Acarolinensis wrote:Having read all the posts, I'm impressed with the amount of misinformation and how strongly the authors believe it to be true. Also having a Y chromosome, I will join in.
Good post, you've made some good points, but I have a few nitpicks:
So this leaves the mantra of red to red and black to black. You get the lowest resistance connection by connecting to all four battery terminals. By Ohm's law: lower resistance -> less voltage drop -> more voltage applied to starter -> more current flows in starter -> more torque to turn over engine.
It's true that you're looking for the lowest resistance, but since you're trying to power the starter of the dead car (not the battery), if you're really looking for the lowest resistance connection you should clip the black lead to where the battery cable connects to the car chassis or engine block -- not to the battery terminal. (or clamp it to the starter itself, but good luck finding the starter on any modern car!). Of course, this ignores any effect on resistance of having the cable bite into the soft lead of the battery terminal -- anyone know if that's significant?
So what do you do when when you can't figure out the polarity of a battery? The nice answer is to get out a voltmeter. But nobody has one in their car. The ugly answer is to hookup three cable connections and *LIGHTLY* brush the last one with the smallest and briefest of contact. Note the result and change the connected cable to the other terminal and repeat the experiment. Obviously you want the boring connection. The trauma experienced may motivate you label the battery, even if it's a stranger's. This procedure is not recommended if you don't know what you're doing, but I would guess 100% of today's participants have accidentally clamped their cables down wrong at one time or another and lived to talk about it.
Since you risk damaging one or both cars with this technique, unless you're in an emergency situation, call for help if you can't tell which terminal is which.
If you're dealing with a common negative ground car (which is every major brand American and Japanese car built in, say, the past 20 years), and you can't make out the positive and negative terminals on the battery, if you can trace the cables and find the one that attaches to the car's chassis, that's the negative cable. (or find the big cable that goes to the starter - that's the positive)
One key to battery cables is to remember that like charges repel each other. Electrons try to get away from each other. Because of this most current flow will be on the surface of a cable. Stranded wire is good because it has more surface area for a given amount of copper and thus more current carrying capacity.
Sorry, I don't buy this -- skin effect is not a factor in DC power. But even at 60 Hz AC, the skin depth is 8mm, so unless your cable is over 1/2 inch thick the whole conductor is involved in conduction. The reason they use stranded cable for jumper cables is so it is flexible -- number 2 cable is 1/4 inch in diameter -- not very convenient when it comes time to roll up the cables to stow them away.
Even if the cables were stranded to defeat the skin effect, each individual strand would have to be insulated, otherwise they'd all be shorted to each other and act as a single, larger conductor.