How to charge a valve regulated lead acid battery
Sealed lead acid battery is designed to prevent electrolyte loss through evaporation, spillage and gassing and this in turn prolongs the life of the battery and eases maintenance. Instead of simple vent caps on the cells to let gas escape, valve regulated lead acid battery (VRLA) have pressure valves that open only under extreme conditions.
Driven by diverse applications, two sealed lead acid battery types have emerged. They are the sealed lead acid (SLA), and the valve regulated lead acid (VRLA). Technically, both batteries are the same. Engineers may argue that the word ‘sealed lead acid’ is a misnomer because no lead acid battery can be totally sealed.
The charge voltage setting on VRLA is generally lower than SLA. Heat is a killer of VRLA. Many stationary batteries are kept in shelters with no air conditioning. Every 8°C (15°F) rise in temperature cuts the battery life in half. A VRLA battery, which would last for 10 years at 25°C (77°F), will only be good for 5 years if operated at 33°C (95°F). Once damaged by heat, no remedy exists to improve capacity.
The cell voltages of a VRLA battery must be harmonized as close as possible. Applying an equalizing charge every 6 months brings all cells to similar voltage levels. This is done by increasing the cell voltage to 2.50V/cell for about 2 hours. During the service, the battery must be kept cool and careful observation is needed. Limit cell venting. Most VRLA vent at 0.3 bar (5 psi). Not only does escaping hydrogen deplete the electrolyte, it is highly flammable.
Water permeation, or loss of electrolyte, is a concern with sealed lead acid batteries. Adding water may help to restore capacity but a long-term fix is uncertain. The battery becomes unreliable and requires high maintenance.
VRLA battery is generally used for stationary applications. Their capacities range from 30Ah to several thousand Ah and are found in larger UPS systems for power backup. Typical uses are mobile phone repeaters, cable distribution centers, Internet hubs and utilities, as well as power backup for banks, hospitals, airports and military installations.
Unlike the flooded lead acid battery, both the SLA and VRLA are designed with a low overvoltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging would cause gassing and water depletion. Consequently, the SLA and VRLA can never be charged to their full potential.
Source: www.leoch.com
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