Originally Posted By: eljefino
You want a surface charge on the dead battery. ~14 volts coming in on the cables does this faster than ~12.5. Having the electrons "that much closer" is beneficial.
Agreed. This will act to dampen the draw from the donor vehicle, since the highest current draw will want to come from the surface charge of the now not-entirely-dead battery.
I've actually only had to jump several vehicles, so my experience is limited. I used to hook-up jumper cables with the donor car running and then immediately try to start the dead vehicle, in order to minimize the time the two charging systems were hooked-up in parallel. This never seemed to work on the first try, but did after charging for several minutes. It's important to note that most vehicles I've encountered that need jumping are dead as a doornail (e.g., a light has been left on for an extended period of time or the battery is simply too old or deficient, period.) I've never encountered a vehicle that was so low that it couldn't operate any electronics, but I've also never encountered a vehicle that could ALMOST turn over a vehicle.
So, my new COA for jumping cars, should I need to, is how you mention, eljefino. That is, unless I have a multimeter on me that proves the dead battery has a shorted cell or something.
The best solution: carry a jumper pack. I haven't had to use it, but I got one for Christmas a couple of years ago. I use it to charge my phone while traveling, but it's usually charged enough to use.
I also have a smart-charger that has a starting feature. I hope to never need to use the feature, but we'll see.
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Also of note, is I've seen people "jump" vehicles that were turning over perfectly. I've personally been asked for a jump by two people whose batteries were fine and the starter was turning over (at least initially) just fine. I think some people think that if the car won't start, it needs to be jumped. With that said, I've heard that people have actually started vehicles with a low battery that seemed to turn over fine, but with batteries that were actually too low to start the vehicle; so, unless you've got a DMM, maybe you can't simply rule out a jump being needed based on the other vehicle being able to turn over???