Car battery "grounding"?

Discussion in 'General Electronics Chat' started by diogenes, Apr 8, 2009.

  1. diogenes

    Thread Starter Member

    Feb 17, 2009
    I am wondering why the negative terminal of a car battery is connected to the chassis. The car is insulated from ground so it doesn't seem analogous to ground in the wiring of a house.

    The only thing I could think of is it allows you to jump start the car without actually connecting directly to the negative terminal, thereby minimizing the chance of a bad battery exploding.
  2. russ_hensel

    AAC Fanatic!

    Jan 11, 2009
    It allows all sorts of junk to run on one wire using the body of the car as the "return". As for why the - side was choose, it is because historic current flows ( does the current in a river flow downhill? ) from.......
  3. beenthere

    Retired Moderator

    Apr 20, 2004
    The term "ground" in a car is an extension from electrical practice. Most surfaces you contact outdoors or in a building are also electrically in contact with the earth. Thus, they are said to be grounded. Most metal you contact in a car is bonded electrically to the chassis, and so is said to be grounded.
  4. Darren Holdstock

    Active Member

    Feb 10, 2009
    Cars from ye olden days used to have a positive earth. This suited the PNP transistors in the radio quite nicely.

    Using the chassis as an earth return can cause problems - ever see a stop light flashing in time with an indicator light? That's a poor ground return - so often body panels with ground returns are connected together with braided earthing straps, just to make sure.
  5. diogenes

    Thread Starter Member

    Feb 17, 2009
    Thanks for the replies. So the main advantage to having a terminal connected to the body is simply to avoid having to run two wires to electrical devices?
  6. thatoneguy


    Feb 19, 2009
    Partly, electronic interference shielding is another reason. Sparks and the onboard electronics make a good deal of interference, which is pretty well attenuated from the Faraday cage round the engine compartment.

    The impedance (resistance) of the ground is lower as well. For a car stereo 150W amp, 1V is lost if using 10 Gauge wire, and that is on the positive wire alone. Running a wire for positive and negative would give a 11.8 working voltage instead of a 13.8.
    When dealing with amplifiers over 100W, use the larger gauge wire. Alwasy match fuse/wire/etc to double the anticipated load.

    Either terminal cna be "Ground" and that ground is entirely unrelated to Earth Ground. The only time they are at the same potential is when dragging chains, which is often seen on gas tanker delivery trunks.

    I would prefer the term "Common" used, so "Ground" always means Earth Ground, but it is looking like that's not gonna happen...