jump starting with donor vehicle running or off?

J

jalikaria24

Joined
Sep 23, 2017
Messages
433
I have always jumped my vehicles with the donor vehicle running in idle, and then starting the dead vehicle. however i heard that some people jump start with the donor vehicle shut off. i assume something to do with the alternator?


anyways, what do yall think/ whats the best way?

thx
 
Engine running, UNLESS the dead vehicle is a motorcycle. I heard in that case you leave the donor engine off. Don't need a lot of juice to kick over a MC engine.
 
Engine off first. Usually if it doesn't crank over, the cables are too small in GA. When you start it, you force more amperage through the cables. You also take the chance that if your alternators regulator is weaker than the running donor car, you can fry it. Motorcycles are especially prone to this. Jump one with engine running and it almost guaranteed to fry the stator on the bike. Its regulator just cant handle it.
 
Not saying it’s the correct way, but I run it with some revs. Around 3,000 RPM. Just jumped a Silverado at about -10F with my MINI Clubman.

Disappointingly, my 800A “Tacklife” brand lithium jump starter pack wasn’t enough to amps for the V8 in the Silverado to even turn over. It was advertised for gas engines up to 7.0 liters. Makes me wish I got a more powerful one.
 
Yes, motorcycle charging systems are unlike cars and will not like voltage forced in from another charging system.

Starting another car usually requires running the donor car. If the donor car is a new model with smart charging system, turn its lights on. This will force the alternator up to a higher voltage.
 
E365 said:
Not saying it’s the correct way, but I run it with some revs. Around 3,000 RPM. Just jumped a Silverado at about -10F with my MINI Clubman.

Disappointingly, my 800A “Tacklife” brand lithium jump starter pack wasn’t enough to amps for the V8 in the Silverado to even turn over. It was advertised for gas engines up to 7.0 liters. Makes me wish I got a more powerful one.
Click to expand...
was it cold? lithium batteries are much more susceptible to cold than lead-acid.
 
Running then bring the RPM up to about 2K to ensure the alternator is charging, works every time. ;)
 
Not running-ever. Don't chance it. That little bit of charging you are getting is totally negligible.
No one here will change their mode. But Scotty (for a change) is right.
 
There is a very small but real chance of damaging the output power diodes of the donor ( good ) vehicle if the engine of it is running.

If jump-starting with the good vehicles engine not running does not have enough to crank a totally dead vehicle, then get all the keys and or fobs for the dead vehicle in your control so there is no way they can try to start it. Then run the good vehicle, connect to the bad with it running for 15 minutes to put some charge on the bad battery. Turn off the good vehicle. Give them there keys and or fobs back and have them try it again with the good vehicle not running but still connected.

If you value your alternator, NEVER allow anyone to try to start a dead vehicle with your good vehicle connected to there vehicle and yours running. NEVER NEVER NEVER. If you do and it damages the diodes of your vehicle they can fail leaky that will slowly discharge your battery while your vehicle is not running. And your vehicle may have to sit for 2 or 3 days to have the battery go dead, but it will go dead every time you let it sit unused for 2 or 3 days until you have the alternator diodes replaced with new ones, or a whole new alternator. What is interesting is this problem can stay hidden for a very long time until you do not use that good vehicle for a few days. It can be months or even years later that this causes a problem if you use that vehicle every day it will not show a problem, until it sits unused for a few days.

It use to be with 1970's and 1980's vehicles that the chance of damage by jump starting with the good vehicle running was much greater. Now days, the pulse with modulation method of output voltage control used in alternators reduces the possibility of this type of damage, but there is still a smaller chance of damaging those diodes. The bottom line is that the risk to the good vehicle does not have to be taken, because the battery of the good vehicle alone should be plenty of enough power to start the dead vehicle. And if it is not the chance of starting the bad vehicle can best be increased by charging the bad battery from the good vehicle for 15 minutes, then shut off the good vehicle before attempting another start of the bad one, and repeat an attempt with the good vehicle not running.
 
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Here’s what the owner’s manual of a BMW E46 says. Engine running is fine.

6. If the battery on the support vehicle is weak, start the engine and let it run
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With this note.
8. Before disconnecting the jumper ca- bles from your BMW, turn on the rear window defroster and set the blower to the highest speed; allow the en- gine to run approx. 10 seconds. This will prevent a voltage surge from the voltage regulator to the electrical accessories
Click to expand...


———————————
2012 Honda Civic

5. If your vehicle is connected to another vehicle, start the assisting vehicle's engine and increase its rpm slightly.
Click to expand...

———————————
2014 Chevrolet Malibu

10. Start the vehicle with the good battery and run the engine at idle speed for at least
four minutes.
Click to expand...

11. Try to start the vehicle that had the dead battery. If it will not start after a few tries, it probably needs service.
Click to expand...


——————-
2014 Ford Focus
Jump Starting
1. Start the engine of the booster vehicle and run the engine at a moderately increased speed.
Click to expand...
 
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I have always connected both vehicles with engine off, let it sit for a minute, then started the good vehicle, let it sit for a minute, then start the dead vehicle.
Never thought about all the electronics and the potential problems from having 2 alternators running at the same time.
I just checked 4 owners manuals (2018 Santa Fe, 2018 Kia Soul, 2012 Scion xB, 2002 F150), and they all say the same thing. Connect batteries with vehicles off, start good vehicle, let sit for a minute while revving engine, then start dead vehicle.
Only the F150 owners manual mentions turning on the heater fans to high to help balance electrical load.

Maybe I will follow what Scotty says, connect, let sit, try to start, if not, start good vehicle, let sit for a few minutes, shut off, try again.

Or just use the battery jump packs that I bought for all my vehicles (just make sure you charge them up periodically). Also as said, if very cold, they may take a minute to "warm up" to full discharge capacity, so connect to battery and turn on the headlights for a bit, then try to start.
 
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I would say running....I tried jumping a Cherokee with my Frontier running, it wouldnt quite crank over the Cherokee even with my Fronty running. But I did not think to give it some throttle while they were trying to crank the Cherokee. Ashame really because the pretty girls were in need ;)

With all the sensitive electronics in vehicles now, I just carry a jump pack around. Easier to connect, less risk to both vehicles, etc. Just gotta charge it up once in a while.
 
pantygnome said:
I would say running....I tried jumping a Cherokee with my Frontier running, it wouldnt quite crank over the Cherokee even with my Fronty running. But I did not think to give it some throttle while they were trying to crank the Cherokee. Ashame really because the pretty girls were in need ;)

With all the sensitive electronics in vehicles now, I just carry a jump pack around. Easier to connect, less risk to both vehicles, etc. Just gotta charge it up once in a while.
Click to expand...
In the old days one had to let it idle for a few minutes, to get the battery charged enough. Idle or preferably fast idle, depended on if the alternator was holding 13.8V or not. In ye olden days starters were a huge huge draw, and one couldn't start over the cables (or I should say, not often, due to resistance). But today the gear reduction starters seem to need far less current.

Went to jump my truck one day, battery was 9V and doing nadda. I hooked up my car and for giggles I hit the key--and it immediately cranked over, fast too, and started. I was rather impressed, all prior vehicles needed to charge the battery up before they would do more than just click.
 
The most over-looked thing when it comes to jump starting is often the most overlooked: current capacity. I don't care how strong the battery is in the running vehicle, if you try to use 8' 8-gauge booster cables in sub-zero weather there's just no chance it's going to start.

In my opinion, the MINIMUM gauge for jumper cables is 4ga. Barely. 1/0 gauge is preferred. On a nice day, depending on the engine size, a starter motor can draw anywhere from 150-300 amps. If your starter is weak or temps are crazy cold, that could jump to 300-450. There's just no way 8ga cables can provide that kind of current. Not through a 2 foot run, not through a 10 foot run.

When the dead vehicle is cranked, the current passing through the cables is coming from the donor car's BATTERY, not it's alternator. Just like when your vehicle cranks on a normal day: the battery is supplying the current, not the alternator. Yes, the alternator is definitely going to max out due to the huge drain that was just placed on the donor car's battery, but with quality cables that provide the lowest resistance possible the risk of damage is fairly low.

I'll say it again: GAUGE GAUGE GAUGE. Yes, the good ones are expensive, but if you really want to rely on your jumper cables when you need them most you shouldn't skimp. Something like this:

Forney 52877 pure copper jumper cables
 
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