A further life limiting factor is electrolyte water loss. VRLA batteries are designed to operate during their entire service life without any topping-up
of the electrolyte with water. For this purpose, an oxygen recombination cycle is introduced so to reduce the loss of electrolyte water to a minimum.

However water loss does nevertheless occur and has following magnitudes:

•   recombination efficiency < 100 % approx. 0.016 g per 100 Ah cell
    and day
  
  
•   excessive float current 0.336 g per ampere hour excessive float   current   and cel
  
  
•   positive grid corrosion 0.17 g per gram of lead oxydized to PbO
  
  
•   cell case transpiration approx. 0.06 g per 100 Ah cell and day at   +40 °C
  
  
•   self discharge reaction approx. 0.0336 g per 100 Ah cell and day
    at 20 °C
  
  
  Excessive water loss by one or more of the above mechanisms can cause the premature failure of a VRLA battery by a process called dry-out. Dry-out results in reduced electrolyte volume and capacity loss.