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Trying to understand the car battery.

Discussion in 'General Electronics Chat' started by foolios, Nov 9, 2009.

  1. foolios

    Thread Starter AAC Fanatic!

    Feb 4, 2009
    Why can this car battery's negative terminal be grounded to the frame like so?

    The positive terminal of the battery doesn't have enough electrons? So it wants to pull/draw electrons from the negative post.

    Isn't the frame of the car a path for current to parts of the electrical system? Why doesn't this cause a short-circuit?
  2. someonesdad

    Senior Member

    Jul 7, 2009
    It puts the whole metal frame of the car at the same potential as the negative battery terminal. This allows the frame to be used as a large "wire" as a return to the battery. Thus, for example, to power a light distant from the battery, the manufacturer just needs to run a single wire to the light. The other side of the light's filament gets connected to the frame where the light is mounted. This saves the cost of running another wire back to the battery.
  3. foolios

    Thread Starter AAC Fanatic!

    Feb 4, 2009
    That light when turned on:
    The circuit is now complete, the resistance of the lamp will now allow but limit the positive terminal of the battery to draw some current from the negative terminal via the frame of the car. So electrons will travel along the frame to the lamp, then after the lamp will travel along a wire to the positive.

    In this DC circuit, the positive terminal is using voltage to draw the current.
    The negative side has the excess of electrons that the positive wants.

    Why don't positive terminals draw electrons out of other things besides the negative terminal? Is it a matter of voltage? Would it draw electrons from something else if there was more voltage applied?
  4. thatoneguy


    Feb 19, 2009
    The battery produces voltage and current through a chemical reaction.

    If you were to put the negative terminal of a AA 1.5V battery on the positive terminal, and measure the voltage between + and - of the AA battery, you would get about 1.5 V (the voltage of a AA battery).

    If you were to measure voltage from vehicle ground to the positive side of the AA battery, you would measure about 15V, but the amount of current that could be drawn is limited by the AA battery, which could explode if short circuited due to the AA battery's internal resistance generating heat.

    Voltage and current only flow where the positive and negative have something in common, be it a battery, or a generator.

    If you were to connect a second battery, say a 6V lantern battery to the car, with the 6 Volt battery's ground connected to the car batteries ground, you would measure 6V from the positive terminal of the 6V battery, and 13V (or so) from the Car battery's positive terminal, even though the two negatives are "common". Thus, you would have two separate sources relative to the ground, a +12V-14V (depending if engine is running or not) from the car battery, and +6V from the lantern battery.

    In the case above, when only using the 6V power, that is the only battery that will supply power. When starting the engine, only car battery would supply power, as the 6V battery is not in series with the car battery.

    The first case, with the AA battery, the two are in series, and the voltages add. In the second case, the two are in parallel, and the voltages are independent unless connected. if the +12V was connected to the +6V, the car battery would attempt to "charge" the 6V battery in order to make the voltages equal. This would also result in the 6V battery self-destructing from heat. Car batteries have a very low internal resistance, and can therefore supply a tremendous amount of current. Portable batteries have much higher internal resistances.
  5. davebee

    Well-Known Member

    Oct 22, 2008
    Inside the battery, chemical activity creates the electron imbalance and sets the positive terminal positive and the negative terminal negative. So when wires connect the two terminals, quite a lot of current can flow as the chemical action maintains the charge imbalance inside the battery.

    If you touch the positive terminal, your body and the terminal will exchange electrons until there is no voltage between you and the terminal. A small current will flow for an instant, but after that, you and the positive terminal will be at the same potential (voltage). The other terminal will then be 12 volts negative with respect to your body.

    The burst of current that flows depends on the capacitance of the battery terminal and your body and the amount of charge that must flow in order to charge or discharge those capacitances as their voltages are brought to being equal.

    If the battery was made of thousands of cells in series then the terminals could measure thousands of volts. Here, if you were to touch one terminal, a larger current would flow to the extent that you'd probably feel a shock and see a spark.

    But in both cases, there is no chemical action maintaining the charge imbalance between you and the battery, so once the charge flows that overcomes that initial imbalance, no more charge will flow.