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Why does a car jumpstart work?

  1. Jul 8, 2012 #1
    For reference, the sequence is positive [on good batt.] to positive, and negative [on good batt] to ground metal.

    I would like to know how to car jumpstart is able to work. First, if you are connecting positive to positive, how can current flow if both sides are absent of electrons, and thus can not transfer electricity? Second, what role does connecting to ground have to do with charging the battery?

  2. jcsd
  3. Jul 8, 2012 #2


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    Staff: Mentor

    What you're really doing is bypassing the dead battery. Current flows from the positive terminal of the good battery, through the jumper cable to the positive terminal of the dead battery, then through the wire from the positive terminal to the starter. The positive terminal of the good battery is nothing but a convenient place to hook into the wire between that terminal and the starter; it's the starter that we're delivering current to.

    The ground connection is the same thing. You could (and many people do - it's a bad practice but that doesn't stop them) connect the ground terminals of the two batteries together. The current would flow through the starter, into the chassis of the car, then through the wire that connects the chassis to the ground terminal of the dead battery, then back through the jumper cable to the ground terminal of the good battery, thereby completing the circuit. Again, what we're really doing is bypassing the dead battery, using its terminal as a convenient place to connect to. But it's just as easy and much safer to connect to the chassis of the dead car directly, instead of doing the extra step through the wire from chassis to dead battery groun terminal.
  4. Jul 8, 2012 #3
    Thanks! It all makes sense now!
  5. Jul 8, 2012 #4


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    Homework Helper

    The reason it's safer to connect ground to chassis after connecting the postiive battery terminals is to avoid having any sparks igniting any hydrogen fumes that might be expelled by either battery.

    Regardless of where the connections are made, some of the current is going "backwards" through the bad battery (attempting to charge it) at the same time it's going through the starter. The starter has much less resistance than the bad battery, so most of the current will go through the starter, allowing a jump start to work. If the bad battery is shorted, then it will have to be disconnected before a jump start will work.
  6. Jul 10, 2012 #5
    good answers above. kudos!

    It makes me wonder just how well 'jump starting' an AGM battery would work as they have extremely small internal resistance. Probably still ok because a typical gas engine starter does not use much energy...way less than 1% what a typical battery can supply except in sub,sub zero weather.
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