A jump start or boost is a term for a method of starting an automobile or other internal combustion engine-powered vehicle which has a discharged battery. A second battery (often in another vehicle) is temporarily connected to the "dead" (discharged) battery, to provide the initial charge required to start the engine. Once the engine is running, it will recharge the battery, so the second battery can be detached. However, if the engine stops running too soon, the battery may not have built up enough charge to restart the engine, so a further jump start may be required.
Most passenger vehicles use a 12-volt battery which provides power to a starting motor for the engine. When the engine is running, electrical power from its alternator restores the charge to the battery in preparation for the next start. When a battery 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 insulated wires with alligator clips at each end. Jumper cables are marked by black and red handles, representing the two polarities of the automobile's direct current system. Interchanging the polarities will cause a short circuit and a hazard to people and damage to one or both automobiles. Good quality jumper cables will have large copper conductors and well-made alligator clips, and insulation that remains flexible at low temperatures. Longer cables require less maneuvering of the boosting vehicle to allow connection of the two batteries.
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