How Valve-Regulated Lead-Acid Batteries Work
The key structural difference between a valve-regulated lead-acid (VRLA) battery and a conventional flooded battery is that the electrolyte is absorbed in the separator and plates rather than being free-flowing.
At the end of charging, oxygen evolved at the positive plate passes through the microporous channels of the separator to the negative plate, where it reacts with active lead to form water again. This process is called oxygen recombination. Because the oxygen is consumed inside the battery, net water loss under normal float conditions is very small, so these batteries generally do not need topping up with distilled water during service.
"Valve-regulated" refers to the one-way safety valve fitted to the battery. When internal pressure rises to the set value due to abnormal overcharge or high temperature, the valve opens to release gas and closes again once the pressure falls. Its purpose is safety, not to allow continuous venting during normal operation.
It should be emphasised that VRLA sealing is valve-regulated sealing, not absolute sealing. If the charging voltage is set too high or the ambient temperature stays high, recombination cannot absorb all the gas evolved, the valve opens frequently, water loss accelerates and service life is shortened. This is why float voltage needs temperature compensation.
