Batteries generate hydrogen as part of the electrochemical reaction that produces power. When mistreated this hydrogen evolution can be very rapid, leading to hydrogen concentrations exceeding the safe limit of 4,000 ppmv in air. Please note that this limit can vary depending on many factors and the reader is referred to the literature to further their understanding of the issues at hand. In addition to the literature, the U.S. Department of Energy’s Pacific Northwest National Laboratory keeps a database of reported hydrogen related incidents at h2incidents.org.
The explosive nature of hydrogen has long been understood. However, in recent years, with the proliferation of battery driven devices at the consumer level, the number of incidents have risen dramatically. While this increase is commonly noted in sealed devices like water proof children’s bathtub toys or immersible scientific instrumentation, it is also ever more common in unsealed units like computers or smart phones.
Batteries themselves are designed to rupture in a hydrogen excursion incident. For unsealed devices that is often enough to prevent serious injury to the user, albeit the damage to the device is often terminal. The risk increases significantly when a device is sealed to make it waterproof. The hydrogen no longer can escape through leakage or rupture of the battery and is contained within the device. Hydrogen concentrations can rapidly exceed 4,000 ppmv, creating an explosive atmosphere. All it takes for a potentially serious explosion to occur is a small spark or arc.