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Negative wire when jumping start a car

  1. Aug 30, 2010 #1
    Hello, I've always heard that when you will jump start a car you connect both cathodes (positive terminal) and you connect the anode of the good battery to the chassis of the stranded car. Why is the setup like this and not connect both negatives with each other too?
     
  2. jcsd
  3. Aug 30, 2010 #2

    turbo

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    You want your negative connections (I do both!) to be on grounded metal some distance away from the batteries, so that any sparks creating when disconnecting the jumpers won't ignite hydrogen liberated by the battery.
     
  4. Aug 30, 2010 #3

    Integral

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    I have always connected the negative terminals, never had any problem. In the days of metal bumpers you could just touch bumpers and run a jumper lead only between the positives , in many cases it is easier to get to frame connections then the battery, in reality it does not make any difference. Just get the best connection you can.
     
  5. Aug 30, 2010 #4
    Do modern sealed batteries still liberate hydrogen or anything else?
     
  6. Aug 30, 2010 #5

    turbo

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    I learned from my grandfather, back in the 60's. He was a heavy-equipment mechanic and he insisted that I learn HIS way of doing things when I was learning to drive and learning how to fix automotive stuff. My father was less anal about the negative-connection, but I respected my grandfather's experience and skill. I don't know if more modern sealed batteries are less dangerous in this regard, but it's an easy rule to remember and follow so I still do it.
     
  7. Aug 30, 2010 #6
    Huh. Yeah, I have jumped many older car batteries terminal to terminal and never had a problem. I recognize the possibilty of hydrogen gas ignition, I just never experienced any problems. Maybe just lucky.
    Also, I thought that lead acid cell batteries emit hydrogen only during charging. So, doesn't a dead/weak battery have nearly zero hydrogen gas discharge above it when connecting jumper cables?
     
  8. Aug 30, 2010 #7

    turbo

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    I think you may have tumbled to the problem that made the old-timers come up with the frame-to-frame rule. If the person with the dead car is anxious to get moving and removes the ground clamp from the negative terminal first, there could be a "pop".

    It's not so common these days, but some time back, there were vehicles that had chassis at positive instead of negative. Probably another reason that my grandfather was so anal about how to approach jump-starting.

    Unrelated, when I was 11, my father bought a Jeep and he had told me to change out the wheels/tires for some better ones that he'd picked up cheap. I broke 3 studs off one wheel before I figured out that I ought to stop and wait for him to get home. He didn't know that Willys' of an earlier era had left-hand threaded studs and nuts on one side and right-hand on the other. Being 11, I didn't know either, but after soaking the nuts and studs with penetrating oil, beating them with a heavy hammer and busting 3 studs with a lug wrench and pipe cheater, I had gotten about to the limits of my tire-changing expertise.
     
  9. Aug 30, 2010 #8
    Ah, yes Turbo!!!!!!!!
    It's the "sparked" disconnect that can cause the problem. Not the connect spark.
    Nice.
     
  10. Aug 30, 2010 #9
    Hmm, that's interesting. I always thought it was to add a bit more resistance to the circuit. Never considered the explosion aspect. Looking through a couple of Google searches I actually found an incident of someone blowing the top off their battery because of this.
     
  11. Aug 30, 2010 #10
    I was thinking too about resistance, never thought that charging batteries release H gas. So eventually if you charge a lot your battery you will end up with no hydrogen and no power right?
     
  12. Aug 31, 2010 #11
    Sure, if you don't replace the distilled water from time to time.
     
  13. Sep 2, 2010 #12
    Distilled water??? Dont those batteries have sulfuric acid in the small vents?
     
  14. Sep 2, 2010 #13

    turbo

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    You top off the cells with distilled water, not more acid.
     
  15. Sep 2, 2010 #14

    DaveC426913

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    I had no idea. But have always wondered too. I assumed it was some circuit thing, not some spark/tible gas thing.

    Thank you.

    After decades of storage, a tiny volume of my mind has has been freed up for other things.

    I wonder what's for dinner? D'oh! It's filled back up...
     
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