The name “valve regulated” does not wholly describe the technology, these are really "recombinant" batteries, which means that the oxygen evolved at the positive plates will largely recombine with the hydrogen ready to evolve on the negative plates, creating water and so preventing water loss. The valve is a safety feature in case the rate of hydrogen evolution becomes dangerously high. In flooded cells, the gases escape before they have a chance to recombine, so water must be periodically added.
VRLA battery is designed for less maintenance lead acid battery which doesn’t required addition of water to the cells. And it are commonly further classified as absorbed glass mat (AGM) battery and GEL battery (gel cell). While the AGM and gel battery designs share many of the same components, such as containers, pressure relief valves and plates, they have different separator systems and electrolyte immobilization systems which result in significantly different high rate performance, heat dissipation and cycle life characteristics.
The AGM VRLA battery typically contains more electrolytes and is of slightly higher specific gravity than the comparable gelled electrolyte battery. Consequently it will provide slightly more long duration capacity within the same container volume. Also, the AGM VRLA battery has a slightly more efficient oxygen recombination cycle and a low resistance than the gelled electrolyte VRLA battery. As a result it will draw slightly more float current resulting in greater internal heat generation. Note how the float current is affected by temperature, increasing with increasing temperature. To prevent premature failure and possibly catastrophic thermal runaway, it is important to operate the VRLA battery in an environment in which it can dissipate heat at a rate faster than it is internally generated.
A battery is in float service when it is continually connected to the power source and the load so as to provide instant uninterrupted power in the event of failure of the primary power source. The float service life characteristics at 77°F are essentially the same for the AGM and GEL VRLA batteries.
In cycle service the battery is deeply discharged as the primary power source for the application such as with wheelchairs, golf carts and photovoltaic systems. While the AGM and Type A gelled electrolyte batteries will provide good cycle service, the Type B gelled electrolyte battery is designed specifically to provide the longest service life in deep cycle applications.
No on design of VRLA battery is optimum for all the various types of applications. The type of electrolyte and its specific gravity and separator systems as well as the electrolyte immobilization technique utilized greatly determine the battery’s suitability to provide maximum power density, superior high rate performance, extended life at elevated temperatures and extended cycle life.
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