Absorbent material made of 100% microglass fibre is well-known as a separator for valve-regulated lead-acid (VRLA) batteries and has been in use for about 20 years. As the VRLA battery market and demands on battery performance continue to grow, the electrical characteristics, productivity and reliability of VRLA batteries are being enhanced. Both the reliability and the electrical performance of the battery are linked to the AGM glass material. The properties of the AGM material impact the assembly route, defect rates, productivity, product cycleability, life, reliability, and recharge performance. An AGM product range which contains synthetic fibres up to 30% in mass (the remaining 70% is still glass) has been developed. The reinforcement of the material with a specific organic fibre is showing very positive effects, both on intrinsic separator characteristics and, as a consequence, on the manufacturing, electrical properties and cycle-life of batteries. The AGM product has been used in the battery market now for more than 20 years. Based on this experience, new testing methods such as the fatigue test and internal pressure measurement have been developed. It is clear from such measurements that reinforcement with synthetic fibre has a positive impact on battery characteristics.