During the charging cycle, hydrogen gas is generated and accumulates in the head space above the electrolyte level, prior to venting. Hydrogen gas has a wide range of explosive limits in air, ranging from 4 to 72-percent hydrogen in air and is easily ignited by a flame or spark. If the hydrogen is ignited inside the battery, then it typically blows off the top of the battery case, showering sulfuric acid in the immediate vicinity along with fragments of the battery case. The explosive energy generation is so rapid that the vents cannot relieve the pressure in time to prevent an explosion.
Figure 2 to the upper right is a view of a two-year-old battery that exploded, causing personal injury from acid burns. This occurred when a standby generator was starting during its normal maintenance test cycle. The top of the battery was blown away, suggesting that hydrogen was ignited inside the battery.