Basics of VRLA Technology

Source: BAE Energy From Batteries

In the gaseous phase, oxygen gas migrates to the negative plate where it is reduced  to oxygen ions (O2). In the meantime, hydrogen protons (H+) are being transported  in the electrolyte to the negative plate as well. At the negative plate the oxygen  and hydrogen combine again to form water. Known as the internal oxygen cycle, this  process is called recombination of the electrolyte and defines the principle of VRLA methodology. It is based on the principle that “the oxygen evolution rate at the positive electrode and the oxygen-reduction rate at the negative electrode balance each other.”5

Not to oversimplify or appear to be too repetitive, but as most who work with  batteries on a regular basis know, in a vented lead acid (VLA) flooded cell, during  float and discharge, oxygen gas is released from the cell and the hydrogen ions  move in the electrolyte to the negative plate. There it is reduced to hydrogen gas. 

Some of the oxygen gas does find its way to the negative plate where it can be recombined with the hydrogen gas and reform into water. But both the oxygen and the hydrogen gasses not recombined are released and leave the cell via the openings in the flame arrestor. This action results in a slow water loss, but eventually these cells have to be refilled with distilled or ionic water. Of course, one of the advantages of a VLA cell is that this event can be monitored and controlled, assuring that the cells remain filled with the correct amount of electrolyte, contributing to their extended service life. 

With valve-regulated lead-acid (VRLA) cells, there are actually two distinct actions: [1] a liquid one in which the hydrogen ions are transported via the electrolyte and [2] a gaseous one. Both play an important role in the performance of the VRLA cell. 

In the valve regulated cell, the water is diffused back into the mat or gel. However, it is important to note that the internal oxygen cycle generates water from the oxygen and the hydrogen protons, not the hydrogen gas. This is important because it shows that hydrogen gas is not removed by the internal oxygen cycle.6

Dr. Rusch points out that while this is true, 5-10% of the hydrogen ions are still reduced to a gaseous state, and this small amount of hydrogen is released through the pressure sensitive release valves

In the VRLA process, the electrolyte is mobilized in one of two ways, depending  upon the type of VRLA being considered. 

1. The Absorbed Glass Mat (AGM) VRLA utilizes a microporous felt known as a  glass mat to immobilize the electrolyte. The electrolyte then fills the smaller pores of the mat by capillary action, but a good portion of the larger pores remain unfilled. Specific gravity also plays a role here, but we’ll deal with that in a moment. 

2. The Gelled Electrolyte (GEL) forms by the addition of silica dioxide (SiO2) to the sulfuric acid (SO4) and water (H2O). In this type, the gel shrinks which open crevices that run through the electrolyte. Void space is available around the electrodes, allowing fast transport of the oxygen gas. 

Obviously, the question we need to ask and answer today is: what are the differences between these two forms of VRLA cells, and what are the resulting pluses and minuses of each in today’s context?