Jump Start (vehicle) - Limitations

Limitations

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.

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.

Current from the boosting vehicle will charge the dead battery if it is still capable of being charged. After several minutes enough energy is transferred to allow cranking of the engine. 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 (or where the battery is not capable of holding a charge) if the cables are heavy-duty. Cranking current drawn through light-gauge cables in this way will damage them by overheating; electrical resistance of a smaller wire is higher than that of a larger wire, and resistance dissipates electrical energy into heat.

If the discharged battery case is cracked, or the battery has a low electrolyte level, or is frozen, a jump start will not restore normal operation. 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. 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.