Give me the the Straight Dope on jump-starting cars

I’ve jumped cars maybe two or three times in my life. Each time we look at the instructions on the jumper cables, and also at the instructions in the dead car’s owner’s manual. Occasionally friends or passers-by will offer advice from past jumps of their own.

There are always conflicting instructions.

So, my questions are:

  1. Should the live car have its engine running before attaching the cables? Does it really matter?

  2. Does the order in which the clamps are attached actually matter? (That is, does it need to go dead-car-positive, live-car-positive, live-car-negative, ground?)

  3. This past weekend, when jumping my fiancee’s car, our downstairs neighbor insisted that grounding the fourth clamp onto a piece of metal inside the “dead” car wasn’t necessary, and that one can simply attatch the negative clamp to the dead car’s battery, forming a complete circuit between the cars. Would this actually work?

  4. After the dead car is jumped, how important is it to unclamp the clamps in the exact reverse order that they were applied?

  5. Can/should the clamps be removed while either car is still running? Both cars? Only the dead one? Only the live one?

I’ve read/heard conflicting answers to all of the above. Given that one result of careless jumping is (apparently) a great big explosion, you’d think there would be more set-in-stone procedures for this operation. Or does it really vary by automobile or battery type?

Any wisdom on the subject would be greatly appreciated.

thanks,

-P

This seems to be a pretty good overview.

Also:
http://www.womanmotorist.com/sfty/sp-jump-start.shtml
http://www.csaa.com/global/articledetail/0,8055,1004010307%7C2065,00.html

They all seem to agree on the steps.

Living in the far north for many years has necessitated many jumpsatrs for me and I’ve done and seen it done a variety of ways. It’s supposed to go like this. Start with both cars off. Connect the positive clamp to the dead car and then ground the negative clamp. It actually will work just on the terminal of the battery, but it works better if you find a big hunk of metal. Go to the live car and basically do the same thing. Clamp the positive and ground the negative. Make sure that you do not ever let a positive and negative clamp touch each other as long as the other end of the cable is hooked up to a battery. Start the live car, then start the dead car. Sometimes it takes a little while for the dead car to fire. When the dead car is going, unhook the positive then the negative clamps from both cars. You don’t have to turn off the cars.

I have seen people hook and unhook clamps in all sorts of order and it doesn’t really seem to make that much difference as to effectiveness, you just want to be careful of touching a negative clam if the hot side is connected.

I appreciate the links, Telemark, but they do serve as an example of conflicting messages. Specifically, the first link warns you to have the live-car engine running from the get-go, while the second link instructs the jumper to attach all the cables before starting the live-car. Which is it, I wonder. Or does it matter at all?

-P

Parthol, as a matter of effectiveness it doesn’t really matter. As a matter of safety, it’s better to start with the live car off.

You’re going to get three types of answers to a question like this:

  1. People who feel they have to answer on car questions, but really don’t know shit, and just want to post. This seems to happen a lot in GQ lately.

  2. People who will tell you the “by the book” method.

  3. People who will tell you the “possibly more dangerous but more practical IRL methods”.

I’ll try to stick to categories (2) and (3)

(2) Many people say for safety to have the car off, so you are not messing around moving belts and so forth. (3) Personally, it does not make any difference. Just don’t get your scarf or hair caught in the accessory belt, and you’ll be fine.

(2) You should connect positive to positive, and then live car negative to dead car ground. (3) In practice, it can be difficult to find a good ground on the dead car - both from a resistance standpoint, and a “not having alligator clamp falling off” standpoint. I typically go to the dead car negative.

(2) Yes it works, but it is potentially dangerous, because it typically makes a spark, which can ignite hydrogen or other flammable gases in the engine bay. (3) The negative of the battery is connected to the car’s body. They are all one circuit. But due to corrosion, or the potential of choosing the wrong piece of metal, the piece of metal you choose may not be a good connection. Thus, I typically go from negative to negative. See prior answer.

(2)(3) Unless you want to play “home mechanic arc welder”, you should typically disconnect the positives first, then the negatives. This is because if you disconnect the negative on one side, sometimes you lay the clamp on something still attached to the body, and still have a circuit. Whereas if you take the clamp off of the positives first, there is no other place that goes to them, and thus no circuit.

(2)(3) Most manuals say to remove the clamps as soon as the dead car is started. You don’t want to immediately turn off the dead car, as you’ll just be re-jumping it again. So take the clamps off of it. And you don’t want two live clamps still attached to the love car, unless you want to play arc welder (see above), so you want to do it as in item 4 above.

You know, I was a mechanic for a while. Been an engineer for much longer. I knew several people who also work as mechanics, and in the auto towing/emergency road service field.

Not one of them, in a combined total of more than a century of experience, ever had a battery explode from a spark.

Yes, sometimes you do hear of them. And since this is the SDMB, someone will either post of an experience, or make up one, to prove me wrong. But I cannot help but feel that it is a highly unusual situation, which more results from jumping a battery which has sat for a while, charging, while in an enclosed space (garage) with no air circulation.

OR…I did know of a person who set fire to some oily/ether-soaked rags which were being stored under the hood of a car once. But if you’re going to store flammable ether-soaked rags on your engine, well, you kinda get what you deserve…

All you’re doing is hooking up the dead cars starter to the live cars battery. As far as function goes, there’s no difference in sequence, car on or off, etc. The only order that counts is:

Hook up cables.
Start dead car.

Generally, people like to leave the dead car hooked up to live car for a few minutes - this is to facilitate charging the dead cars battery. Realistically though, it’s not strictly necessary - the alternator on the dead car should be taking care of it.

I hate to make a simple think like jumping a car problematic, BUT it depends on your car.
In most cases, simply making sure you don’t cross the connections will do. Where you ground the neg. clamp usually doesn’t matter, as long as it is solid. Sure you can use the neg. post in most cases. Whether the jumper car is running or not isn’t a big deal. Unless the dead car pulls so much off the battery you may wind up w/ two dead cars.

I say refer to your owners manual because since the introduction of computer systems into vehicles, it is very easy to damage the system by jump starting it.

I usually hook up w/ both clamps to the “hot” car first.
Then clamp the ground to the dead car next. With the key “off” in the dead car, I touch the pos. clamp and notice if there’s a spark or not…if there is you have a drain on the battery while the vehicle is off…it could be just a minor electric connect ie: radio, clock, accessories…etc. BUT IF THE SPARK is very noticable then you have a problem, either way hook the pos. clamp to the pos. post on the dead car…idle the hot car up a bit , give it a minute or two and see if it’ll start.

If it does AND you’re interested…leave the “dead car” running and then disconnect the pos. terminal from the battery.

IF the “dead” cars dies again…it’s an electrical problem…usually an alternator. IF the car runs w/ the battery disconnected, you simply have a bad battery. Get it checked, most autopart store will do this for free.

What is the year model and make?

(possibly TMI here)

First of all, some terminology clarification. “Jump-starting” has a long history of meaning “starting the car without the starter, by getting it rolling and then popping the clutch with the transmission in gear and the ingition switched on.” What you are asking about is properly called “using jumper cables,” popular abbreviated to “jumping” and, unfortunately, sometimes “jump-starting.”

1. Should the live car have its engine running before attaching the cables? Does it really matter?

It doesn’t really matter, but it eliminates the possibility of making the live car harder to start due to a drain caused by the dead battery.

2. Does the order in which the clamps are attached actually matter? (That is, does it need to go dead-car-positive, live-car-positive, live-car-negative, ground?)

Not in terms of the procedure working. These instructions are an attempt to maximize safety and minimize confusion for people who really don’t quite understand what they’re doing. (In other words, it’s a lot quicker and easier to list a few specific steps than to educate about all the details involved.)

3. This past weekend, when jumping my fiancee’s car, our downstairs neighbor insisted that grounding the fourth clamp onto a piece of metal inside the “dead” car wasn’t necessary, and that one can simply attatch the negative clamp to the dead car’s battery, forming a complete circuit between the cars. Would this actually work?

Sure. The recommendation to attach the clamp to a hunk of metal has two advantages: it gets around a possible poor connection of the negative cable end to the negative battery terminal, and it places any possible spark farther from the battery. But in most situations, it doesn’t make a practical difference.

4. After the dead car is jumped, how important is it to unclamp the clamps in the exact reverse order that they were applied?

Not very. Again, it’s stated that way to maximize safety and minimize confusion.

5. Can/should the clamps be removed while either car is still running? Both cars? Only the dead one? Only the live one?

There’s no reason to shut off either car. There is a reason to NOT shut off the dead car–it probably won’t restart if you do.

I’ve read/heard conflicting answers to all of the above. Given that one result of careless jumping is (apparently) a great big explosion, you’d think there would be more set-in-stone procedures for this operation. Or does it really vary by automobile or battery type?

Batteries give off hydrogen gas (very easily ignited) when they are being charged. The amount is generally negligible on the live car, but sometimes can be significant on the dead one. Creating a spark near the battery is potentially dangerous and one should be quite careful to avoid this. The main thing is to never let the jumper cable ends touch anything (each other, a battery terminal, part of the car) that could result in a spark (boom!) or a short (goodbye, alternator). What’s important is to keep them from touching the wrong thing at the wrong time. The typical instructions are trying to achieve this.

*If it does AND you’re interested…leave the “dead car” running and then disconnect the pos. terminal from the battery.

IF the “dead” cars dies again…it’s an electrical problem…usually an alternator. IF the car runs w/ the battery disconnected, you simply have a bad battery.*

Disconnecting the battery with the engine running can result in frying the alternator. Don’t do it.

If I’m controlling the operation, ie., my car is the “live” one:

Leave it running.

Hand one end of cables to other driver, strongly reminding them to not let the clamps touch.

Connect both connectors to battery terminals on mine.

If other driver seems to know what they’re doing, tell them which color you stuck on which terminal, and let them connect their own car.

Otherwise, go take the other end of the cable from them and do it myself, connecting to the appropriate terminals of their battery in either order.

Have them start theirs. Once it starts immediately disconnect both clamps and either hand the end to somebody with another admonition not to let the clamps touch, or carefully keep them apart myself while I disconnect my car.

Most of the “procedure” is to assure that you don’t inadvertently create a short - I figure the best thing to do is go ahead and deal with having a “hot” pair of connectors that you are aware of, and are consequently careful with. I connect battery-to-battery to avoid having to look for a good ground, and to be sure you’re getting positive-to-positive, negative-to-negative. Who knows, you might wind up jumping an old Jaguar.

There’s some misinformation here with regard to the negative connections.

1. You have two negative connections: one on the good car and one on the dead car. One of the negative connections should be to something other than the battery’s negative terminal. I usually use the alternator bracket, but just about any unpainted metal bracket or chassis item will work (assuming it’s somehow connected to the battery’s negative terminal). Just make sure you get a good connection.

2. With regard to #1, it is not necessary to do this to both cars. On one car, you can connect directly to the negative terminal. And it does not matter which car you choose.

3. Most important – the ground connection to the unpainted metal bracket should be made last.

4. When disconnecting the cables, the ground connection to the unpainted metal bracket should be made first.

by the way… in such case, you should take your car home and put it on the battery charger immediately…

the alternators job is not to charge a battery from being almost dead, its to keep the battery topped off.

if you repeatedly kill your battery (like if you are very forgetful and leave your lights on all the time) and just let the alternator charge it up you should expect to have alternator problems soon…

very true, i damaged an ecm in my car jump starting a friend at school a few years ago… no big sparks, i know what im doing. the ecm in my car is real sensitive to stuff like that, so i usually dont help out other people (sounds selfish… but if it costs me $50 then its not worth it), this time though it was a friend and i hoped for the best. oh well. I dont do it anymore though :slight_smile:

There’s some misinformation here with regard to the negative connections.

What misinformation, exactly?

      • Modern cars generally don’t have the extra electrical capacity to jump start another car. Doing so can damage the running car’s alternator or other related parts, as something clever and I have noted in earlier posts. I myself will only jump start another car if I know that it has been left with its lights on or similar reasons–there’s some simple reason the battery got drained. If somebody’s car “just ups and dies and won’t restart”, there’s something else wrong with the electrical system, and jump-starting it isn’t going to fix that.
        What happens is:
  1. Their alternator fails for whatever reason, and the alternator can’t recharge the battery as fast as the engine depletes it, so engine runs ignition system off battery voltage until battery is depleted, and car dies.
  2. You come along, hook your good alternator up to a dead battery and attempt to recharge it (and alternator install instructions say you’re not supposed to charge a dead battery straight off an alternator, by the by…)
  3. Their car starts right up, so everything’s just ducky, right? Wrong–their deal alternator still don’t work, so their car is only running off the battery voltage you charged their battery up with. You have seriously shortened the life of your alternator, and they’ll get perhaps another five miles down the road at best.
    To that I say, there’s no point.
    ~

A piece of advice from the “No good deed goes unpunished” file:

On a very (sub-zero) cold night on the street where I lived I agreed to give a jump start to someone who couldn’t get their car started. We hooked up the jumper cables and they tried and tried to get the engine to fire. We never got it started and gave up. I pulled my car into the garage and turned it off. My car would not restart. The battery was dead.

Think of it this way. By jump-starting you are essentially hooking up a good battery to the engine without replacing the dead battery. When you try to start the engine you are drawing electrical power off the good battery and draining it. Even though the good engine/battery is running you can still draw the good battery down to the point of it almost being dead.

If you give somebody a jump-start and it doesn’t catch immedietly, make sure you give your battery time to recover before you shut the engine off (or use a recharger to bring you battery to full recovery). Also you should moderately rev your engine during the recharge so that your alternator is providing full electrical charge to the system.

Just a helpful hint.

Guess work is not a good idea with electricity. Follow rules. Carefully, especially if you are not usually fooling around with voltage.

The thing you have to remember is this:

Car batteries generate Hydrogen, sometimes in enuf quantities that the spark that results from attachin the clamps to the batteries sets off an explosion strong enuf to crack the battery and spill acid on the person putting on the cables. It doesnt happen ofetn but it does happen. I had a battery explode on me strong enuf to put a small dent on my SUV hood. Fortunately, I was well away from my hood.

The safest way is to connect both positive cables first. there is no closed circuit here so no sparks. Connect to the the metal frame of the live car, then to the metal frame of the dead car. Any spark that gets created while doing this is now well away from the battery and any hydrogen that is generated is away from the spark. The best, safest and most convenient way is to get jumper cables that connect to cigarette lighter outlets.

As far as the live battery having its engine running, if the dead car just starts up and keeps running, having the live car’s engine running isnt a factor. If the dead car doesnt start up or if it does and it immediately dies after the cables are dettached, then having the live car running would be better because you are now using the live car’s alternator to put enuf charge into the dead cars battery to sustain it after starting.

I have better luck connecting directly to the soft, lead battery clamp than the frame. That’s why (in a previous post) I recommended only connecting to the frame at one car (not both). It should be safe as long as this is the last connection made when hooking everything up, and the first broken when removing the cables…

Slight hijack. If you are ever called upon to jump-start a motorcycle, do it with the engine OFF. The miniscule current needed to start a motorcycle is easily supplied by the car’s battery alone; having the alternator running appears to fry motorcycle electrical systems.

My motorcycle’s battery delivers about 200 cold crank amps, and normally runs at 1.4 amps.