What Happens to a Car When Jumping It and You Touch the Cables Together and Cause a Spark?

by John Cagney Nash
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Most people’s first thought when discovering their car’s battery is discharged is to get a jump start from another vehicle. This practice is so common that familiarity can breed contempt but -- unfortunately -- the process involves some potential dangers, especially from sparking. When using jumper cables, always follow the instructions from the manufacturers of the car, the battery and the cables.

Deliberately Causing Sparks

Some people are in the habit of tapping the free ends of jumper cables together when the other ends are connected to a battery; this can be extremely hazardous. Volts can be compared to the pressure in a water hose, and amps to the volume of water. The 12 volts carried by a car battery are unlikely to cause injury, but the amperage can be extremely dangerous. A fully charged battery can discharge hundreds of amps through a spark, producing a current similar to that of a welding arc; like a massive amount of water that is not under great pressure, this can still kill.

Accidental Sparking

Jumper cables are universally color-coded red for positive and black for negative. Reversing these polarities will almost certainly result in a massive spark that can cause severe burn injuries. The complex electronic systems of modern vehicles can also be destroyed by reversed polarity. When jumping a car, always double-check that the red cable is connected at both ends to the larger terminals marked with “+” signs, and the black cable is connected at both ends to the terminals marked with “-” signs. Always connect both ends of the red cable first; this prevents accidental sparking to the body of either car as the second cable is connected.

Dangers of Creating a Spark

When batteries charge, they give off hydrogen gas, and a discharged battery connected to another vehicle via jumper cables is charging. Hydrogen is extremely explosive when mixed with combustion air, and this dangerous mix typically occurs close to the charging battery. Causing a battery to explode with a spark from jumper cables can create flying shards of plastic that can easily blind anyone standing nearby. Aside from the obvious dangers of being near an explosion, remember the electrolyte in lead-acid batteries contains sulfuric acid, which aggressively attacks human tissue. When a battery explodes, it can shower nearby people with acid.

Hybrid Vehicles

Some hybrid vehicles carry a voltage much higher than anything delivered by regular utility mains. The Toyota Prius and the Ford Escape both have battery packs that can yield in excess of 300 volts. Making a circuit through the human body with 300 volts is likely to be lethal. Making any connection to the battery pack must only be done while wearing heavy-duty insulated gloves and in strict accordance with the vehicle manufacturer’s instructions.

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