Really Long Car Battery Jump-Starting Instructions (was: "RE: Inflammable vs. flammable")

[Andrew Warren]

Tariel, Lauren R wrote:

> I thought the car with juice had the cables attached to both
> terminals and the car with the dead battery had positive to
> positive and negative to ground.

    Yeah, that's correct... On the good car, both cables are clamped
    directly to the battery terminals; on the bad car, the positive
    cable is clamped directly to the battery while the negative cable
    is clamped to the chassis or engine block, away from the battery.

> knowing that I should begin with the negative cable on the ground
> of the dead vehicle has nothing to do with my experience as a
> technical writer.

    Actually, you SHOULDN'T begin there.  The safe order is:

    1. Positive to the bad car's positive terminal.
    2. Positive to the good car's positive terminal.
    3. Negative to the good car's negative terminal.
    4. Negative to the bad car's engine block, away from the battery.

    It's easiest to understand the reason for this order if you think
    about the final connection first:

    As the fourth cable connection is being made -- no matter
    where -- it will complete the circuit and sparks will probably
    be produced between that fourth clamp and whatever it's being
    clamped to.

    The sparks themselves are harmless, but hydrogen gas forms around
    a lead-acid battery when it discharges.  It's not a lot (and it'd
    be hard to contain it under an open automobile hood and
    concentrate it enough to make it dangerous, anyway), but there's a
    SLIGHT danger of explosion if sparks are generated right at the
    battery... So the final circuit-completing connection must be made
    NOT to the battery itself, but to a part of the car that's
    electrically connected to the battery by the car's wiring, but
    physically some distance away from the battery.

    All modern cars wire the negative terminal of the battery to the
    car's chassis/engine block through a thick, reliable cable, so we
    make the final connection to the bad car's engine block instead
    of to its negative terminal.

    Next, think about the first and second connections:

    If you made the first two connections to the good car's positive
    and negative terminals, then the two clamps dangling at the other
    end of your jumper cables could touch each other and short-circuit
    your good battery.  That'd be bad, so the first two connections
    can't be to the good car's battery.

    We're saving the bad car's negative connection for last, so THAT
    can't be one of the first two, either... Which leaves these
    choices:

    a. Good positive, then Bad positive
    b. Good negative, then Bad positive
    c. Bad positive, then Good negative
    d. Bad positive, then Good positive

    Choice "a" has a disadvantage: Because the good car's negative
    terminal is wired to the car's chassis, there'll be a short-
    circuit if the clamp dangling at the end of the positive cable
    touches any metal part of the good car before the second
    connection is made.

    Choice "b" means having to run back and forth between the cars:
    Good for the negative, then Bad for the positive, then Good for
    the other positive, then Bad for the final negative.

    Choices "c" and "d" have the same problem as "a" -- the dangling
    positive-cable end could short-circuit the battery -- but the
    danger is less because the bad car's battery is already dead and
    therefore not as likely to be damaged or to damage anything else
    as a result of a short-circuit.

    Choice "d" is slightly preferable to "c" because it may be a
    little easier to remember and perform without errors.

    Anyway... After you've connected the cables with both cars turned
    off, start the good car and run it for a while with headlights,
    radios, cellphone chargers, climate-control fans, interior lights,
    etc., turned off in both vehicles.  Optionally rev the good car's
    engine to a moderate speed to help charge the bad car's battery
    faster.

    To avoid damage to the good car's alternator, DON'T try to start
    the bad car while the good car's engine is running -- the diodes
    in the good car's expensive alternator can easily be destroyed
    by the high current required by the bad car's starter motor.
    Instead, turn off the good car's engine so the current will flow
    only from the batteries rather than from the alternator.

    If you're worried about draining the good car's battery and being
    left with TWO cars that won't start, disconnect the cables before
    trying to start the bad car.  Note that if you do this, you'll
    have to charge the bad battery for more time than if you start
    with the two batteries connected.

    When you're done, disconnect the cables in the reverse order: Bad
    negative, then good negative, then good positive, then bad positive.

    -Andrew

=== Andrew Warren  - awarren at synaptics.com
=== Synaptics, Inc - Santa Clara, CA