it is not a myth that hydrogen gas comes out of the battery vents if you are charging a battery. similar to charging, jump starting a vehicle, the weak battery produces hydrogen gas. there is always that risk. that is why the battery top warns you not to smoke, have an open flame and avoid sparks when connecting or disconnecting a battery. i once had a co worker in a construction company who was charging about thirty 7D batteries for heavy equipment who was a smoker. he would check batteries charging with a lit cigarette between his fingers. i told him about the risk. and he would brag that he had been doing this for decades. one lucky day, two batteries being charged exploded because of his lit up cigarette. he was lucky enough only two batteries exploded. he came running out of the shop screaming in pain from the cuts he sustained from the battery casings and the acid on his face. accidents are avoidable specially if you the risks. experience has it, batteries do explode when they are gassing and there is a source of ignition or spark. that co worker has since passed away, rest his soul but the lesson i learned from his indifferent mistake lives on
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