Patricinus, thicker cables can carry more current w/ less heating due to internal resistance. If the dead battery is really dead, you could be pulling a few hundred amps through the cables and cheap ones can get pretty hot.
MC, hey, believe whatever you want.
Everybody else, I encourage you to put people before the possiblilty of damage to a peice of hardware.
Geeze, pmh, you sure know how to make a guy feel like a piece of shit. 
Ok, I’ll read my owners manual. And if it doesn’t warn against jump starting, I’ll consider helping next time I’m asked.
Thing is, I haven’t been asked to jump a car for years. I think it may be a geographical thing. You know, cold weather and all.
50 times? Goodness.
Peace,
mangeorge
50+ sounds like a lot, but spread over 6 years, it’s not all that much really.
And yeah, it’s mostly brutal winters, but also:
cheap friends w/ really crappy cars,
being the only “car guy” at work,
and having a reputation for doing maintenence/repair in exchange for baked goods :).
Also, to be fair, my “current setup” is probably a little beefier than a production one. It includes:
875CCA 30 min reserve battery,
90A (rated) alternator (20% overdriven),
aftermarket regulator w/ bigger home-made heat sink.
Still, if I can do this many, you should be able to do 2 or 3 easily (if your owner’s manual permits).
Yeah, I don’t know what MC has in mind, but his wording, in electrical terms, doesn’t make any sense at all.
Here’s a Web page on car batteries, including jumping them:
http://nyquist.ee.ualberta.ca/~schmaus/batfaq.html
Ray
I’m trying to figure out this “no load” business.
If the dead battery has an internal short, I agree there’s “no load”, because you’ve basically just created a short circuit.
Otherwise, wouldn’t reversing the natural lead/acid reaction (that is, charging the battery) be a “load”? There’s certainly a counter EMF associated with it.
Is the prohibition there on the off chance that the other guy’s battery is dead because of an internal short (in which case jumping it is the LAST thing you want to do), or is my understanding of what constitutes a load incorrect?
I’m not sure I understand MC’s problem exactly. I’ve never been able to end up at the parts store with a bad alternator, I always watched my lights get dimmer as i tried to get home and had to hitch a ride to the parts store the next day. But electrically, no load is an open circuit, maximum load would be a dead battery or a short circuit. If you put a volt meter on a dead battery, it’ll probably read 12 volts until a load, like headlights, are put on it. The only way I can see that there would be a problem connecting a alternator to a dead battery would be if the engine were running at the time. Otherwise, you are going to need to have a jump start to get the engine running in the first place and if your battery and the other car’s battery have the voltage high enough and charged enough to handle the 300 or more amps that the starter motor draws, then I don’t see why the alternator would see anything differently than normal operation once the engine started.
Now back to the original post, a few others have said also, getting someone out of trouble is worth the few bucks that are at risk. I absolutely agree. And when done properly, I don’t think there is much risk. The original answer by voltaire seemed to cover the right way to jump start a car. The reason, as I know, for connecting the last connection to the frame is that if it was not connected right, the last connection would complete the circuit and make a major current draw ( the instantaneous current from a lead-acid battery is near infinite ) and that arc would not be something you would want in the vicinity of a battery that is trying to pump out that infinite current.
Can anyone give an example of an owner’s manual that warns owners to never jump start another car? I am really trying not to come off as flaming (especially not at anyone here), but the claim is absurd. Jump starts are a part of driving A car company would have to be run by idiots to release a new car that was damaged by this routine procedure.
An alternator’s voltage is produced by rotating an electromagnet (the armature) inside the charging coils. Voltage is regulated by controlling the current to the armature with a voltage regulator. The alternator’s output is rectified with a diode network, typically a bridge rectifier (there may be different types of, but diodes do the job.)
After the car is started, the regulator will normally run at full current. It’s designed to and will not blow because it can’t handle the job it is designed to do.
The rectifier can blow if the alternator’s current is too great for the diodes to handle, but output is limited by the size and design of the coils and the alternator (engine speed is also a factor.) So, for the rectifier to blow, the guys who designed the alternator (Delco, Bosch, Mitsubishi, etc.) had to intentionally design the alternator with diodes that could not handle the output of the regulator that they designed. Considering that the big diodes cost just a few cents more than the little ones, and considering the customer relations meltdown that would ensue, they would be stupid to even consider cutting costs with lower current diodes.
With a dead battery, there is a full load on the alternator, but alternators are sized to maintain the battery’s level of charge. For example, I once replaced the belt driven fan on my import truck with an electric fan. The added eight amps of load was enough to drain the battery in a months worth of regular driving. The free charge allows them to test the battery and prevents the situation where a customer returns an alternator because his car wouldn’t start after he swapped the part, jumped the car and drove a few miles. I just called a chain aftermarket parts store; they confirmed this.
As the ualberta.ca link says, your own car battery acts as a filter for the car’s electrical power; high voltage spikes, if any, will be absorbed. If the jump is properly done, the battery’s voltage may drop, but not excessively. Even if it did drop below 5-8 volts, to where the ignition would cut out, it would be no worse for your car than if you removed the battery with the car not running. I’m not an expert on engine management systems, but the data needed to run the car is stored in nonvolatile memory. What else could happen except for maybe the clock resetting on the dash? Were the people at the drive-in within hearing distance of one another? My guess is that either the first person made up a story to get out of doing the jump and the others heard it and didn’t want to hurt their car, or someone mentioned this on the radio or TV.
You may (should) charge the other battery for a while before you do it, but you jump the other car off of your battery, not the alternator. Most alternators don’t provide enough current.
Patricinus Scriblerus, give me a call whenever you need a dead battery jumped (caveat: I live in Central Texas.) I’ll be glad to do it.
I think cornflakes pretty well finish that one off. I’m in North Texas and I’ll be glad to jump start you.
I’ve jumped plenty of people, and been jumped many times. never had a problem. Once, when i was 16, I did it backwards because the really old battery designated + and - with a sticker that had worn away a long time ago. I hooked it up and everything started smoking. I quickly removed it and redid it the correct way and it started right up. Although I don’t recomment this and consider myself lucky i didn’t blow my hands off, it tells me that cars may not be as fragile as we think. Or maybe because it was an older american car, it didn’t have as many computer components.
Has anyone contacted tom and ray, the car guys about this?
Also, I heard someone introduced a battery a couple of years ago that had a reserve that had enough juice to start your car, if it drained because you left you lights on or something. Assuming there was nothing else wrong, this seems like a really good idea. why did it never catch on?
The only way to rid yourself of temptation is to yield to it–Oscar Wilde
I didn’t mean to kill it. I was only playing. Really. 
Sorry for getting obsessed, but automotive and roadside assistance both rate pretty high up there for me. If anyone does have an owners manual that advises against jump starts, I’d like to hear about it. FWIW, my wife’s Neon manual does advise against jumping with a tired battery, but the local dealership does it all the time.
On the other hand, I did just get off the phone with Daimler-Chrysler’s tech line. They gave the “computer surge” story, but when I asked for particulars, they put me on a hold line that hung up (this happens, but it didn’t boost sales with me.) The next time I called (I told you I was obssesed about this), the level one advisor said that she thought that the car receiving might be damaged by the jump (and how is this different from installing a new battery??? I said that I was obssesed, didn’t I?), but that they may have included the warning for liability reasons.
I’m going to try to get more info on this (ob-SSESED!!!) I’ll post what I find out.
Well, at least I’m not ** obsessed ** about spelling…
FWIW, I do recall seeing one manual that recommended jump-starting w/ the ignition switch off (don’t remember make/model, but probably GM (lots of easily fooled sensors)). This would take the alt/reg and computer out of the equation entirely and should make MC happy, but be careful, or you’ll end up w/ 2 dead batteries…
The “battery w/ reserve” was just a smaller battery in parallel w/ the main one. Consumer Reports gave it an unfavorable review (too expensive and not powerful enough) and it went away pretty quickly after that. There are aftermarket boxes available though that will disconnect a battery when it falls below 11.7V.
as long as I’m here, my above battery spec should be 875/300.
hey, what’s an order of magnitude between friends?
In very cold areas you can short the battery & it causes it to get hot & then you start the car. This works, althought I don’t recommend it but I did read its possible.
Cornflakes,
I was playing too, it seemed, though, you had a pretty good handle on the situation.
I’ve jump started people and been jump started in all kinds of situations, and have never had a problem. Although, when you do hook one up wrong, that big spark is a real wake up call to pay attention. But it never caused any real damage though.
A couple of points some people touched on earlier. I always have the car with the good battery running when I connect the cables. The 2 batteries, without the alternator going, will try to equalize the charge. You could end up with 2 batteries, neither of which can start either car. Also, I let the bad battery charge for a few minutes before trying to start the car. That’s because the low battery is already making a major current drain on the good battery and alternator, without adding the 200 or 300 amps from the starter motor. Which brings up another point about the size and quality of the jumper cables. Since the bad battery is not going to be much help supplying all that current, most of it is going to come from the good battery through the jumper cables. Cheap cables with small wires are going to get hot in a hurry.
What’s all the fuss about? A battery is only about $40 these days. Simple enough & cheap enough to buy. duh.
Although one should have the charging system tested just to make sure its working.
Let me add something. If you are calling a friend to come assist you why not ask them to get a battery for you on the way?
[EAT CROW]
According to a friend who works in the industry, a power surge could ruin the electronics. The warning is there to limit their liability if something does happen to the electronics after a jump. He didn’t have an example of it happening either, but the potential for damage exists.
(Just my opinion, but another good reason for this warning is to protect against dishonesty and human error. If someone smokes because a buddy said that they could hop up their car that way, the manufacturer does not want to cover it, even if the customer sounds very sincere when he claims that it died when he was getting a jump. Alternately, they don’t want to pay if someone gets the cables switched or otherwise makes a mistake that screws up the jump.)
Last week, I said that the battery would filter any voltage spikes. After posting, I realized that sometimes people need jump starts because their battery connections have corroded and can’t pass enough current to start the car. This would also limit how well the battery acts as a filter. The good battery should still filter when the cables are connected together, but there might be a spike in the dead car’s power when the cables are taken off (in this case, the car giving the jump would still be as safe as its battery connections are good.)
For added safety, they do make jumper cables with built-in surge protectors (I’m still surprised that a high value surge protector isn’t in the car’s wiring harness), or you can just switch on the headlights of both cars.
In summary: it could happen, but it isn’t a common problem. I’m not worried.
[/EAT CROW]
Sorry, I did proof read it…
“If someone smokes ** the car’s electronics ** because a buddy said that they could hop up their car that way, they don’t want to pay.”
Obviously, this should be checked before jump-starting. True, though, that most people dont even think of it. Free tip: After installing the battery, spread a thin coat of grease over the cable connections. This seals out the air and minimizes oxydation/corrosion.
Actually, the wiring harness does contain a ‘fusible link’ (or equivalent), A high resistance wire (or slo-blo fuse) designed to melt at about 1.5X the max normal operating current. Sadly, though, it does nothing for short duration pulses.