Given the recent improvements in valve-regulated batteries, lead-acid technology is nowadays considered to be well-suited for stationary power applications. Gel and absorptive glass mat (AGM) valve-regulated lead-acid (VRLA) batteries are complementary technologies and provide reliability and efficiency due to progressive optimization of the design and components. Special attention has been paid to the separation system, as its influence on the battery performance has been fully demonstrated. Polymeric calendered ribbed separators are traditionally used in gel VRLA batteries. For this technology, the separator is required to have high pore volume, optimized pore size, low acid displacement and low electrical resistance. It must also support efficient and controlled oxygen transfer. Glass-microfibre separators are presently the preferred material for AGM batteries. In addition to the properties listed for the polymeric type, glass-microfibre separators must not allow any drainage or stratification of the liquid electrolyte, and be able to retain their initial thickness after filling and during the battery life in order to sustain the initial compression in each cell. The Amersorb separator is well adapted to both technologies, for example: (i) the ribbed and corrugated patterns provide improved porosity (pore volume and pore-size distribution); (ii) the flat membrane is not only able to wick and retain the acid, but has also optimal compression properties (low compressibility and excellent springiness). (c) 2004 Elsevier B.V. All rights reserved.