What would make a battery explode?

Answer 1

Exploding Car BatteriesSince this is in the "Car Batteries" section, this answer will be limited to automotive batteries only. However, almost all batteries can explode under the right set of conditions and circumstances.

Almost all automotive batteries are of the "Lead-Acid" type. The acid in Lead-Acid batteries is Sulphuric Acid [H2SO4] and in the process of discharging and/or charging of these batteries HYDROGEN gas [H2] is created inside the battery. Some of this gas is always present inside the battery, and some always escapes to the outside via the vents in the battery cell caps.

IF the "mixture ratio" [a proper amount of air mixed with the Hydrogen gas] happens to be within the "explosive range," and if any spark occurs within that gas envelope, either inside or outside of the battery, then the gas-air mixture will ignite explosively.

Ignition of the gas-air mixture can occur inside the battery, or outside, and if outside, can "flashback" into/inside the battery causing it to explode.

This is why all instructions regarding working with automotive batteries warn the operator to keep all open flame and/or sparks away from the battery.

AND when making "jumper" wire connections, that the "last" connection to be made should be the negative cable from the jumping vehicle, and that it should be connected to a grounded part of the engine or frame of the vehicle being jump started, well AWAY FROM THE BATTERY.