Most car battery explosions are caused by thermal runaway, which is when the battery overheats, causing its cells to break open. Usually, one cell starts a chain reaction in which other cells rupture. A swollen battery is a less serious manifestation of the thermal runaway effect.
Both older lead-acid car batteries and newer lithium-ion car batteries are susceptible to the effects of thermal runaway, thus both types of battery can explode under the right circumstances. Because of their higher density, lithium ion batteries have the potential to explode more violently than their legacy lead-acid counterparts.
Thermal runaway can occur as the result of several issues. A short circuit in the vehicle's electrical system can overload the car battery to the point at which it overheats. Other batteries contain manufacturing defects, which make them more prone to trouble. Blocked or absent safety vents on sealed cells can cause them to overheat and rupture.
When one compartment in the battery overheats, it starts a domino effect. The overheated cell breaks apart and causes the cell next to it to overheat. In turn, all the cells in the battery overheat until the entire unit bursts into flames or explodes.
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.