All lead based battery’s give off hydrogen when charging… even “sealed lead acid” battery’s have a small vent.. From: owner-personal_submersibles@psubs.org [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Alan James Does anyone know if any battery can be just covered with a thin layer of fiberglass & the attached cables potted for external use.? I was thinking maybe AGM or Glass matt, but I'm not sure whether there are air voids in them. If they have no air voids & don't off gass possibly they are reasonably pressure resistant as they are. Just read this on Wiki About Lithium batteries. Pure lithium is very reactive. It reacts vigorously with water to form lithium hydroxide and hydrogen gas is liberated. Thus a non-aqueous electrolyte is typically used, and a sealed container rigidly excludes water from the battery pack. Alan |