Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Jump starting a car

  1. Dec 15, 2005 #1
    Why in the world would you hook the two positive terminals up, and then you put 1 side of the wire on the negative and the other on the car.... I dont get this. I would think that if you had them directly set up so 1 wire goes from negative to positive and the other wire goes from negative to positive also, it would drain the the jumping cars battery instantaneously. So I think thats why you hook it up to a metal part of the car, simply because you want the cricut you are creating to have some resistance. But I cant figure out for the life of me why it goes positive to positive.
     
  2. jcsd
  3. Dec 15, 2005 #2

    Danger

    User Avatar
    Gold Member

    All that you're doing to jump a car is hooking a functional battery up to the same circuit as the dead one, in parallel. If you try to reverse the polarity, you'll fry the system (some of those batteries put out over 1,000 amps). The reason for making your last connection one of the negatives to a ground point rather than the negative terminal is that there's almost always some sparking involved. Lead-acid batteries produce hydrogen, and a spark could blow the thing up quite violently if it occurs in the venting area.
     
  4. Dec 15, 2005 #3
    I have a picture here, hope this explains it. http://www.geocities.com/paintballfan8/untitled.bmp

    (top battery is the dead one)
    (bottom battery is the live one)
    (the resistor is all the car parts that draw power from the battery; spark plugs, kicking sound system, etc.)



    but he is right by saying if you were to hook them up postivie-negative, negative-positive, and they weren't leaky batteries, they would drain instantaneously.
     
    Last edited: Dec 15, 2005
  5. Dec 15, 2005 #4

    Danger

    User Avatar
    Gold Member

    You know, it never occurred to me until just now that the OP might not be aware that the chassis/engine/body of the car is the return side of the circuit.

    edit: Hmmm... just saw your edit there, Wishbone. The instantaneous draining thing is outside of my experience, but I sure do remember a melted set of cables and blown regulator from my youth. (And no, it wasn't me who did it.)
     
    Last edited: Dec 15, 2005
  6. Dec 15, 2005 #5
    Ya i think thats what would happen (although im not in any rush to try it), because like you said there would be really high amps caused by very little resistance. So you would have a huge rush of amps that would quickly drain the battery (if it didn't burn through the wire first like "your friend" experienced :wink:)
     
  7. Dec 15, 2005 #6

    Danger

    User Avatar
    Gold Member

    Honest, man... it wasn't me. That was before I could even drive.
     
  8. Dec 15, 2005 #7
    Truthfully I pretty much always just connect them positive to positive, negative to negative. I've tried doing it the way the cables say to, but it hasn't ever worked for me.
     
  9. Dec 15, 2005 #8

    Danger

    User Avatar
    Gold Member

    There's so much plastic in modern cars that it's quite possible you tried a clamp point that was insulated from the electrical system. I usually use the alternator bracket myself, but a sure bet is the bolt where the ground cable is connected to the vehicle.
    As a side note, I'm pretty sure that modern sealed batteries don't present an explosion hazard. If it has vent caps, however, be very careful.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Jump starting a car
  1. Jumping in a train (Replies: 11)

Loading...