One of the most common reasons why automotive batteries explode is when the hydrogen gas that is produced during the charging cycle builds up inside the case and is ignited by a spark or a flame. Exploding car batteries send sulfuric acid and fragments of the battery case flying everywhere and these can cause injuries to anyone in the vicinity.
A lead-acid battery system produces hydrogen gas through the electrolysis of water when overcharged. Car batteries have vents on each battery cell to allow hydrogen to dissipate. However, the hydrogen gas may not be vented out properly for several reasons and these may include structural flaws in the battery, the vents are inadequate for proper dissipation or the battery may be extremely overcharged.
Sometimes, an automotive battery may not explode but the accumulation of hydrogen gas may cause the ends of the battery to become swollen and too hot to handle. This is also an sign of a malfunctioning charging system of the vehicle. The swollen ends are indicative of extreme overcharging of the batteries creating hydrogen in such large amounts that the ventilation system is no longer able to handle the gas build up. A hot and swollen battery is also dangerous.
Another potential cause of battery explosion is not related to hydrogen build up, but is caused by the build up of water vapor from a boiling electrolyte. The electrolyte may be accidentally heated when the terminals of the battery are short circuited.
Another explosive risk for older lead-acid batteries is sheer age. As lead-acid batteries age, they lose water, causing the lead plates in the batteries to warp. Sometimes, the energy requirement of the vehicle upon ignition causes the warped plates to flex and touch one another, which can lead to a sudden explosion.