The fundamentals of operation of the gas recombination in a VRLA lead acid cell is described below:

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Schematic view of the oxygen recombination cycle in a VRLA cell in which the electrolyte is immobilized/absorbed in a glass mat (AGM).

During float charge the current decomposes water and oxygen is evolved on the positive plate according to the reaction:

• electrochemical oxydation of water with oxygen evolution
  
  2H₂O -----> O₂ + 4H⁺ + 4e^-
  

The oxygen is diffusing via the gas phase through unfilled pores of the glass mat separator to the negative plate, and reacts there chemically with the formation of lead oxide, lead sulfate and water:

• reaction of oxygen with lead
  
  O₂ + 2Pb -----> 2PbO
  
  
• reaction of lead oxide with the electrolyte and formation of water
  
  2PbO + 2H₂SO₄ -----> 2PbSO₄ + 2H₂O
  
  

The float current passing through cell then reduces, instead of evolving hydrogen, on the negative electrode the formed lead sulfate back to lead and sulfuric acid so that a closed cycle (oxygen recombination cycle) is obtained.

• electrochemical reduction of lead sulfate
  
  2PbSO₄ + 4H⁺ + 4e^- -----> 2Pb + 2H₂SO₄
  
  
• The overall oxygen recombination reaction is hence
  
  O₂ + 4H⁺ +4e^- -----> 2H₂O
  
  

Due to the oxygen recombination reaction essentially no hydrogen evolution occurs on the negative plate.