One of the major failure mechanisms with VRLA cells used primarily is premature loss of the electrolyte within the cell, or accelerated water loss. This often results in a dry out condition within the cell which can lead to thermal runaway in extreme events if not detected prior to the event.
A major advantage for the “Gel” is the extra water in the battery. As long as recombination is good, the extra water simply translates to extra life. Dry out can be a major failure mode in VRLA batteries and can be countered by good recombination efficiency and adding water to the battery. Unfortunately, just adding water to the battery will lower the efficiency of recombination so that it approaches that of flooded batteries. The gel facilitates recombination, since it crack sand provides pathways for the oxygen to reach the negatives.
Measuring the polarizations in VRLA batteries is difficult, but it was found that the values in Gel showed a very small decrease in positive polarization (3 mV) that would point to a decreased corrosion rate and, thus, if the polarization difference remains, then the growth rate would also slightly favour increased life.
The gel components does not participate in the electrochemical reactions and can be viewed as a dilutant for the sulphuric acid. The lower the concentration of gel, the higher is the battery capacity.
The heat capacity of a VRLA battery resides mostly in the electrolyte, and this provides an opportunity to increase the overall heat capacity by using gel. The electrolyte has over 65 % of the heat capacity and that can be increased, since the gel can be added in differing quantities. This extra heat capacity can help the battery survive fluctuating outdoor temperatures, since the excursions will be minimized and the battery temperature moderated.
The gelling time depends on the particle size and concentration of the colloid along with the concentration of the acid present and their resulting temperature. The gelling times can range from seconds to many tens of hours. Temperature has a rate effect .there is also an appreciable heat of mixing with the sulphuric acid. Cooling the sulphuric acid can slow the gelling, but, generally, the colloid cannot be frozen and this limits the increased time achievable.
The goal of permeating the active materials with gel can be achieved by choosing combinations that gel very slowly. Thus, diffusion has time to work, and the increase of acid concentration at the end of formation actually finishes the gelling. This provides a uniform gel that, since it permeates the battery, provides protection against acid stratification.
Since the gel permeates the active material and separator of the battery, its structure will decrease the acid stratification. The gel behaves like a very fine fibre glass separator and thus, the battery can cycle more effectively.