I’m trying to convince Anagramless Guy that jumpstarting my car isn’t harmful to his. He got really pissed off last night when I went to go urinate while we were in the process of jumping my car, because everything was attached, and he didn’t want to leave the cables attached longer than they needed to be.
Help?
I haven’t kept up with this but I don’t think the cautions about jump starting are because of possible damage to the batteries. Rather I think they are about possible damage to the many electronic devices in modern cars. Not the least of these is the electronic voltage regulator in the alternator circuit.
As for me, I always hook up the positive lead first, on a negative ground circuit, and disconnect it last. Other than that I take no special precautions although I must admit, jump starting is a rare event with me.
Here’s a site that contains jump start procedures and another from battery maker Exide.
The only caution that both mention is to make sure that the ground connection on the car being jumped is on the engine block away from the battery. This is so that the spark that always occurs when the connection is made or unmade doesn’t ignite any hydrogen that might be around the battery.
One of the sites does say that some cars might not tolerate the surges that occur in jump starts and to check the owner’s manual for any jump start cautions.
Assuming that only the jumper is running, and the jumpee hasn’t been started yet, leaving the jumper cables connected is just adding a second battery in parallel with the first. It doesn’t really change the dynamics of the circuits in the jumping car outside of placing a load on the alternator. This is no different than if that car were started up with a lowish charge in its own battery.
It’s my understanding that the cautions about damage are, as the esteemed David Simmons suggests, due to the many electronics in modern cars, and in addition that the particular danger arises when the jumpee attempts to start the engine while the jumper is running. The extremely large draw of a starter motor can apparently mess up some of the electronics of the running car. To avoid this, one hooks up the jumper cables, charges the jumpee’s battery for a while, and then either disconnect the cables before hitting the ignition, or shut the jumper’s engine off before hitting the jumpee’s ignition. Or so it has been explained to me.
Dave that first link has some really bizarre directions for jump starting. Things I have never seen in over 30 years of dinking around cars.
Remove the battery caps? WTF for? In case the battery starts to gas? That’s why a vent is built into the battery.
I agree with making the last connection away from the battery. Battery explosions are impressive, and can spray acid into your eyes.
The only caution I would add is do not disconnect the jumper battery the instant the jumped car fires, wait a few seconds and allow the idle to stabilize before disconnecting the jumpers. This will allow the charging system to settle down and may prevent a voltage surge when the jumpers are disconnected.
I agree. The only possible reason I can think of would be that the vent’s might be dirty or hydrogen might be generated at too high a rate. Maybe just to be doubly sure, like wearing a belt and suspenders. And on a lot of current battery designs the caps aren’t all that accessible.
Just hooking the batteries up is bad as well–discharging a fully-charged battery too fast (like, connecting a good battery to a uncharged battery) can damage the good battery as well.
And often it’s not really much help anyway, unless you KNOW that the dead car was simply left with the engine off and with its lights on until the battery drained. If a car’s electrical system can’t hold a charge, then either the alternator or the battery is already bad, and if either one is bad it tends to ruin the other by over-taxing it. If you “jump” the dead car, all you are doing is storing a charge in the battery that the engine really runs off of for a short time–a mile or two–before dying and “needing a jump” again. No car normally “just needs a jump”: if it does at all, something in the electrical system is bad and needs to be replaced anyway, and jump-starting it won’t fix that.
The right way used to be to disconnect the +terminal from the dead car’s battery, jump start the dead car by hooking the jumper cables to the -terminal and +cable, and then disconnect after the dead car was running–and then the dead car’s +cable could be reconnected to their own dead battery. This way, they were burning up their alternator but not yours, because the “jumping” car was never attached to any dead battery. Many newer cars caution against running them with no battery attached however.
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So much poor information in one post, where to start?
Do you have a cite that jump starting a car is harmful to the good car? I have never seen this in over 30 years in the auto repair business.
–You don’t hook up an alternator to a fully dead battery, because alternators are not designed to charge a dead battery, they are designed to keep a fully charged battery fully charged, or to top up an almost fully charged battery.
–The reason new batteries come fully charged is because your old battery is dead. It would be pretty stupid to buy a dead battery. With that said, back in the day when batteries were stored dry (dry charged) after filling, it was necessary to top up the charge with a battery charger.
–I would like to note that you are not hooking your alternator to just a dead battery, you are hooking it up in parallel to your (presumably) fully charged battery. Electrically this is kinda like hooking your alternator to a very large 1/2 charged battery
Again Cite? Unless you had a dead short in the dead car and the jumper cables from hell there is no way in hell you are going to conduct more current through those cables that your starter draws every single time you start your car.
Now this I agree with, and you are 100% correct.
NO, NO, NO! Do Not do this
This is dangerous advice and can make a very large paperweight out of a perfectly good car that had it lights left on.
The battery acts as an electrical shock absorber in the system. If you disconnect a battery cable with the engine running the alternator no longer has the battery to act as a shock absorber and the voltage can run wild damaging the alternator and various electronic control units in the car. While this was an OK procedure back in the days of DC generators, doing this on a modern car is tempting Mr. Murphy to come kick your ass. Don’t get me wrong, you might not kill every control unit in the car the first time you spike them, but do you really want to gamble a few thousand dollars worth of onboard electronics that it won’t happen?
There are some cases where jumping is the obvious solution that you are neglecting. Around here, the #1 circumstance in which cars are jump-started is -40 degree mornings. In extremely cold weather, batteries don’t produce anywhere near as many cranking amps as they do when they’re warmer, and to compound the issue engines are harder to turn over when they’re colder. A good battery (i.e., one that has gobs more cold cranking amps than are normally needed) and a block heater are ordinarily sufficient to resolve this issue (and oilpan heaters and battery warmers are good too), but there are certainly times when perfectly functional cars simply won’t start on the charge stored in their batteries. Jumpstarting them is the obvious solution, as the alternative might be waiting till March.
As Rick said a charged battery acts as an capacitor so removing the cable as quick as possible is the wrong way to go,leaving them in place for a while allows the flat battery to collect some charge and reduces the chance of a spike.
…Now knowing these two facts and assuming you have only regular jumper cables, there is no way to connect a good car’s electrical system to a dead car without overloading either the good car’s battery or alternator. When you connect the jumper cables, you are connecting the good battery in parallel to a dead battery–and there is a HUGE rush of current as the two equalize, and this far exceeds the battery’s rated current draw, damaging it. And then the running car’s alternator is left trying to charge up two batteries (both approximately 50% charged, essentially the same as one battery 0% charged!) as well as operating another car’s starter–and this likely exceeds the alternator’s current capacity, damaging it.
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::: sigh::: perhaps you did not read what I posted
Starter draw (depending on the car of course) varies from about 100 Amps to as much as 300 Amps under normal conditions. To properly load test a battery a technician will load the battery to 1/2 of it’s cold cranking amp rating for 15 seconds. For the 800CCA battery in my car that is a 400 amp load. Cite Cite #2 Cite #3 . So the question becomes just how many amps do you think you are going to draw through those jumper cables? Most cables won’t transmit enough power to start a truly dead battery, just a mostly dead one. They are probably allowing 50 Amps or so to go through the cables. The cheaper the cables, the less current that will flow. The training center I am driving to later this morning has one car with a dead battery (bad cell) Just because this is GQ when I drag out the cables, I will put an ammeter on them and report back the results.
You can’t have it both ways, either the battery is being drastically discharged (it isn’t) or the alternator is being overloaded. Which side of this argument are you going to take? Again to properly test an alternator a technician will connect a carbon load pile to the car run the engine and load the system to see if the alternator will produce its rated output. Again for my car that is 180 Amps. Do you really think that jumpstarting a car is going to exceed that value? Cite (PDF!)
How do you figure that the battery on the good car is at a 50% state of charge? If the huge rush of current you describe did in fact take the good car’s battery down to 50% that would require hundreds and hundreds of amps to flow. That is arc welding not jumping a battery. In reality what happens is you attach the jumper cables there is a flow of current. (Off the top of my head, I’d SWAG it at about 15-25Amps) this is for sure in the realm of normal for a car’s electrical system, and will do no damage. When the starter is engaged on the dead car, the amp flow will increase up to the lesser of
A) What is necessary to start the car.
B) The capacity of the cables.
Rarely would I expect the current flow to exceed 100 amps. But as I said, in the interest of fighting ignorance I will connect the VAT 40 and see just how many amps it takes to start a dead car.
When come back bring cite (and pie)
Many cars today have a delay built into their alternators so that they alternator does not start to charge the instant the car starts, and the charge ramps up over several seconds. Jumping one of these cars requires the cables to be left in place for a few seconds.
I wouldn’t want to leave your stone dead battery connected to my good battery any longer than necessary either. I don’t want my battery getting drained trying to charge up yours.
As far as I’m concerned, jumping a car means you hook up 4 connections, start the dead car, and unhook 4 connections. Once the connections are made, all you need to do is start her up and disconnect, you can’t wait all of 20 seconds to take a leak? If it takes longer than that to get your car started, then there’s something wrong with your car, and you’ll drain my battery as sure as you drained yours.
OTOH, what he could have done is disconnect the ground on your car and hold it until you got back, that way the circuit is not complete you can wait all day without a problem.
I’ve jumpstarted many vechicles. Plow trucks and heavy equipment with my ’93 Pathfinder.
In the case of an absolutely dead battery, I have had to leave them hooked up for a few minutes before trying to start the dead vehicle. And of course the car giving the jump will be running.
I’ve got good quality jumper cables, and make sure to make a good connection on both vehicles. I usually ground the negative to the ground cable that is often hooked up to the alternator mount. It sometimes seems that an absolutely dead battery in the hook up is draining to much power getting charged and not enough amps can be diverted to properly turn the starter. After a few minutes, it’s usually no problem.
As long as the alternator on the ‘dead’ but now running vehicle is in good shape, I will pull the cables off right away. Then let it run or drive it for a good 20 minutes or so.
I really doubt there is any problem leaving them hooked up for 20 seconds to take a pee.
But, I prefer to never leave two running unattended cars in this position. Vibration can make one of the clamps fall off.
So this might support my idea that is a good idea to hook the running car up to the dead one and wait at least a few minutes before trying to start the dead car. At least it will pick up a little charge before the starter kicks in. This has worked for my tractor a number of times.
I’ve used my Pathfinder to jumpstart completely dead vehicles probably 30 times.
No problems so far. In fact, my 200,000 mile 13 year old Pathfinder has never not started, even in -20 f. I’m on my second battery.
DougC, you do realize that Rick has been in the auto repair biz for a bunch of years, right? A look at the cites he’s provided prooves that at least one of your long held beliefs is flat out wrong. I suspect more will fall by the wayside soon.
Thanks Rick. I’m learning some good stuff today.
I’m back with results. OK, the donor car is a 06 Volvo with a fully charged battery. The dead car is a '98 Volvo with a bad battery. The last time I was at this training center I diagnosed a dead cell in this battery. Today, I tried the key and it cranked very slowly. So I disabled the ignition system and cranked the car until it would not crank any longer, and I just got a click when I turned the key.
Hmm about that huge rush of current. So here is what I did, I parked the electron donor car next to the front of the dead car. I hooked the jumper cables to three of the connections. I hooked up the Sun VAT 40 with the amp clamp set to register the load placed on the system by the dead battery. With the keys off in both cars, I hooked up the last connection. The draw was, are you ready for this? ONE Amp. One single solitary amp. About what a taillight bulb draws. So I started the donor car. As the voltage in the system when up so did the current flow. With the donor car running, the current flow was 8 amps. When I turned on the ignition of the dead car the amp flow increased to about 18 amps. This is 10% of the rated output of the alternator of the donor car.
So with the ignition disabled I cranked the dead car and observed an amperage flow of 60-65 Amps. Since this car normally takes about 100 Amps to start this tells me that the donor car was supplying about 65 Amps and the dead battery about 35 amps.
With the ambient temp about 80F these numbers are not shocking to me in any way. With the exception of the amp draw from when both cars were off, the numbers were about what I expected.
Now if I were to leave the car with the bad battery alone for say a month, these values would increase, but in no event will the dead car draw more amps out of the good car than its starting system would with a full battery.
None of these numbers show the battery or charging system of the donor car in any danger or risk of premature failure.
I hope this clears this up.
I know, it’s not pan fried semen, but then again what is?
Unless you buy the cables with really good clamps I not sure I would even put their current capacity that high.
Ive see a few things happen:
I’ve seen people blow the rectifier out in the Alternator in either vehicle. If a problem is going to take place it’s going to be at the connection if a surge takes place or if the car with the dead battery puts too much load on the alternator. Sparks have blown up leaky batterys that are producing a gas.