Jump start (vehicle)
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A jump start, also called a boost, is a method of starting a vehicle that is unable to be self-started due to insufficient power to the starter motor from a discharged starter battery (also known as an SLI battery). A temporary connection is made between the discharged battery's terminals and those of another running vehicle, or to some other external power source. When the connection is made, the external supply of current effectively bypasses the battery cells, passing directly into the vehicle's electrical system through the terminal connections and providing the current needed to operate the starter motor. Once the vehicle has been started (and barring any other malfunction in the electrical system), the vehicle's alternator will naturally recharge the battery over a running period ranging from 30 minutes to 3 hours.
The most common cause of accidentally discharged SLI batteries is leaving headlights or other electricity-consuming accessories in operation for too long while the engine is shut off, which eventually drains the charge below the level necessary to run the starter motor. Jump starts usually require a set of specialized heavy-gauge two-wire cables, known as jumper cables, with connectors designed to clip securely to both sets of battery terminals.
Jumper cables (jump leads)
Jumper cables, also known as booster cables or jump leads, are a pair of insulated wires of sufficient capacity with alligator clips at each end to interconnect the disabled equipment/vehicle with an auxiliary source, such as another vehicle or equipment with the same system voltage or to another battery. The alligator clips may be covered in insulation to prevent inadvertent shorting. Clips may be made of copper or steel. Alligator clips are generally marked by black (-) and red (+) to indicate the polarity.
Operation of a lead-acid battery may, in case of overcharge, produce flammable hydrogen gas by electrolysis of water inside the battery. Jump start procedures are usually found in the vehicle owner's manual. The recommended sequence of connections is intended to reduce the chance of accidentally shorting the good battery or igniting hydrogen gas. Owner's manuals will show the preferred locations for connection of jumper cables; for example, some vehicles have the battery mounted under a seat, or may have a jumper terminal in the engine compartment.
Jumper cables should not be used to interconnect between different voltage systems. Connecting 6v and 12v systems together may cause damage.
If the dead battery is physically damaged, has a low electrolyte level, decayed, or is frozen, a jump start will not repair the battery. A vehicle with a frozen battery should not be jump started, as the battery may explode.
A hand-portable battery, equipped with attached cables and charger, can be used similarly to another vehicle's battery. A self-contained jump box contains a battery and connects directly to the battery of the engine that needs a boost.Portable boosters may automatically sense the battery's polarity prior to sending power to the vehicle, eliminating the damage that can result from reversing the connection. 
Cigarette lighter outlet
An alternative to jumper cables is a cable used to interconnect the 12 volt power outlets (cigarette lighter outlets) of two vehicles. While this eliminates concerns with incorrect connections and generation of arcs near battery terminals, the amount of current available through such a connection is small. This method works by slowly recharging the battery, not by providing the current needed for cranking. The engine cranking motor current draw will exceed the fuse rating in a cigarette lighter outlet. Many vehicles turn off the cigarette lighter outlets when the key is turned off, making the technique unusable unless the ignition key is turned to the accessory or on position to connect the cigarette lighter outlet to the battery.
Battery booster and jump starter
Some battery chargers are designed with a capability to remain connected during starting for a limited duration to jump start the engine. Motorists and service garages often have portable battery chargers operated from AC power. Very small "trickle" chargers are intended only to maintain a charge on a parked or stored vehicle, but larger chargers can put enough charge into a battery to allow a start within a few minutes. Battery chargers may be strictly manual, or may include controls for time and charging voltage. Some chargers are equipped with "boost" settings that supply a large current to assist in cranking the engine. Battery chargers that apply high voltage (for example, more than 14.4 volts on a 12 volt nominal system) will result in emission of hydrogen gas from the battery, which may damage it or create an explosion risk. A battery may be recharged without removal from the vehicle, although in a typical roadside situation no convenient source of power may be nearby.
A vehicle with a manual transmission may be push started. This requires caution while pushing the vehicle and may require the assistance of several persons or another vehicle. If the vehicle battery cannot provide power to the ignition system, push starting will be ineffective. Most vehicles with automatic transmissions cannot be started this way because the hydraulic torque converter in the transmission will not allow the engine to be driven by the wheels.
Generally referred to as "slave starting" in military parlance, the jump starting procedure has been simplified for military vehicles. Tactical vehicles used by NATO militaries possess 24-volt electrical systems and, in accordance with STANAG 4074, have standard slave receptacles for easy connection. A slave cable is plugged in to the receptacle on each vehicle, and the dead vehicle is started with the live vehicle's engine running.
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. 
Formerly, especially in cold climates, some jump starts were done with two series-connected batteries to provide 24 volts to a 12 volt starting motor.
Heavy vehicles such as large trucks, excavation equipment, or vehicles with diesel engines may use 24-volt electrical systems. They usually have a 24V supply using two 12V automotive batteries in series: it is therefore possible to jump-start a vehicle with a 12V electrical system using only one of the two batteries.
Vintage cars may have 6-volt electrical systems, or may connect the positive terminal of the battery to the chassis. The methods intended for boosting 12-volt, negative-ground vehicles cannot be used in such cases.
Passenger vehicles with 42-volt electrical systems may not be possible to "boost" from other vehicles; professional assistance would be required to prevent severe damage to the vehicle and possible personal injury (see tow truck). Hybrid vehicles may have a very small 12 volt battery system unsuitable for sourcing the large amount of current required to boost a conventional vehicle. However, as the 12-volt system of a hybrid vehicle is only required to start up the control system of the vehicle, a very small portable battery may successfully boost a hybrid that has accidentally discharged its 12-volt system; the main propulsion battery is unlikely to also have been discharged.
- 2004 Owner's Manual,`Toyota Camry Solara, Toyota Publication No. OM33596U, for an example of an owner's manual.
- http://new.volvocars.com/ownersdocs/1986/1986_240/86240_03b.htm On-line version of a 1986 Volvo 240 owner's manual, page 64, shows jump start procedure
- Bauer, Horst (1996). Bosch Automotive Handbook 4th Edition. Stuttgart: Robert Bosch GmbH. pp. 806–807. ISBN 0-8376-0333-1.
- Schultz, Mort (December 1979). "What you may not know about jump starting". Popular Mechanics 152 (6). ISSN 0032-4558.
- http://www.duracellpower.com/documents/tech-specs/DS20070719_dcell-jumpstart-17a.pdf Duracell jumpstart 17a
- For example, http://www.vat19.com/dvds/auto-jumper-jumpstarts-car-without-cables.cfm, one maker of a cigarette lighter booster, says it won't work if the car switches off its cigarette lighter with the engine off.
- "Injuries Associated with Hazards Involving Motor Vehicle Batteries". Road Management and Engineering Journal and TranSafety. Retrieved August 2, 2007.
- Organizations such as Prevent Blindness America recommend use of splash-resistant safety goggles to protect the eyes while connecting cables. "Prevent Blindness". Prevent Blindness. Retrieved August 10, 2007.
- "Injuries Associated with Hazards Involving Motor Vehicle Batteries". NCSA. Retrieved 11 July 2013.
- "Tech Article:24 Volt Systems". BJ Series Land Cruisers. Retrieved January 5, 2012.
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- Breakdown Advice: Using Jump Leads Safely - The AA (United Kingdom)
- How to Jump Start A Car Using Jumper Cables by Matthew Wright, About.com
- Official Car Talk Guide to Jump Starting Your Car by Car Talk's Tom and Ray Magliozzi, "Click and Clack, the Tappet Brothers"
- Quick & Easy Directions to Jump Start by Car Care Council