Separator Materials for Valve-regulated Lead-Acid Batteries

Publisher Summary This chapter discusses new separator materials for valve-regulated lead–acid (VRLA) batteries. There are two major VRLA battery technologies: absorptive glass mat (AGM) and gel designs. The most commonly used AGM separators are microfiber-glass fleeces without organic components, such as binders or polymeric fibers. In the battery, the acid saturation of AGM separators is usually in the range of 85 to 95%. This increases the effective electrical resistance over that of fully saturated separators by 20% for 95% saturation and by 90% for 85% saturation. AGM batteries are predominantly used in applications that do not involve frequent service under deep-discharge cycles. In gel batteries, the addition of 5–8 wt% of fumed silica (pyrogenic silica) with a specific surface-area of 200–300 m 2 g -1 serves to immobilize the electrolyte. Because of the very effective immobilization of acid in the fumed-silica gel, the batteries are less prone to electrolyte drainage and acid stratification than AGM batteries. Separators for gel batteries have ribs positioned not only toward the positive plate but also toward the negative plate to facilitate the gel-filling process. Synthetic wood-pulp (SWP) separators are used widely for automotive batteries. These are fleeces made from fibrillated polyethylene or polypropylene and small amounts of additional fibres (e.g., polyester fibres); sometimes, the fleeces are also filled with silica. The organic sheets are laminated with a glass mat, which serves to stabilize the positive active-material. Recently, a microporous separator known as the “acid jellying separator” (AJS) has been developed. This is a highly filled polymer separator with fumed silica as the major filler component.