Valve-regulated lead–acid batteries operating under the oxygen cycle have had a major impact on the battery market over the last 25 years. They differ from conventional flooded batteries in that the electrolyte level is controlled to ensure that some gaseous porosity remains in the separator.
The valve-regulated version of this battery system, the VRLA battery, is a development parallel to the sealed nickel/cadmium battery that appeared on the market shortly after World War II and largely replaced lead-acid batteries in portable applications at that time.
The valve regulated lead-acid (VRLA) battery functions by means of an internal oxygen cycle (or oxygen-recombination cycle)\. During the latter stages of charging and overcharging of the positive electrode, oxygen is evolved.
The valve-regulated lead-acid (VRLA) battery is a type of lead-acid battery that requires no replenishment of the water content of the electrolyte solution and does not spill liquids. It can be used in any desired orientation.
Moreover, acid is immobilized in the new design and this endows the cell with the additional advantages of being ‘spill-proof’ and able to operate in any orientation (upright, on its side, or even upside down). The change to the so-called ‘valve-regulated lead–acid’ (VRLA) technology has not, however, been accomplished without some difficulty.
Charge profiles for new 6 V 100 Ah valve-regulated lead–acid (VRLA) batteries at different charge voltages and temperatures. Reproduced from Culpin B (2004) Thermal runaway in valve-regulated lead-acid cells and the effect of separator structure. Journal of Power Sources 133: 79–86; Figure 1. Figure 9.