Almost all drivers know that a car with a dead battery can be started by jumping it from a car with a charged battery. Automotive batteries are designed to produce the high electrical current required to start the engine. This routine procedure can become dangerous if the jump leads are connected improperly -- if the positive terminal on each battery is mistakenly connected to the negative terminal on the other battery. Damage will result from very high current flow, and possibly from incorrect polarity on the "dead battery" vehicle.
Damage to the Batteries
Connecting the positive terminal of each battery to the negative terminal of the other battery will result in a huge surge of electrical current between the two batteries. This will cause the batteries to heat very quickly, and in lead-acid type batteries -- the most common type -- it will result in the generation of a large amount of hydrogen gas within the charged battery. The heat can melt internal and external battery parts, while the pressure from the hydrogen gas can crack the battery casing. Once the casing is cracked, escaping hydrogen can potentially ignite and explode.
Damage to the Jump Leads
Jump leads are not designed to carry the huge surge of electrical current, and will quickly heat up to very high temperatures. This can melt the insulation on the cables and potentially expose people to direct contact with the electrical cables. The heat can also melt solder and other components that hold the cables and clamps together.
Other Possible Damage
The surge of electrical current can blow the fusible link or fuse element that protects the vehicle's main electrical system. If the engine of the vehicle with the charged battery is left running, the electrical surge can damage this vehicle's alternator.
Damage Due to Incorrect Polarity
When the jump leads are incorrectly connected, the polarity of the electrical system on the vehicle with the dead battery will be reversed for a few seconds. This can irreparably damage many of the sensitive electronic components that are common on today's vehicles, such as on-board computers and electronic sensors.