO there's a difference between a "dead" battery which is discharged but still good and a failed battery which isn't capable of absorbing charge any longer. The failed battery isn't going to provide the damping effect, much like a capacitor, which would damp a voltage spike. What we need is an opinion from someone who does automotive electrical system design.


Agree on both counts. Need someone from Bosch or elsewhere to chime in. Id be interested in the specs/tests that automakers invoke for worst case abuse protection.
 
Originally Posted By: JHZR2
Originally Posted By: wrcsixeight
Jhzr2

Think Jumper cables jumping modern vehicles could benefit from ferrite chokes and twisted leads, or are such practices only for RFI?


Dunno...

Twisting drops inductance from 0.4 to 0.08uH/ft according to this:

http://www.nationalwire.com/pdf/cat07_design_guideV10.pdf

Which is good reading btw, including for conductor configurations, etc.

I think thar cheap power electronics will show up (like the michelin cables) before twisting heavy 100% copper becomes the norm.

There is no inductance in a DC powered conductor because you need AC frequency for that to happen, which is inductive reactance. Just buy some good jumper cables and not worry about electrical theory. The resistance of the wire is affected by the length and diameter of the conductor. The clamp connections is where you get the biggest voltage drop and lose current flow. If you used 2 inch diameter cables it wouldn't help at all, if you have poor contact at the clamps. All the current trys to flow through the little contact area of the clamp.
 
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A chain is only as good as its weakest link. So all it takes is one of those computers to decide to have something not quite right and blow to ruin one's day. That said, I like how Goss says "millions" of cars are damaged each year from jumping. Hyperbole, eh?

I'd think electronics downstream are pretty well isolated from any spikes; the battery is a pretty big capacitor. If the alternator power goes to the battery, and then outward, it'd take a very big spike in current to bounce above 15V. But then it's up to the OEM to spec how each module is to withstand overvoltage. They may or may not be designed to take continuous overvoltage, or they may take it for small periods of time, or they might take it continously. All a matter of specmanship. I know the sensors I've seen go both ways. Some are spec'd to run on 24V, some will blow at 7V.

If you want to know what tests, I'd start at " load dump ", which is the method to test for battery disconnect. That should lead to the other specs for testing.

Finally, I'm not sure if L*di/dt matters, at least not in the cables. Once the dead car starts, its alternator is going to bring its battery voltage up very quickly. Battery might not be "charged" but I'd think far less current is flowing. If really worried, pull out the DMM and disconnect cables once voltage at either end are nearly the same.

Which is actually an interesting thought... I've always "raced" to disconnect, once started. Never liked to alternators "fighting" against each other, although that's probably a foolish thought now that I've typed it: they cannot sink current, just source it. Whoever is stronger will set the voltage.

Now, I suppose the L*di/dt in the donor alternator may lead to a spike in the donor during cable disconnect; now you have me thinking about that.
 
Originally Posted By: Traction
Originally Posted By: JHZR2
Originally Posted By: wrcsixeight
Jhzr2

Think Jumper cables jumping modern vehicles could benefit from ferrite chokes and twisted leads, or are such practices only for RFI?


Dunno...

Twisting drops inductance from 0.4 to 0.08uH/ft according to this:

http://www.nationalwire.com/pdf/cat07_design_guideV10.pdf

Which is good reading btw, including for conductor configurations, etc.

I think thar cheap power electronics will show up (like the michelin cables) before twisting heavy 100% copper becomes the norm.

There is no inductance in a DC powered conductor because you need AC frequency for that to happen, which is inductive reactance. Just buy some good jumper cables and not worry about electrical theory. The resistance of the wire is affected by the length and diameter of the conductor. The clamp connections is where you get the biggest voltage drop and lose current flow. If you used 2 inch diameter cables it wouldn't help at all, if you have poor contact at the clamps. All the current trys to flow through the little contact area of the clamp.
I suggest you read up on square waves. If thee is no inductance in a DC circuit then ignition coils (fed from the battery) would produce NO spark energy. A transformer, which is what a coil is, can work on a DC pulse, in the old days from a set of breaker points.
 
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Originally Posted By: Traction
There is no inductance in a DC powered conductor because you need AC frequency for that to happen, which is inductive reactance.


The rest of what you said was correct, but the above isn't. Inductance arises from energy stored in magnetic fields, which in turn are generated by flowing currents. You are correct that, as long as current is steady, then inductance does not matter. However there is no such thing as DC: at some point of time there was zero current, and eventually there is zero again. Rapid connection/disconnection of a DC supply (battery, power supply) results in phenomenon best described as "AC effects". Usually when we say "AC" we think 60Hz (50Hz outside of North America), sometimes 400Hz (aircraft) but in reality di/dt and dv/dt do not care if it is a pure sinusoid. Only if the connection/disconnection was controlled by variable resistance means can the inductance be truly minimized, and usually that is not feasible (due to heating effects).
 
The ONLY sources of potential damage that I can think of are:

1) You hook up the cables wrong. That one is on YOU.

2) If the jump-recipient's vehicle has a charging system that has failed in such a way that the alternator is driven to max output all the time- in that case when the recipient car starts, it could potentially drive the voltage on the donor vehicle above the usual 14-15 volt max safe range. But given that there should be a dead battery in the system, I don't think that is likely- especially if the recipient is warned not to rev their engine to the moon right after startup.

As I see it, the biggest risk of damage is exactly the same as it has always been- operator error in hooking up the cables backwards. The only, ONLY, difference between now and 1960 is that the collateral damage now is a lot more expensive to repair. And I don't think its likely to even be the computers- those are easily protected by input diodes to guard against reverse polarity, and zener diodes to guard against over-voltage. Its all the other stuff- relays, servomotors, actuators, sensors, stereos, nav systems, expensive alternators whose diodes could get blown by a reverse hook-up, etc.
 
Originally Posted By: supton
Originally Posted By: Traction
There is no inductance in a DC powered conductor because you need AC frequency for that to happen, which is inductive reactance.


The rest of what you said was correct, but the above isn't. Inductance arises from energy stored in magnetic fields, which in turn are generated by flowing currents. You are correct that, as long as current is steady, then inductance does not matter. However there is no such thing as DC: at some point of time there was zero current, and eventually there is zero again. Rapid connection/disconnection of a DC supply (battery, power supply) results in phenomenon best described as "AC effects". Usually when we say "AC" we think 60Hz (50Hz outside of North America), sometimes 400Hz (aircraft) but in reality di/dt and dv/dt do not care if it is a pure sinusoid. Only if the connection/disconnection was controlled by variable resistance means can the inductance be truly minimized, and usually that is not feasible (due to heating effects).


+1.

L*di/dt voltage spikes from batteries are abdolutely real, Ive observed them firsthand studying batteries for electric weapons.

Its the di/dt part thats critical that im theorizing about and chasing down, as was mentioned; steady current flow is not an issue. And Im OK being wrong in my theory, but im trying to make heads and tails of the stories I keep reading about the possibility of damage.

Its a really low battery being charged by a high battery (low source impedance) and a running alternator. The recipient alternator could be duped into low output due to an artificially high dc bus caused by the donor car battery and alternator. With the right cables, a decent current can flow and be intereupted by pulling a conductor, interrupting a hundred amps in the fraction of a second. Voila di/dt
 
Originally Posted By: JHZR2
The recipient alternator could be duped into low output due to an artificially high dc bus caused by the donor car battery and alternator. With the right cables, a decent current can flow and be intereupted by pulling a conductor, interrupting a hundred amps in the fraction of a second. Voila di/dt


True but wouldn't you expect the BCM-driven "soft start" alternators to be better at this than the old ones that just got field power through the dash "BAT" light?
 
Originally Posted By: eljefino
Originally Posted By: JHZR2
The recipient alternator could be duped into low output due to an artificially high dc bus caused by the donor car battery and alternator. With the right cables, a decent current can flow and be intereupted by pulling a conductor, interrupting a hundred amps in the fraction of a second. Voila di/dt


True but wouldn't you expect the BCM-driven "soft start" alternators to be better at this than the old ones that just got field power through the dash "BAT" light?


The issue in my view would be the reverse.

The donor car is runing full bore because the recipient vehicle is dead and the battery is sucking up charge. On the recipient car, because it was soft started into a 14V-ish bus, its not doing much.

The issue is that the donor car is dumping current from the battery and alternator into the recipient car. Remove the cable from the recipient car and the donor car sees a huge change in current in next to no time. the inductance is IN the cable, so it dissipates back into the donor car.

As I think about it more, the spike would always occur on the side opposite the first clamp being removed. This is why Ive also read that jumping in a system where there is an Anderson plug connected to the donor vehicle protects the best. A professionally, correctly installed connector likely has a TVS or MOV system between the Anderson plug and the donor vehicle, protecting it; and the first connection removed would be on the recipient vehicle.
 
Originally Posted By: JHZR2
Originally Posted By: eljefino
Originally Posted By: JHZR2
The recipient alternator could be duped into low output due to an artificially high dc bus caused by the donor car battery and alternator. With the right cables, a decent current can flow and be intereupted by pulling a conductor, interrupting a hundred amps in the fraction of a second. Voila di/dt


True but wouldn't you expect the BCM-driven "soft start" alternators to be better at this than the old ones that just got field power through the dash "BAT" light?


The issue in my view would be the reverse.

The donor car is runing full bore because the recipient vehicle is dead and the battery is sucking up charge. On the recipient car, because it was soft started into a 14V-ish bus, its not doing much.

The issue is that the donor car is dumping current from the battery and alternator into the recipient car. Remove the cable from the recipient car and the donor car sees a huge change in current in next to no time.


That's why i would always recommend hooking up the cables, and unhooking with the donor car not running. Shut it off before unhooking, also in case any fumbling shorts the ends, blasting the diode.
 
Originally Posted By: 901Memphis
This is the chart I saw before for sizes. Is there another way they gauge it?


Seems about right...

The number of stranded conductors in there can vary based upon quality and need for flexibility.

Also the conductor material can be aluminum, copper, or copper coated aluminum.
 
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