I understand the safety concerns leading to grounding to the frame or engine for the car with a dead battery. From what I've read the dead battery is the one with a chance of venting hydrogen, but every source I've seen says to make the ground connection there last. If sparking at the last connection is the concern then why not ground the good battery last?

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    Related:Why should I connect the red/positive terminal first when jump starting?. As you mention tho, I'm not sure why the dead car should be connected last. – JPhi1618 Apr 11 '16 at 18:02
  • I was hoping for an answer other than mine. No one else knows? – JPhi1618 Apr 12 '16 at 13:48
  • I'm with you on your points JPhi. The only thing I'm not clear on is why not make the final connection (neg(-)) on or near the good battery since the dead battery is the one that is most likely to vent hydrogen gas and we're trying to avoid sparking there. – Pepper Apr 12 '16 at 19:15
  • Because we're really trying to avoid sparking near either battery. Hydrogen is given off as the battery charges whether its a good or bad battery, and maybe the good battery is really in worse shape physically, but still has the charge to start the car? I've certainly jumped off much nicer cars than mine that had new-looking batteries. – JPhi1618 Apr 12 '16 at 19:18

Fair warning, this is a guess. Vote appropriately...

General accepted order:

  1. Dead +
  2. Good +
  3. Good - (directly to the terminal)
  4. Bad - (to a good engine ground, but not the terminal)

Avoiding the last connection directly to the dead negative is to avoid sparks near the dead battery, but this also avoids sparks near both batteries since the good one is securely connected at this point.

Connecting Positive first is good because doing it the other way makes it easier to accidentally short out the battery.

Now, why not start with the Good battery positive? I think the reasoning is that you want the best possible connection to the Good battery, so you really want the alligator clips of the cables connected directly to the terminals.

To connect the positives first and end with a negative connection away from the battery, the "recommended" method is the only order that satisfies everything.

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