Rethinking Jumper Cables

[440Magnum]

Originally Posted By: JHZR2

You sure? IIRC, the value provided by the reference I linked to in this thread (cable manufacturer) indicated ~0.8uH/ft. A 20' set of cables would be 40' of cable length... So more like 32 uH.


Is that for a single cable, or a pair laid side-by-side? Because the fact that most jumper cables are close-coupled significantly reduces the inductance of the circuit compared to two widely-separated cables.

But let's assume that it is 32 uH and do some back-of-the napkin math. How fast is the circuit actually broken by disconnecting a cable? What is the relative voltage difference at the time of disconnection?

I would argue that the circuit actually "breaks" relatively slowly when you un-clamp a cable, because the teeth retract from where they've sunk into the terminal and the resistance ramps up over maybe a millisecond as they do so. Its not like snapping a relay open, which itself is on the order of 10s to 100s of microseconds. I'll be conservative and call it 500 uS. Now the next thing to consider is the current flow. The worst case would be if the recipient car's alternator is actually not working AT ALL, even after that car starts, so that the donor car is both running the recipient car's electronics (maybe 10A) and chargning its battery (full alternator output initially, but even a very, very dead battery will quickly build enough surface charge and internal resistance to drop that down to about 20a. So let's be generous again and say that 50 amps are flowing at the time of disconnection:

So from that, we can guesstimate that dI/dT is on the order of 50A/500uS.

So the voltage "spike" due to the cable inductance will be (50/500e-6)*32e-6. By my trusty HP15c, that gives me a voltage spike due to the cable inductance of 3.2 volts.

That's not going to damage ANYTHING. Even if some of our guesstimates are off by a factor of 2, its still not an issue.

So I'm back to my first claim: the biggest risks in jump-starting are a) operator error, and b) the recipient car having a full-field charging system fault that makes it's system voltage surge high when it first starts.

 

[Shannow]

Originally Posted By: 440Magnum
By my trusty HP15c,


Still got mine, and love it.

 

[supton]

Originally Posted By: BalticBob
Once I was helping jump start a vehicle, the owner had small gauge cables, they were not working. I also had small diameter cables, but we paralled them, and it worked quite well.
After that event, I purchased some large gauge cables.


Bet if you had waited long enough the thin cables would have gotten the job done. Start the car, wait 5 minutes, try; if no go wait another 5.

 

[440Magnum]

Originally Posted By: Shannow
Originally Posted By: 440Magnum
By my trusty HP15c,


Still got mine, and love it.


If it were a car, I could register it as an antique. But it works like new- built when HP was an instrument company, not a computer house.

 

[jim302]

Originally Posted By: 440Magnum
Originally Posted By: Shannow
Originally Posted By: 440Magnum
By my trusty HP15c,


Still got mine, and love it.


If it were a car, I could register it as an antique. But it works like new- built when HP was an instrument company, not a computer house.



At this point HP exists to sell printer ink. The original instrumentation that the company started with are now with Keysight. And now they have separated servers, networking, and storage (which are all still pretty good products) to Hewlett Packard Enterprise. Sad to see a once great business with such humble beginnings being sent to the scrapyard to be sold piece by piece.

Those HP 15C's are woth a fair amount of money now. There's a modern remake I found that I'm thinking about getting but it's still about $100 by the time you get it shipped to the US from Switzerland. For now I use an app on my phone that emulates the HP 48... it's tough for me to use a calculator without RPN now.

In regards to jumper cables vs jumper packs, I think there's a place for both. Some of the better jumper packs have things like reverse polarity protection so they're a great option for someone who is not knowledgeable or just not comfortable with the process of jump starting. The bigger the jump pack, the more likely it is to start a car in very cold weather or if the battery is in very bad shape.

My biggest concern with the jumper packs is quality and whether or not they meet their published specs regarding storage capacity and maximum/sustained current draw. The small and light ones use Lithium Ion or Lithium Polymer batteries, and from what I have seen in teardowns posted on the Internet, the batteries aren't necessarily from top suppliers (though even the big names have had failures... see: Samsung Galaxy Note 7). These batteries are very mean when they fail, and can start quite a large fire. Over all these batteries are great and have made cordless tools, mobile phones, and laptops much more enjoyable and practical to use... but they do need proper safety protection and there's clear evidence that this protection is lacking in a lot of products, including some jump packs I have seen stress tested on YouTube.

Also, these jump packs have a lifespan like any other battery. Lithium Ion and Lithium Polymer batteries are sensitive to heat will have substantially reduced capacity after a few years... so when your 5 year old car battery dies, your 5 year old jump pack may not be much help.

Jumper cables are still a good thing to carry around for anyone who understands how to use them. Risk of damage is very low as long as the owner's manual is consulted to find any special requirements and the cables are hooked up properly. You don't have to worry about good quality jumper cables going bad from old age or being damaged in the heat. As long as they are heavy gauge copper, they can jump start most things that a jump pack fails on. Automotive electrical stuff seems to be fairly robust. Automotive forums are not filled with stories of jump starts using traditional cables causing damage.

 

[Y_K]

Waiting for study in hysteresis and Ruhmkorff coil to enter this 'scholarly' nonsense. Recalled hearing on public radio about a physics prof, who made a name for himself by writing a book that debunked the prevailing aero wing theory of the time: that lift force is created by the delta in lower/upper pressures. Really?
Luckily, engineers who designed real things didn't get poisoned by this 'science'.

 

[JHZR2]

Originally Posted By: 440Magnum

So from that, we can guesstimate that dI/dT is on the order of 50A/500uS.

So the voltage "spike" due to the cable inductance will be (50/500e-6)*32e-6. By my trusty HP15c, that gives me a voltage spike due to the cable inductance of 3.2 volts.

That's not going to damage ANYTHING. Even if some of our guesstimates are off by a factor of 2, its still not an issue.

So I'm back to my first claim: the biggest risks in jump-starting are a) operator error, and b) the recipient car having a full-field charging system fault that makes it's system voltage surge high when it first starts.



Ive seen 3x nominal voltage on quick turn off of batery-fed loads before, when running nearly 1000A through short conductors.

Id agree with your assessment though, the spikes shouldnt be that large. I said Im happy to be wrong if inductance isnt the issue.

Which goes back to my original point - lots of places saying that jumper cables are obsolete, are saying that you can possibly see 100+VDC. That's not hooking cables in reverse or some such thing... So are they lying? Inaccurate? What?

 

[Rand]

I retired my battery cables I have a little stanly "boost" pack. IF that wont start it.. they can call AAA.
It will work on low batteries--left lights on etc.. but not shorted/frozen kaput batteries.
I had a full size jump pack but it self destructed last year.

If I needed something absolutely reliable I'd get a clore JNC660.

I have jumped plenty of co-workers and that works fine as long as I basically tell them
"do nothing sit in your car. Follow my instructions"

Seen a couple people blow up their car from jumping people in the parking lot.
One guy in particular was leaning on the ignition key before the cables were even attached.

Nothing like seeing the "rescue vehicle" being flat-towed off to put a damper on helping people.

I will still air up their flat tires if its an elderly person or otherwise normal person..
not so much if its the guy who has a total beater from spending all this money on meth etc.

 

[Jarlaxle]

I jumped a truck at work a while back.

People were amazed that a car could make enough power to start a Cummins M11.