Gassing Batteries
The release of hydrogen and oxygen during the charging of a lead acid battery is normal. However, in a non-sealed battery, such as a typical flooded cell, the hydrogen release is the potential problem. When the concentration of hydroden in the atmosphere reaches or exceeds 4.5%, it is explosive. Actually, it is VERY explosive. The only lead acid battery currently available that cannot and will not ever produce enough hydrogen to reach the magic 4.5% is the Lifeline AGM family of batteries. Even under massive and long term over-charging, they will never produce more than about 2.5% hydrogen, and under any normal charge protocol, basically will release only trace amounts of anything.In any flooded battery area that has either heat sources or spark sources, ventilation is the only thing between you and a fireball.