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> 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 -dot- com
=== Synaptics, Inc - Santa Clara, CA
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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