venting is required on ALL batteries.
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Updated June 2017
All lead acid batteries, AGMs and gel cells, generate explosive gas. Even though most are sealed, makers stress that battery ventilation is vital still. Confusion exists over this. Around 2000, some battery makers began to claim that no ventilation was required. Or, ventilation, is advisable but not necessarily essential. They withdrew this advice, however, shortly after. Many batteries thus have a warning notice as below.
Battery ventilation notice
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