BMW owner looking at Lexus

Okay, you've made the case you can sometimes jumper a dead car without starting the donor car. We get it.

What you haven't made the case for is why it doesn't help to have the donor car's engine running.

You're right, the majority of the power to turn the dead car's starter is supplied by the donor car's battery, but a modern car's electrical system can typically supply 50+ amps of additional power, significantly raising the voltage the starter motor in the dead car will see. And you're right, the alternator is not designed to handle starter motor current levels, but it is designed to put out its rated output into a load and this output will significantly reduce the battery voltage required to overcome the extra esistance in the jumper cables.

You continue to state that jumping a car from another battery is the same situation as starting a car its internal battery. It is not. A typical run from the batter to the starter motor is less than 3' while jumper cables are typically 12'+. Any you've got the resistance of the two connections. Finally, many jumper cables are smaller gauge wire than the starter motor wiring in the car. All in all, there could be 10x as much resistance in the jumper cables and connections than the internal car wiring. So just because a battery will start a car doe NOT mean that it can start another car through jumper cables. It needs all the help it can get.

And you haven't made the case that there is anything that will be harmed by having the donor car running. You realize, don't you, that after starting, the dead car will be running? So it's sensitive electrical system with all those "diodes" is hooked up the the donors car's.

There is a lot of urban legend running around about how the electrical systems on today's cars are exceeding fragile and you shouldn't hook up anything without risking damage. I am unaware of anybody who has ever had an issue with having the donor car running during a jump, assuming everything is hooked up properly.

Here are a few net references talking about proper jumping procedure. While some say you can attempt to do the start without the donor car running EVERY ONE suggests starting the donor car to assist the jumping process:

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- Mark

Reply to
markjen
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I have always been one to jump a car with the engine running on the donor car. What I have found is that sometimes the dead car will only crank slowly with the donor car not running. When I start the donor car, the dead car now cranks faster. In my experience, there IS a difference if the car is started or not.

- Phil

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>> - Mark

Reply to
Phil

Because of the damage you could cause to the donor car's electrical system.

Reply to
Elmo P. Shagnasty

For example, from

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> Try this method before you try Method #2. A guy named Doug > from Canada sent in this tip. He's boosted cars a lot in > winter and has found that it's critically important to shut > off the boosting car¹s engine during the moments of actual > cranking of the car with the dead battery. This does however > reduce available power to the dead car because the boosting > car's alternator isn't running, Doug has had a number of > "live" boosts make the boosting car's alternator diodes > either fail outright or die very prematurely from the > extreme current draw on the alternator while cranking the > engine on the bad car. So the safest method is to have the > good engine running a few minutes to charge the dead > battery. Then shut off the good car's engine and disconnect > the cables and start the bad car's engine.

Not to mention that nowadays, with everything in the car being controlled by the computer, you put the donor car's computer at risk with this process.

Reply to
Elmo P. Shagnasty

OK, call me ignorant if you wish, but I found this statement in your referenced URL interesting. "...Jump starting a battery can crank 300+ volts through your system and the transients can destroy equipment...".

300+ volts? How, on a 12 volt system? Maybe that was supposed to be 300 amps?

In any event, I checked several Internet sources. The bottom line is that some manufacturers have specific guidelines on whether their car, as a donor, should be running or not when jumping a car. It appears that there may be some risk to the charging system in some cases, and no risk in others. For me, 100% of all jumps have not resulted in any kind of failure or harm to any part of the electrical system. Doesn't mean it can't happen to me on some other car I might own.

- Phil

Reply to
Phil

From what I have found, the key word in your statement is "could", or perhaps more appropriately, that word followed with the word "possibly".

- Phil

Reply to
Phil

When you start your car with your own battery you do NOT simultaneously try to charge a dead battery which is exactly what you propose we do. Bad analogy. If fact, none.

Reply to
GRL

Not exactly sure what you're getting at. Nobody ever said "start the donor car while you're starting the dead car".

All I said was, it doesn't take an engine to start your car in the garage, and it doesn't take an engine to start a car in the parking lot. All it takes is a battery. The bit about "start the donor car, give it more juice" is just random people not thinking it all the way through.

Add in the potential for damage to the donor car, and what do you have? Just hook up the cables and start the dead car--that's what you have.

Reply to
Elmo P. Shagnasty

Exactly. And the more complicated the auto electronics get, the more risk you take.

Some would say "it never happened to me, therefore there's no danger". Those same people would say the same thing about seat belts. "I've never been in a wreck, so I don't ever have to wear a seat belt."

Reply to
Elmo P. Shagnasty

And if you're in a wreck and aren't wearing a seatbelt, you could be hurt severely. Possibly.

Reply to
Elmo P. Shagnasty

"...The bit about "start the donor car, give it more juice" is just random people not thinking it all the way through...". Well OK, but explain why in my experience, having the donor car run makes the dead car crank faster.

- Phil

Reply to
Phil

Your analogy could be applied to anything, including walking outside and being hit by a meteor. Depending on what it is, it is silly to act in a way to protect oneself from everything that could or possibly could happen. If you did, you would be a hermit, and surely not jumping starting a car for fear of a hydrogen gas explosion.

As an aside, I ride and raced sport motorcycles. I know all about accidents, wrecks and safety gear. I just think your analogy is a bit of a stretch. Possibly.

- Phil

Reply to
Phil

I'd like to know HOW the electronics are at risk. I would say that if the car maker says there is no risk, then there is no risk, while if they say there is, then I will abide by that. But, I know you're point. Same thing for people driving around with missing lug nuts, bald tires, or other risks that don't happen to result in serious consequences. In the jump start case, the results can be more predictable with car manufacturer information, and/or knowledge of the electronics (unlikely for mere mortals). I'll stick with the manufacturer says.

- Phil

Reply to
Phil

Fair enough. At least you have something to hang your hat on if something goes wrong.

I wonder if any manufacturer says "if you're using this car to jump start someone, go ahead and start it first".

Likewise, I don't know how often they'll say "be sure to leave your car off".

Reply to
Elmo P. Shagnasty

Well, sure. But people are arguing for "you HAVE to turn the donor car on!" which is flat-out silly.

That's all I'm saying.

If the donor battery can start the donor car by itself, it can start another car by itself. It's really simple. And if it can do that, and if there's some chance of damaging the donor car by having it running, why bother running it if you don't have to?

I'm curious why people would WANT to run the donor car if they didn't have to, if the donor battery by itself was capable of starting the dead car--as undoubtedly it is, if it starts the donor car itself.

Reply to
Elmo P. Shagnasty

But is faster cranking your concern, or is getting the dead car started your concern?

I think getting the dead car started is the concern.

How about this: try to start the dead car first WITHOUT starting the donor car. Take the safe route. If that doesn't work, boost the available power by starting the donor car--while knowing that you've opened the door to possibly--not certainly, but possibly--damaging your alternator, engine control computer, etc.

Reply to
Elmo P. Shagnasty

Of course the object is to start the car, and to do that, reasonable cranking speed is needed, which I find sometimes is not available just by connecting the donor car without engine running.

- Phil

Reply to
Phil

I checked the owner's manual in my 1998 BMW M3 and 1995 Honda Odyssey van.

The Odyssey manual says to start the donor car, after cables are connected. The M3 manual says to start the donor car if needed (dead car cranks slowly). The M3 manual also says to turn on a lot of electrical accessories when disconnecting the cables to prevent surge at the voltage regulator. The manual does not clearly state if this should be done for the donor or dead car. The manual also says the process is the same whether the BMW is the dead car or donor car.

Other than limiting electrical surge on the BMW, apparent little risk with the car running. Can't speak for other cars.

- Phil

Reply to
Phil

I have understood your point. More often than not in my experience, I can not get the dead car started unless the donor car is running. My cables are long, but are also very heavy with robust connectors. I am well aware of resistance in electrical wiring. Your logic makes sense, but in practice, I find significant power is lost between the good battery and the starter motor on the other car, even with high quality cables. Odd, since on the BMW, the equivalent of jumper cables runs from the rear mounted battery all the way to the front of the car where jumper terminals are located. This may work due to heavy cabling, superior connections, or..? Depending on the situation, it appears that sometimes that sufficient battery power is available to start the dead vehicle. As an example, I have jumped my motorcycle with a car, and in that case, the car battery by itself is more than sufficient.

- Phil

Reply to
Phil

I'm not sure if this helps but I had the brain blow cuz of using a wrong procedure jumpstarting a car, they used one of those plug in transformer things, get this it was a BMW tech at the dealership while i was pcking up my car after having some work done to it, great huh go to pick up your car after spending 3 grand on something that should have been covered under warranty and its filthy dirty and has a dead battery then the bone head jumps it wrong and blows up the curcit board, THAT S WHY I'LL NEVER OWN ANOTHER BMW AS LONG AS I LIVE!!!!! THIER CUSTOMER SERVICE SUCKS,

mIKE

Reply to
Simmonsmc

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