Automobile Repair/Jump start

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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 direct current (DC) Lead-acid 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 large crocodile clips at each end.

Good quality jumper cables will have large copper conductors, well-made alligator clips, and insulation which remains flexible at low temperatures. Longer cables require less maneuvering of the boosting vehicle to allow connection of the two batteries.

Detailed Procedure[edit | edit source]

Preparation and cautions[edit | edit source]

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.[1][2] 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.[3] Organizations such as Prevent Blindness America recommend use of splash-resistant safety goggles to protect the eyes while connecting cables.[4]

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[5] 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.

Current from the boosting vehicle will charge the dead battery. After several minutes enough energy is transferred to allow cranking of the engine.[6] 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.

A jump start is only effective for a discharged battery and cannot resolve other faults including a lack of fuel, a failed battery, or other mechanical problems. 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 themself with the effects of a dead battery; booster cables may be unavailable if the keyless entry system won't unlock the trunk (boot).

How to[edit | edit source]

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.
A "safety" jumper cable, in two pieces. The red and black leads of each piece are connected to a car's battery, and then the yellow connectors are plugged together.

With a safety cable, such as the one shown in the picture, the two pieces are connected together. The safety cable has colour coded crocodile clips and the connectors are polarised 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 flammable 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 circuit.
  • 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 cars, such as a different sequence of attaching or detaching jumper cables. For your safety, please read your owner's manual!)

The "good" car is then started, and the dead battery is allowed to charge for a few minutes. Then the car with the dead battery can be started, the cables carefully detached, and the formerly-stranded motorist goes on his or her way. If the original cause of the dead battery was simply a drain such as the headlights being left on overnight, then the car's generator should take care of finishing the recharge and keeping the battery charged; but if the battery is damaged or old or there is some problem with the car's electrical system, then the motorist should keep his engine running until he can buy a new battery or reach a service station.

Note that unlike a battery charger, a car's alternator lacks the current-limiting circuit needed for the proper trickle-charging of deeply discharged batteries, and should not be used for that purpose. Charging a battery at too high a current is not good for either the alternator, or the battery.

In localities or situations lacking in Good Samaritans, there is always the auto club, for its members. Roadside assistance vehicles, dispatched by them or not, can be expected to carry equipment at least as handy as a self-contained and portable jump-start unit consisting of a battery and two leads to connect it to the stopped car's starter circuit, substituting for the dead battery.

Step-by-Step Guide[edit | edit source]

  1. Clamp one cable to the positive (+) terminal of the dead battery. Don't let the positive cable touch anything metal other than the battery terminals.
  2. Connect the other end of the positive cable to the positive terminal of the good battery.
  3. Connect one end of the negative (-) cable to the negative terminal of the good battery.
  4. Connect the other end of the negative cable to metal on the engine block on the car with the dead battery. Don't connect it to the dead battery, carburetor, fuel lines or moving parts.
  5. Stand back and start the car with the good battery.
  6. Start the stalled car.
  7. Remove the cables in reverse order.

see safetycenter for more details.

Alternatives to jumper cables[edit | edit source]

Cigarette lighter outlet[edit | edit source]

An alternative to jumper cables is a cable used to interconnect the 12 volt power outlets (cigarette lighter outlet) 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 through very slowly charging the battery, not through providing the current needed for cranking. Engine cranking should not be attempted as the starter motor current will exceed the fuse rating in a cigarette lighter outlet. Not all vehicle cigarette lighter outlets remain on while the ignition is turned off,[7] making the technique unusable unless the ignition key is turned to the accessory position to connect the cigarette lighter outlet to the battery.

Battery booster and jump starter[edit | edit source]

A hand-portable battery, equipped with attached cables and charger, can be used similarly to another vehicle's battery (it is called a battery booster and jump starter). It can reach 1700 peak amps.

Portable boosters may automatically sense the battery's polarity prior to sending power to the vehicle, eliminating the costly damage that can result from a simple jump-starting mistake.

Battery charger[edit | edit source]

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 allow source of a large amount of current to assist in cranking the engine. Battery chargers that apply high voltage (for example, more than 16 volts on a 12 volt nominal system) will result in high emission of hydrogen gas from the battery and may damage it. A battery may be recharged without removal from the vehicle, although in a typical roadside situation no convenient source of AC power may be nearby.

Push starting[edit | edit source]

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. 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 (some very old automatic transmissions, e.g., General Motors' two-speed Powerglide transmission, do leave a solid connection between the engine and wheels, and cars equipped with such transmissions can be push started).

Notes[edit | edit source]

  1. 2004 Owner's Manual,`Toyota Camry Solara, Toyota Publication No. OM33596U, an example of an owner's manual 
  2. On-line version of a 1986 Volvo 240 owner's manaul, page 64, shows jump start procedure
  3. Injuries Associated with Hazards Involving Motor Vehicle Batteries, Road Management and Engineering Journal and TranSafety,, retrieved August 2, 2007 
  4. Prevent Blindness, Prevent Blindness,, retrieved August 10, 2007 
  5. Bauer, Horst (1996). Bosch Automotive Handbook 4th Edition. Stuttgart: Robert Bosch GmbH. pp. 806-807. ISBN 0-8376-0333-1. 
  6. Hybrid vehicles have slightly different power requirements, there is no need to 'crank the engine' for instance. The donor vehicle only has to supply enough power to start the hybrid system. Automobile Repair/Toyota/Prius for example.
  7. For example,, one maker of a cigarette lighter booster, says it won't work if the car switches off its cigarette lighter with the engine off.

References[edit | edit source]