The Straight Dope

Go Back   Straight Dope Message Board > Main > General Questions

Reply
 
Thread Tools Display Modes
  #1  
Old 03-03-2006, 09:03 AM
Phlosphr Phlosphr is offline
Member
 
Join Date: Jun 2001
Location: in a Moot
Posts: 11,619
Why did my Jumper Cables burn in half after attempting a jump?

We had a nasty storm last evening... and all of our vehicles were obviously burried in snow this morning. I have an 87' Land Cruiser that likes to go dead after letting it sit for a couple days. I only got it a month ago and it has been giving me grief ever since. I bought it as a beater car for the winter so I could garage my Avalanche. I found out quickly that I was killing the battery by leaving the ignition switch on ACC over night every night... I quickly learned to switch it to OFF and have not had too much problems with it since.

This morning I wasn't getting anything upon trying to start it so I grabbed the truck, pulled it up to the Cruiser and tried jumping it. Here is the exact sequence of events and subsequent issues.

Pulled truck to cruiser and left it running. Attached cables to truck then to cruiser... Tried jumping cruiser to no avail. Nothing came from the jump so I hopped out to check the connections. Positive on Positive Negative on Negative check...

I noticed smoke coming from both connections on both vehicles, I tried pulling the cable off my battery and it burned through the protective rubber and snapped off leaving the bare wire still burning the rubber casing on the cable line. I tried removing the cable from the truck and the same thing happened, it burned through the rubber casing in the line and snapped off.

Here's the weird part. I then tried starting the cruiser without the cables and it started right up?

Do I have a Stephen King mobile or am I doing something wrong? I've jumped the cruiser plenty of times... what gives today?
Reply With Quote
Advertisements  
  #2  
Old 03-03-2006, 09:23 AM
Sam Stone Sam Stone is offline
Charter Member
 
Join Date: Jun 1999
Posts: 25,531
Is it possible you've got something shorting to ground? It sounds like essentially the jumper cables just tied the battery terminals together.
Reply With Quote
  #3  
Old 03-03-2006, 09:41 AM
Rick Rick is offline
Charter Member
 
Join Date: Aug 1999
Posts: 15,701
Too much current and too small of a cable.
Before they burned up, you managed to get enough amps into the battery to start.
Jumper cables come in three flavors
Very Crappy (cheap)
Crappy (not very cheap)
Good (very spendy)

For what you are trying to do, I would expect that you need to pay in the $50-$75 range for cables that will last.
Reply With Quote
  #4  
Old 03-03-2006, 11:10 AM
Kevbo Kevbo is offline
Guest
 
Join Date: May 2005
I've been on this soapbox too often, but here goes again:

The vast majority of jumper cables sold are junk. Good ones exist, but you have to seek them out, and as previous respondant said, they are not cheap. Thin wire with thick insulation so that it looks "heavy duty". This is actually detrimental, as that thick plastic provides thermal as well as electrical insulation. Hot thin wire has even higher resistance than cold thin wire, so generates even more heat.

Somewhere on the package will be the wire gauge. On cheap cables it will be in the fine print, and will be AWG (american wire gauge) 10 at best. Like most things measured in "gauge" smaller numbers mean larger wire.

You can't start a cold engine with #10 wire. You can, at best, charge the battery enough over time so the dead car might start.

Actually starting a car requires at least #6 wire. Make that #4 if you live somewhere where it gets really cold. Make that #2 if the dead car is a diesel. Make it #0 or 2/0 if it is a diesel truck.

Places that cater to truckers sell heavy jumper cables. The auto parts chain with the blue and gold motif also does. Welding equipment dealers sell wire and clamps so you can make your own.
Reply With Quote
  #5  
Old 03-03-2006, 12:56 PM
Ol'Gaffer Ol'Gaffer is offline
Guest
 
Join Date: Jul 2002
I will just echo what the learned Rick and Kevbo have already said - crappy cables.

Plus, welcome to the Land Cruiser club ('78 FJ55 here). At least you got one of the "real" Cruisers before they turned into a luxury SUV!
__________________
"It's such a fine line between stupid and clever." - David St. Hubbins
Reply With Quote
  #6  
Old 03-03-2006, 01:10 PM
Praetor Praetor is offline
Guest
 
Join Date: Jan 2006
Quote:
Originally Posted by Phlosphr
Attached cables to truck then to cruiser... Tried jumping cruiser to no avail. Nothing came from the jump so I hopped out to check the connections. Positive on Positive Negative on Negative check...
Your problem is that you're not supposed to connect both terminals on the car to be jumped. You connect positive to positive, but then negative on the car doing the jumping to the engine block (or some other grounded section of the car) on the car to be jumped. By connecting positive-positive and negative-negative, you created a short circuit and the live battery forced electricity through the dead one.
Reply With Quote
  #7  
Old 03-03-2006, 01:13 PM
Phlosphr Phlosphr is offline
Member
 
Join Date: Jun 2001
Location: in a Moot
Posts: 11,619
I wonder if I should even bother having a mechanic look for a short in the lines... It would seem there is probably miles of electricals that couls be shorting something to ground. Oh and I did have a cheap pair of cables...
Reply With Quote
  #8  
Old 03-03-2006, 01:16 PM
Phlosphr Phlosphr is offline
Member
 
Join Date: Jun 2001
Location: in a Moot
Posts: 11,619
Quote:
Originally Posted by Ol'Gaffer
I will just echo what the learned Rick and Kevbo have already said - crappy cables.

Plus, welcome to the Land Cruiser club ('78 FJ55 here). At least you got one of the "real" Cruisers before they turned into a luxury SUV!
Oh hell yeah! That was one of the reasons I bought it, it's not all decked out with leather, ultra sound proofiing etc...etc... It's a hog! And it can pull just about anything I hitch onto it. I love it! I'm contemplating having the exterior body done to perfection and retoring the looks to full luster and using it full time. I get tons of complements on it.
Reply With Quote
  #9  
Old 03-03-2006, 01:21 PM
Gorsnak Gorsnak is offline
Charter Member
 
Join Date: Jan 2003
Location: Saskaboom
Posts: 8,029
Quote:
Originally Posted by Praetor
Your problem is that you're not supposed to connect both terminals on the car to be jumped. You connect positive to positive, but then negative on the car doing the jumping to the engine block (or some other grounded section of the car) on the car to be jumped. By connecting positive-positive and negative-negative, you created a short circuit and the live battery forced electricity through the dead one.
The block and any grounded section of the car are electrically common with the negative post on the battery - in fact, the negative post on the battery is the reference ground for the entire car, given that there's obviously no "grounded to earth" reference on a vehicular electrical system. There is absolutely no difference in terms of the circuit created between connecting your negative clamp to the battery terminal and any grounded chunk of metal on the car. The reason it is sometimes not recommended to connect the negative clamp to something other than the negative post is to avoid creating sparks in the vicinity of the battery - something to do with flamable fumes given off by the battery acid or something.
Reply With Quote
  #10  
Old 03-03-2006, 01:45 PM
LiveOnAPlane LiveOnAPlane is offline
Guest
 
Join Date: Sep 2005
Quote:
Originally Posted by Gorsnak
...something to do with flamable fumes given off by the battery acid or something.
Yes. Batteries can give out hydrogen gas, which you *don't* want any sparks around.
Reply With Quote
  #11  
Old 03-03-2006, 02:54 PM
BubbaDog BubbaDog is online now
See, I had a dog named Bubba
Charter Member
 
Join Date: Mar 2003
Location: KC MO or there abouts
Posts: 4,438
Quote:
Originally Posted by LiveOnAPlane
Yes. Batteries can give out hydrogen gas, which you *don't* want any sparks around.
Oh the humanity!
Reply With Quote
  #12  
Old 03-03-2006, 03:21 PM
astro astro is offline
Guest
 
Join Date: Jul 1999
Here

Mize 20' 2-Gauge Jumper Cable (600-Amp)
Reply With Quote
  #13  
Old 03-03-2006, 03:46 PM
Chronos Chronos is offline
Charter Member
 
Join Date: Jan 2000
Location: The Land of Cleves
Posts: 55,260
Quote:
Hot thin wire has even higher resistance than cold thin wire, so generates even more heat.
This is incorrect. Given a constant (or nearly so) voltage source like a battery, less resistance generates more heat. Think of it this way: Before you snap on the cables at all, there's an air connection between the two batteries. Air has a huge resistance (effectively infinite, for most purposes), far higher than even the crappiest jumper cable. And the air between the batteries doesn't heat up at all.
__________________
Time travels in divers paces with divers persons.
--As You Like It, III:ii:328
Reply With Quote
  #14  
Old 03-03-2006, 03:50 PM
Gorsnak Gorsnak is offline
Charter Member
 
Join Date: Jan 2003
Location: Saskaboom
Posts: 8,029
Quote:
Originally Posted by LiveOnAPlane
Yes. Batteries can give out hydrogen gas, which you *don't* want any sparks around.
I'd be curious to know if there's any actual evidence of this being a real concern. Even if a battery gives off a little hydrogen, it would be a fairly small amount and disperse pretty much immediately. And in the unlikely event that there were a high enough concentration to ignite, it'd be just one little poof of flame and go out. I can't imagine that it would be any more likely to ignite a grease fire or anything that might actually burn in something like a sustained fashion than the spark itself would be. I dunno. I could be wrong here, but the concern as stated doesn't make much sense to me.
Reply With Quote
  #15  
Old 03-03-2006, 04:20 PM
t-bonham@scc.net t-bonham@scc.net is offline
Guest
 
Join Date: Mar 2003
Quote:
Originally Posted by Gorsnak
I'd be curious to know if there's any actual evidence of this being a real concern. . . . I dunno. I could be wrong here, but the concern as stated doesn't make much sense to me.
Me, either.

I've heard this nonsense for years, often with scary stories (urban legend type) of what will happen if you do direct battery-to-battery connections.

Yet in 50 years of living thru Minnesota winters, and seeing a lwhole ot of dead batteries being jumped, I have probably seen less than a dozen cases where real people used anything other than a direct battery-to-battery connection.*

And I've never seen, or even heard of anyone who had a 'hydrogen gas explosion' while jumping a battery.
I suppose it could happen, but I'd bet being hit by a lightning bolt is more common!


* (Heck, on most cars it'd be real hard to find a part of the engine block that is connectable that isn't so dirty with grease, mud, or snow as to be a poor conductor.)
Reply With Quote
  #16  
Old 03-03-2006, 04:43 PM
Rick Rick is offline
Charter Member
 
Join Date: Aug 1999
Posts: 15,701
Quote:
Originally Posted by Gorsnak
I'd be curious to know if there's any actual evidence of this being a real concern.
Yes this really does happen. Iíve seen it happen.
Quote:
it'd be just one little poof of flame and go out.
Allow me to correct this statement. It should read "It would be just one small explosion that although it probably won't burn the car to the ground it will blow a hole in the top of the battery, and spray sulfuric acid all over the nearby area. This includes the unlucky guy holding the jumper cables. Sulfuric acid seems to be almost magnetically attracted to peopleís eyes.Ē
Likely damage from an exploded battery include:
New battery
New clothes
Visit to the Dr. for your eyes
Trip to the body shop to repaint fender
Quote:
I could be wrong here, but the concern as stated doesn't make much sense to me.
You are. The last battery I saw explode threw a large chunk of the top of the battery about 20 feet into the air. It sent the guy that was standing there to the hospital due to acid in the eyes.
When the battery charges it generates Hydrogen. It also generates pure oxygen. Put a spark to that mixture and it will EXPLODE. Not burn, explode.
Reply With Quote
  #17  
Old 03-03-2006, 05:34 PM
CC CC is offline
Charter Member
 
Join Date: Apr 2000
Location: not elsewhere
Posts: 3,849
I'm glad to read that story. I'm another person who has always believed that stuff about hydrogen danger around a battery but I never heard about anyone actually getting hurt by an actual explosion. Fortunately, one story from a person who saw it happen is enough for me. (Jeez, I hope this doesn't count as a friend of a friend kind of thing). I'll take it for the case that disproves the hypothesis. And that's all I need.
Reply With Quote
  #18  
Old 03-03-2006, 05:47 PM
Bryan Ekers Bryan Ekers is offline
Guest
 
Join Date: Nov 2000
Of course, now I'm wondering if the cables I keep coiled up in my trunk could get brittle in the cold and snap inside the insulation if I pull them out to use them on a particularly chilly winter morn. I haven't had a problem over the last seven or so years with occasional use, though, and these (as I recall) were decent but not super-high-end cables.
Reply With Quote
  #19  
Old 03-03-2006, 05:56 PM
A.R. Cane A.R. Cane is offline
Guest
 
Join Date: Oct 2005
Location: The great PNW.
Posts: 3,636
Your jumper cables burned because you got too impatient. When jumping from one vehicle to another it's advisable to wait a few minutes, after hooking up the jumper cables, to allow the low battery to absorb some charge. If the low battery is in good condition, this won't take long.
When you hook up the jumpers and immediately try to start the second vehicle, you are bypassing the low battery and send current directly to the starter motor. That means that your passing a couple hundred amps. through those jumper cables. Now compare the size of the wire in the jumpers to the size of the wire going from the battery to the starter, the cheaper the jumpers, the bigger the difference. There's also the issue of the aligator clamps, most have little teeth to grap onto the battery post, or other point of contact. This is, in effect, the "weak link" in the circuit. This weak link cause more resistance and faster heat rise in the jumper wires.
So hook up the cables and then up the idle on the donor vehicle for a few minutes before attempting to start the other engine. This will avoid drawing all the required an amperage through the jumpers.
Here's where the generation of gas comes from, as both batteries are accepting a charge it warms and generates the gas in both batteries. After the second vehicle starts, you should properly disconnect the neg. connection from it first. If you have made that connection to the neg. post of the low battery then there may be a spark right there where the gas is venting from the charging battery and BOOM. If you made the neg, connection to a point away from the battery, it may spark, but there's no gas to ignite.
Remember, both batteries can be venting gas, but the low one is charging faster and more likely to be the danger.
If you have quality jumpers w/ heavy cable and strong aligator clip, such as you'll find on a wrecker, or service truck, you may be able to skip the few minutes of charging, but w/ typical Wallyworld jumpers, you should do it as I described.
Reply With Quote
  #20  
Old 03-03-2006, 08:30 PM
Carnac the Magnificent! Carnac the Magnificent! is offline
Guest
 
Join Date: Jan 2001
Quote:
Originally Posted by Phlosphr
Oh hell yeah! That was one of the reasons I bought it, it's not all decked out with leather, ultra sound proofiing etc...etc... It's a hog! And it can pull just about anything I hitch onto it. I love it! I'm contemplating having the exterior body done to perfection and retoring the looks to full luster and using it full time. I get tons of complements on it.

If you think of selling, keep me in mind. Top dollar, Joe.
Reply With Quote
  #21  
Old 03-03-2006, 09:24 PM
racer72 racer72 is online now
Charter Member
 
Join Date: Mar 2002
Location: Covington, WA
Posts: 5,380
Quote:
Originally Posted by Gorsnak
I'd be curious to know if there's any actual evidence of this being a real concern. Even if a battery gives off a little hydrogen, it would be a fairly small amount and disperse pretty much immediately. And in the unlikely event that there were a high enough concentration to ignite, it'd be just one little poof of flame and go out. I can't imagine that it would be any more likely to ignite a grease fire or anything that might actually burn in something like a sustained fashion than the spark itself would be. I dunno. I could be wrong here, but the concern as stated doesn't make much sense to me.
The problem isn't the little bit of gas that has leaked out of the battery, it's the pressurized gas in the battery. Batteries are designed to slowly vent the hydrogen gases that are generated during the charging cycle. If the battery is under a heavy charging load, such as while on a good battery charger or while being jumped, the gases build up much faster than can be vented. The deader the battery, the more gases that are generated. Add a spark and kaboom. I use to have an 18 volt, 400 amp battery just for jumping dead batteries, I also blew up a couple of regular 12 volt batteries with it when I was a bit careless. Many tow truck carry this type of battery just for that purpose. Look in the cab of any tow truck and you will find boxes of baking soda just in case a battery does explode, the baking soda will neutralize the battery acid.
Reply With Quote
  #22  
Old 03-06-2006, 12:31 AM
Excalibre Excalibre is offline
BANNED
 
Join Date: Nov 2003
Location: Michigan
Posts: 7,585
Quote:
Originally Posted by racer72
The problem isn't the little bit of gas that has leaked out of the battery, it's the pressurized gas in the battery. Batteries are designed to slowly vent the hydrogen gases that are generated during the charging cycle. If the battery is under a heavy charging load, such as while on a good battery charger or while being jumped, the gases build up much faster than can be vented. The deader the battery, the more gases that are generated. Add a spark and kaboom. I use to have an 18 volt, 400 amp battery just for jumping dead batteries, I also blew up a couple of regular 12 volt batteries with it when I was a bit careless. Many tow truck carry this type of battery just for that purpose. Look in the cab of any tow truck and you will find boxes of baking soda just in case a battery does explode, the baking soda will neutralize the battery acid.
But how could there be oxygen to support combustion in the inside of the battery? Pressurized hydrogen and oxygen inside could easily catch, but what chemical process is producing oxygen?

I don't doubt Rick, but it seems to me that it's more likely to be something else that caused the battery to explode rather than hydrogen catching on fire.
Reply With Quote
  #23  
Old 03-06-2006, 01:10 AM
Gorsnak Gorsnak is offline
Charter Member
 
Join Date: Jan 2003
Location: Saskaboom
Posts: 8,029
Quote:
Originally Posted by Excalibre
But how could there be oxygen to support combustion in the inside of the battery? Pressurized hydrogen and oxygen inside could easily catch, but what chemical process is producing oxygen?

I don't doubt Rick, but it seems to me that it's more likely to be something else that caused the battery to explode rather than hydrogen catching on fire.
I did a little reading up on it, and according to what I found it's electrolysis of water in the electrolyte that's the source of hydrogen, and if that's correct it would of course be a source of oxygen as well.

My ignorance on this matter has been successfully fought, it would seem.
Reply With Quote
  #24  
Old 03-06-2006, 01:51 AM
Rick Rick is offline
Charter Member
 
Join Date: Aug 1999
Posts: 15,701
Quote:
Originally Posted by Excalibre
But how could there be oxygen to support combustion in the inside of the battery? Pressurized hydrogen and oxygen inside could easily catch, but what chemical process is producing oxygen?

I don't doubt Rick, but it seems to me that it's more likely to be something else that caused the battery to explode rather than hydrogen catching on fire.
Battery electrolye is H2SO4 and H20. (it's late and I am too tired to do sub tags. deal with it. ) Anyway as the battery discharges the SO4 ions are deposited onto the plates and the peroxide ions are released into the electroyte. The hydrogen ions combine with the peroxide ions and water is the result. As a battery discharges its specific gravity gets closer and closer to that of water. When you go to recharge the battery these reactions are reversed. However under some conditions, the water will disassociate instead of the reaction reversing. This give you two parts hydrogen to one part oxygen in and around the battey.
Add a spark and you get a rrather large boom.
Reply With Quote
  #25  
Old 03-06-2006, 01:56 AM
Excalibre Excalibre is offline
BANNED
 
Join Date: Nov 2003
Location: Michigan
Posts: 7,585
Well, I'm glad I read this thread. I always figured this was a miniscule risk, since hydrogen released from the battery would most likely dissipate. But obviously hydrogen and oxygen inside the battery are an entirely different matter. I'll make sure to stick the jumper cables on the right spots from now on.
Reply With Quote
  #26  
Old 03-06-2006, 07:51 AM
LouisB LouisB is offline
Charter Member
 
Join Date: Apr 2000
Location: Seminole, FL
Posts: 8,225
Just one more story about exploding batteries; during my VW mechanic days, I managed to explode a battery. In my case, the battery was sitting on a work bench and I was connecting a battery charger. I didn't get acid in my eyes, but I did ruin a set of coveralls. I also had to endure a very cold hosing off administered by another mechanic.
Reply With Quote
  #27  
Old 03-06-2006, 08:23 AM
CookingWithGas CookingWithGas is offline
Charter Member
 
Join Date: Mar 1999
Location: Tysons Corner, VA, USA
Posts: 9,780
Quote:
Originally Posted by Chronos
Given a constant (or nearly so) voltage source like a battery, less resistance generates more heat.
IANAEE, but this can't be right. Using your logic a #10 gauge wire with US house voltage of 120VAC will generate more heat than a tungsten filimant in a light bulb at the same voltage. If this were the case then the walls of my house catch fire when I turned on the lights.
Reply With Quote
  #28  
Old 03-06-2006, 09:09 AM
matt matt is offline
Member
 
Join Date: Aug 1999
Location: A Brit in 'Stralia
Posts: 1,964
Quote:
Originally Posted by CookingWithGas
IANAEE, but this can't be right. Using your logic a #10 gauge wire with US house voltage of 120VAC will generate more heat than a tungsten filament in a light bulb at the same voltage. If this were the case then the walls of my house catch fire when I turned on the lights.
It will generate more heat if you connected a #10 gauge wire directly across a 120V source, which is what is happening to a tungsten filament. The reason why the walls of your house don't catch fire is that your house wiring is in series with high-resistance loads, so the current is limited by those loads and not by the #10 wire.

Or to put it another way, the voltage drop across the #10 wire leading to your bulb is not the full 120V but is only a fraction of a volt, while the voltage drop across the filament is practically the full 120V. So they aren't equivalent situations.
Reply With Quote
  #29  
Old 03-06-2006, 04:07 PM
Praetor Praetor is offline
Guest
 
Join Date: Jan 2006
Quote:
Originally Posted by matt
It will generate more heat if you connected a #10 gauge wire directly across a 120V source, which is what is happening to a tungsten filament. The reason why the walls of your house don't catch fire is that your house wiring is in series with high-resistance loads, so the current is limited by those loads and not by the #10 wire.

Or to put it another way, the voltage drop across the #10 wire leading to your bulb is not the full 120V but is only a fraction of a volt, while the voltage drop across the filament is practically the full 120V. So they aren't equivalent situations.
But Chronos said "Given a constant (or nearly so) voltage source like a battery, less resistance generates more heat." For any particular voltage drop, a 12 gauge wire (less resistance) will generate less heat than a 18 gauge wire (more resistance).
Reply With Quote
  #30  
Old 03-06-2006, 06:46 PM
JimOfAllTrades JimOfAllTrades is offline
Guest
 
Join Date: Mar 1999
No, Chronos is right.

Consider a 100 watt light bulb and a 200 watt light bulb. The 200 watt bulb gives you more light, and so more heat, correct? Now, which one has the higher resistance?

If we assume the voltage drop for both is exactly 120 volts, we can back into the resistance. Since watts = volts x amps, we calculate the amperage draw as follows:

100 watt bulb = 120 volts x ? amps = 100 / 120 = .83 amps *
200 watt bulb = 120 volts x ? amps = 200 / 120 = 1.66 amps *

And since we know that resistance is Volts / amps, so we calculate the resistance as:

100 watt bulb = 120 volts / .83 amps = 144.5 ohms *
200 watt bulb = 120 volts / 1.66 amps = 72.3 ohms *

So we see that for a constant voltage drop (120 volts) a bulb with 72 ohms resistance gives us more light (and more heat) than a bulb with 144 ohms resistance.

The same is true for the 12 gauge wire vs. the 18 gauge wire. For a given voltage drop, lower resistance means more current, more current means more power dissipated, more power means more heat.

The reason most people get confused on this is that youíre not used to thinking of the wire as a load. In a typical circuit, heavier wire (lower resistance) stays cooler because it has less resistance than the load in the circuit, so more voltage is dropped across the load and very little voltage is dropped across the wire. So the wire stays cool.

But decrease the wire size until it the resistance goes high enough to drop a significant voltage compared to the load, and wire starts to heat up. So youíve increased the resistance and made the wire get hot.

But this isnít the situation Chronos was talking about. By increasing the wireís resistance, youíve increased the voltage drop (and decreased the voltage drop across the real load) so now the wire heats up.

What Chronos applies only to a constant voltage drop. This doesnít happen when you increase the resistance of the wire in a circuit where there is a load besides the wire itself.

In the situation of jumping a car, as the wires heat up, the resistance will increase, and the resistance goes up, and the current will start to drop off. As the current drops off, the cable will cool slightly. Eventually, the cable will reach an equilibrium temperature and current capacity, assuming it doesnít melt first. But itís not a runaway situation, rather it is a self limiting one.

Clear as mud?

(* Ok, itís been nearly 30 years since I took this stuff in school, but I think all those numbers are correct.)
Reply With Quote
  #31  
Old 03-09-2006, 12:25 PM
ZenBeam ZenBeam is offline
Charter Member
 
Join Date: Oct 1999
Location: I'm right here!
Posts: 8,634
Without more information, it's impossible to say whether the wire heating up and becoming more resistive will increase or decrease the amount of heat generated in the wire. In the case here, we'd need to know the resistance of the jumper cables and the how much other resistance is in the circuit. To say that less resistance generates more heat, Chronos and RJKUgly are assuming (whether they realize it or not) that the resistance of the jumper cables is larger than all the other loads in the circuit (the internal resistances of the good and dead batteries).

One of the things every EE is taught in their first circuits class is that maximum power to the load occurs when the load resistance matches the source resistance. For those of you who aren't EEs, but want to play along at home, for a battery and a load (the jumper cables), the total resistance in the circuit is
Rjumper + Rbattery,
so current is
I = Vbattery / (Rjumper + Rbattery).

Energy turned into heat in the jumper cable is
I2 * Rjumper = Vbattery2 * Rjumper / (Rjumper + Rbattery)2
It's easy to see that there is zero energy loss in the jumpers in the limits Rjumper = 0 and Rjumper --> infinity. It's not hard to show the energy loss is a maximum when Rjumper = Rbattery.

I don't know what the internal resistance of a good or bad battery is, but my hunch is that they're higher than the resistance of the jumper cables. If this is the case, Kevbo is right when he says that "Hot thin wire has even higher resistance than cold thin wire, so generates even more heat."
Reply With Quote
  #32  
Old 03-09-2006, 01:33 PM
Sean Factotum Sean Factotum is offline
Guest
 
Join Date: Jul 2004
It's also possible that some of the strands inside the cable were broken. Most of the cables people own, in addition to being of smaller overall diameter, are made up of many to dozens of extremely small-diameter cables stranded together (like a rope.)

As the cables get older, flexing it weakens the individual strands, causing eventual breaking of some of those strands. As each strand breaks, the same amount of current still has to go from the good battery end to the bad battery end, but there are less individual conductors available to do this (a broken wire will not pass current.) More current through each individual strand will increase the heat generated in that strand.

As the cable gets older and more strands break, this process increases, and you'll eventually get to the point where too much heat is generated for the cable to dissapate to the surrounding atmosphere.
Reply With Quote
Reply



Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 01:39 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2014, vBulletin Solutions, Inc.

Send questions for Cecil Adams to: cecil@chicagoreader.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Publishers - interested in subscribing to the Straight Dope?
Write to: sdsubscriptions@chicagoreader.com.

Copyright © 2013 Sun-Times Media, LLC.