Charging & Starting Systems

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. more...

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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 (UK usage: jump leads) 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.

Procedure

If the discharged battery is cracked, has a low electrolyte level, or is frozen, then a jump start should not be attempted. Proper jump start procedure are usually found in the owner's manual for the vehicles. Owner's manuals may show the preferred locations for connection of jumper cables; for example, some vehicles have the battery mounted under a seat, and may have a jumper terminal in the engine compartment.

Motorists can be severely injured by a battery explosion. In the United States in 1994, a research note by the National Highway Traffic Safety Association estimated that around 442 persons were injured by exploding batteries while attempting a jump-start. Organizations such as Prevent Blindness America recommend use of splash-resistant safety goggles to protect the eyes while connecting cables.

Operation of a lead-acid battery produces hydrogen gas which is flammable. Caution is in order to avoid a spark which may ignite the gas. The usual recommended sequence is to first connect the positive terminals of the charged and dead batteries with the red booster cable, then connect the black cable to the negative post of the charged battery and to the frame of the vehicle with the dead battery, at a point away from the battery. This method avoids creating a spark near the battery terminals. The boosting vehicle must not touch the boosted vehicle.

Read more at Wikipedia.org