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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 (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, a jump start should not be attempted. Corroded terminals will increase the voltage drop during cranking and will contribute to starting difficulty. Proper jump start procedures are usually found in the vehicle owner's manual. Owner's manuals may show the preferred locations for connection of jumper cables; for example, some vehicles have the battery mounted under a seat or 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 about 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 needed to avoid a spark that may ignite the gas. The recommended sequence is to first connect the positive terminals of both batteries, then connect the negative post of the charged battery and make the last connection to the frame of the vehicle with the dead battery, at a location away from the battery. By not connecting the last clamp on to the dead battery, the risk of ignition is reduced. The described sequence of connections is intended to reduce the chance of accidentally shorting the good battery.
Long heavy-gauge cables make boosting a vehicle easier.

Current from the boosting vehicle will charge the dead battery. After several minutes enough energy is transferred to allow cranking of the engine. If the connections are good and the cables are large, the boosting vehicle battery may also supply part of the cranking current. In an extreme case, it is possible to crank and start a vehicle with no battery in it if the cables are heavy-duty. Cranking current drawn through light-gauge cables will damage them by overheating.
Jumper cable connected to positive post.

A jump start is only effective for a discharged battery. Other faults such as lack of fuel, a failed battery, or other mechanical problems cannot be overcome by a jump start. Even after a successful jump start a vehicle may not be able to resume normal operation if the reason for the dead battery is a failed charging system. Unless the cause of the battery discharge is known, the operator of the boosted vehicle should have the battery and charging system checked.

Loss of voltage from the vehicle battery may have wide-ranging effects--from a trivial loss of radio receiver preset stations to a significant loss of security codes or engine control parameters. A prudent motorist should familiarize himself with the effects of a dead battery; booster cables may be unavailable if the keyless entry system won't unlock the trunk (boot).

Booster Cables

Booster cables consist of a pair of heavy wires, terminated in spring-loaded clamps for connection to battery terminals (or grounded metal parts of the vehicle frame). They are available in varying lengths. The size of copper conductors varys from about #10 AWG for light duty sets, to #1 AWG. The clamps, or jaws, generally have color-coded, vinyl-coated (PVC) insulation to prevent electrical short circuits. Most jaws fit both top- and side-mounted battery terminals. Top-quality jaws are made of pure copper; lesser-quality are made of alloys, such as zinc-plated copper or copper-plated iron or steel.