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A car's battery is designed to perform one task very well, and that is to power an electric starter motor whenever the ignition switch is turned on. Once a car starts, an electric generator called an alternator assumes responsibility for most of the car's electrical needs, including recharging the battery for future starts. If the battery's power level becomes too low to turn the starter motor, the driver may have to use a second battery to provide additional power. The cables used to connect the auxiliary battery to the dead battery are called jumper cables, because they allow electrical power to "jump" from one battery to another.
Jumper cables are insulated lengths of heavy gauge wire which have large spring-loaded alligator clips on either end. Two clips are usually marked "positive" with the use of red insulation and plus marks (+), while the other two clips have black insulation and "negative" marks (-). This is a very important designation, since the positive clamps on each end of the jumper cables should only be attached to the positive terminals of the batteries, and the negative clamps should only be attached to the negative post or grounded area of the engine block.
In order to use jumper cables safely, the two batteries need to be in close proximity to each other, which usually means maneuvering the second vehicle end-to-end or side-by-side with the vehicle containing the dead battery. Jumper cables can be 10 feet or more in length, but the clamps need to be seated securely on the battery posts in order to get the best connection. Once the batteries are positioned properly, the driver can begin to attach the jumper cables to both vehicles.
Jumper cables essentially draw the power from one battery and feed it steadily to a uncharged battery. The driver should take one positive clamp and attach it securely to the positive post of the "good" battery. The corresponding positive clamp should then be attached to the "bad" battery. The negative clamp on the same end of the jumper cables as the first positive clamp should then be attached to the negative post of the "good" battery. At this point the remaining negative clamp should never come in contact with the positive clamps, since it would complete a circuit and create sparks. The last negative clamp should be attached to a piece of metal connected to the engine block but away from the battery to avoid creating sparks.
Now that the jumper cables have been properly attached, one driver can start the "good" car, which sends power from both the alternator and battery through the jumper cables and into the "bad" battery. This recharging process can take several minutes, since car batteries use a chemical process to generate their power. After a certain amount of time has elapsed, a driver can then attempt to start the "bad" car by turning the ignition. If the battery has regained enough power or the power from the second battery is sufficient, the engine should turn over and the alternator should begin charging the battery again. If this doesn't happen on the first attempt, the battery may need more time to build up a charge.
Once the car has started, the jumper cables should be removed in reversed order. The negative clamp on the "bad" car should be removed first, then the negative clamp on the "good" car. The positive clamp on the "bad" car is removed next, followed by the positive clamp on the "good" car. The jumped car's engine should be allowed to idle for several minutes in order to make sure the battery has been charged enough to handle a second start if necessary. Many drivers who have their cars jump started will leave the engine running until they have reached their destination or a repair shop.
Jumper cables are relatively inexpensive and should be stored in a car's trunk for emergency use. If a car breaks down on an isolated stretch of highway, having a set of jumper cables can make the difference between a quick jump start and a long wait for a wrecker or tow truck.