Exploding Batteries
Any lead-acid battery system when overcharged (>14.34 V) will produce hydrogen gas (gassing voltage) by electrolysis of water. If the rate of overcharge is small, the vents of each cell allow the dissipation of the gas. However, on severe overcharge or if ventilation is inadequate, or the battery is faulty, a flammable concentration of hydrogen may remain in the cell or in the battery enclosure. An internal spark can cause a hydrogen and oxygen explosion, which will damage the battery and its surroundings and which will disperse acid into the surroundings. Anyone close to the battery may be injured.
Sometimes the ends of a battery will be severely swollen, and when accompanied by the case being too hot to touch, this usually indicates a malfunction in the charging system of the car. Reversing the positive and negative leads will damage the battery. When severely overcharged, a lead-acid battery produces high levels of hydrogen and the venting system built into the battery cannot handle the high level of gas, so the pressure builds inside the battery, resulting in the swollen ends. An unregulated alternator can quickly ruin a battery by excessive voltage. A swollen, hot battery is dangerous.
Another potential cause of explosion is when the battery terminals are short-circuited via a very low resistance path (like a wrench or other tool dropped or lying across the terminals). Apart from the sparks which usually occur in a short circuit, heating due to the internal resistance of the battery can cause the electrolyte to boil, also leading to explosion due to buildup of water vapor pressure (unrelated to electrolysis).
Persons handling car batteries should wear protective equipment (goggles, overalls, gloves) to avoid injury by acid spills. Any open flame or electric sparks, including lit tobacco products like cigarettes, cigars or pipes, in the area also present a danger of ignition of any hydrogen gas emanating from a battery (this is the reason, when recharging the battery in place in the vehicle or jump starting, that the negative cable of the recharger or attached to the other vehicle's jumping battery negative post is always attached away from the battery to ground on the engine or frame, and is always attached to complete the circuit only after the positive cable has been attached to the battery's positive terminal (and is removed in the reverse order, i.e., negative cable first from the frame or engine, breaking the circuit, then positive cable from the battery) - in this fashion, any sparks which may occur will occur at the more distant location of the negative cable attachment point, away from the battery and potentially explosive gases, and no sparks will occur, as the circuit is no longer complete, when the positive cable is attached or detached from the battery).
Read more about this topic: Automotive Battery
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