check your charging system and water level in the battery ( that hiss was the vent and for sure was pushing out fumes/boiled over water from the battery)but more likely then not you'll have to replace the battery, because it is for sure over charging and if it's been overcharged for awhile it usually turns the battery into scrap. but check into that stuff and see if it will hold charge after the problem has been fixed.
bartamos is right they do blow up if kept overcharging you may even notice your battery has a bulge somewhere on the sides of it now
