Bateries in general do not do well when run down but my understanding is sealled glass matt batteries do worse. If they run down that far I can see where they would get warm depending on the charge rate. I am not suprized they give off gas but I don't think it aproaches the corrosiveness of the sulfuric acid in a lead acid battery. I also suspect the gassing to revive a low battery cannot be good.
One way to make the battery last longer is to get a good alternator. Marginal alternators that cannot keep up at idle allow the battery to discharge. The minute you hit the accelerator a large amount of current reverses direction to the battery.
Gary
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