Cars - Jump start

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	Cars - Jump start

A jump start is a colloquial term for a method of starting an automobile or other internal combustion engine-powered vehicle having a discharged battery.

Most U.S. passenger vehicles use a standard 12-volt DC battery which, when the driver turns the ignition key, an electric motor engages the teeth on the flywheel briefly turning the flywheel which is connected to the crankshaft and powers the spark plugs until ignition is achieved and the engine can produce its own electrical power from its generator or alternator.

When a battery fails or is discharged, such as by inadvertently leaving one's headlights switched on while parked, the car's engine will not "turn over" when the ignition key is turned. Many motorists carry "jumper cables" which consist of a pair of heavy gauge wires with alligator clips at each end. An automobile with a good battery is parked near the car needing the jump start and the cables are attached in this order:

One cable attaches to the positive (+, red) terminal of the dead battery, and then to the positive terminal of the live battery.
The other cable attaches to the negative (-, black) terminal of the live battery, and then to the engine block of the car with the dead battery.

With a safety cable, such as the one shown in the picture, the two pieces are connected together. The safety cable has color coded alligator clips and the connectors are polarized so that they can only be inserted in the correct orientation.

Since the entire engine block is grounded to the negative terminal, the cable need not be connected directly to the dead battery's negative terminal, and in fact it would be unwise to do so as sparks from the connection (when the circuit is completed) could ignite the battery. (Highly explosive hydrogen gas can be given off by a battery, though this is less of a risk with the completely sealed "maintenance-free" batteries unless their case is compromised). A good connection point would be a piece of unpainted metal at least eighteen inches away from the dead battery. Some engines have eyelets which are used to attach chains when the engine needs to be lifted out of the car; these make good connection points for the jumper cable.

If two cables are used that are completely detached from each other, then the grounds of the two vehicles should be connected first using one of the cables, while keeping the other cable out of the way. Then, using the other cable, the positive connection should be made. Once one end of the positive connection is made, that jumper cable is "live" and must not be allowed to touch anything other than its intended positive connection point in the other vehicle. To further reduce the risk of spark around the dead battery, each connection should be made in the stranded vehicle first, and then in the running vehicle. (Even when the grounds are being connected together, and there isn't a complete circuit, there could be an electrical potential difference between the cars, the discharge of which could cause a spark. This is one reason why grounds are connected first, if possible, otherwise such a discharge may later choose a path through the electrical components). It is a basic electrical principle that ground connections are made first and broken last, unless there is a good reason to do otherwise, as described next.

If the two jumper cables are attached together, as is often the case, then the opposite procedure is followed: the positive connection must be completed first, and then the grounding connection. The reason for this reversal is that because the cables are joined, when one clip is fastened, the opposite one is left dangling near it, unless an assistant holds it up. The dangling clip is likely to touch something that is grounded, like the chassis or engine block. Therefore, the clip that is left dangling should be the negative one which is intended to be grounded anyway. If, incorrectly, the negative clip is connected, while the positive one, left dangling, touches something that is grounded, an instant short circuit will result when the corresponding positive connection is later made at the opposite end. For the same reason, it is also important not to connect both clips on one end first to one car, because then the clips on the opposite end of the cable may touch, causing a short circut.

If a safety cable is used, such as the one shown in the picture, then the alligator clips of the two separate cable components are attached in whatever order is convenient. Then the plug-and-socket connection is made to bridge them together.

Later, after the jump start is complete, the connections should be detached in exactly the reverse order of their attachment. In the case of the safety cable, the socket connection between the two cable sections is unplugged first and then the clips are removed.

(Many auto manufacturers specify methods of jump-starting their autos, such as a different sequence of attaching or detaching jumper cables. For your safety, please read your owner's manual!)

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Cars - Jump start

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