Difference between flooded and VRLA batteries

Lead acid batteries are divided into two major technologies, namely Flooded and Valve Regulated.  Valve regulated lead acid batteries are also called VRLA or SLA (Sealed Lead Acid) batteries.  

Flooded Lead Acid Batteries:

• The battery electrodes are immersed in the electrolyte (Sulfuric acid).
• When the cell or battery is punctured acid will leak.
• Lower float voltage compared to VRLA (SLA) batteries. 2.17 to 2.22V per cell.
• Float voltage changes depending upon the acid concentration used.
• Water loss with time occurs, and hence it is required to water as part of the maintenance.
• Usually bulkier than SLA batteries.
• Cycle life is lower than SLA batteries since the electrode material will shed with cycle life.
• In general the capacity will reduce with time except in certain designs (Eg., Round Cell batteries).
• Shipping is expensive because of large volume of free acid.
• Normally not allowed in commercial air carriers.
• Old/matured technology and well understood.
• Higher ventilation is required in the battery room.
• Spill containment is required around the battery stand as per the Uniform Fire Code (UFC).
• Thermal runaway is not a concern.
• Requires monthly maintenance in large standby applications.

Valve Regulated Lead Acid Batteries (VRLA/SLA): 

• The battery electrolytes are held in glass mat also called separator.
• When the cell is punctured there should not be any acid leak. Some manufacturers use excess acid and could have small acid leaks when the cells are new.
• Float voltage is higher than flooded batteries (2.25V/cell plus).
• No need to add water since the water loss is significantly lower than flooded batteries.
• Takes lower volume compared to flooded batteries.
• The electrodes are warped with separators and held tight.  The electrode shedding is generally reduced and hence the rate of capacity decay with cycle is also reduced compared to flooded batteries.
• Higher cycle life compared to lead acid batteries.
• Lower amount of hydrogen is release during normal operation and hence generally lower ventilation is needed compared to that of similar size flooded battery.
• Thermal runaway is a concern in VRLA batteries.
• Maintenance requirement is low compared to flooded batteries.
• In general Spill containment is not needed - However, in UFC there is no differentiation between flooded vs VRLA batteries and depending upon the local fire marshals you may have to put spill containment for valve regulated batteries.
• Thermal run away is a concern.  
• More and more flooded installations are being with VRLA batteries.

If you have any question regarding this feel free to contact us.

Battery Consulting
(877)394-3941