Charging a car battery question.

  1. Hey. Was jump starting and charging a car battery a couple of days ago for the first time. I connected the positive of the bad battery to the positive of the good battery, then connected the negative of the good battery to a solid metallic, non-painted part of the engine on the bad car, or the body frame.

    My question is: How is this a closed circuit? Is the negative terminal of the bad car's battery connected to its "solid metallic parts/body frame?"
  2. jcsd
  3. marcusl

    marcusl 2,153
    Science Advisor
    Gold Member

    Yes, the negative is grounded just as you said.
  4. Thank you for the quick reply. My mind is now at ease.
  5. Ok now a follow up question if you don't mind. Why then not connect the good battery's negative directly to the bad battery's negative?
  6. marcusl

    marcusl 2,153
    Science Advisor
    Gold Member

    Batteries produce hydrogen gas bubbles as they charge, and this can be vigorous for a fully discharged battery. Connecting to chassis ground keeps any sparks away from the battery and flammable gas.
    Last edited: Feb 10, 2013
  7. Thank you.
  8. jim hardy

    jim hardy 5,460
    Science Advisor
    Gold Member
    2014 Award

    Follow your battery wires.
    Most GM cars have a positive post near the battery for jump-starting. It'll be underneath a red plastic pop-up cover.
    Sure easier than trying to grab that battery side-post terminal, and safer too as marcusi pointed out.

    I like to connect negative to a substabntial piece of metal on the engine. I figure starter is bolted to engine block, so starter's heavy current doesn't have to flow through a potentially weak frame wire on its way back to my jumping battery's negative..
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