During the charging of a lead-acid battery, which gas poses a risk of explosion?

During the charging of a lead-acid battery, hydrogen gas is produced as a byproduct of electrolysis. This gas poses a significant risk of explosion if it accumulates in sufficient quantities and comes into contact with an ignition source. Hydrogen is highly flammable and can ignite easily, leading to potentially explosive reactions. The production of hydrogen gas increases especially when the battery is overcharged, which can result in a dangerous buildup of gas within the battery housing or surrounding area.

In contrast, the other gases listed—oxygen, nitrogen, and carbon dioxide—do not present similar risks in the context of lead-acid battery charging. While oxygen is produced during the charging process as well, it is not flammable in the same way hydrogen is and does not pose an explosion hazard under normal conditions. Nitrogen is an inert gas that does not react with the elements present in a battery, and carbon dioxide is a byproduct of combustion and does not occur in significant quantities during battery operation. As such, hydrogen's properties are what make it the primary concern for explosive risks during the charging of lead-acid batteries.