Overfilled battery now have rotten egg smell
Hydrogen doesn''t smell of anything, the rotten egg smell is Hydrogen Sulphide, H2S . The Sulphuric Acid H2SO4 has lost oxygen and what is left is Hydrogen Sulphide H2S I would say the battery has come to the end of it''s life, and if connected in parallel with any other battery can cause other serious issues with the connected batteries.