believe it or not, that most likely is your batteries flash arrester. we put them in our batteries (die-hard, odyessy, stinger, SBS 145s, etc) but the ones we use are little styrofoam pellets or flat disc that sit beneath the top valve cover label. it appears to have the same function, if in case it flames out(?) it keeps it from spreading. If anyone doesnt have one I wouldnt worry all that much about it