A new modified AGM (MAGM) separator for VRLA batteries and the respective technology for its production have been developed. The new product is obtained by processing absorbent glass mat (AGM) separator with polymer emulsion, which is adsorbed onto the surface of the glass fibres and concentrates at the sites of contact between the fibres. Thus, a continuous porous network of interconnected glass fibres is formed. The two surfaces of the AGM sheet are treated with polymer emulsion of different concentration in view of the different processes that take place at the positive and the negative plates, respectively. As a result of the above treatment the surface properties of the AGM are modified to balance hydrophobic/hydrophilic properties. The MAGM separator has improved tensile strength as compared to untreated AGM, a slightly reduced H2SO4 absorption and wicking rate, and increased chemical and thermal stability. It does not disperse in H2SO4 Solution as does AGM. The influence of the MAGM separator on the processes in the valve-regulated lead-acid cell has been investigated. An experimental method has been elaborated and used to establish the existence of horizontal electrolyte stratification between the plates. This stratification is a result of the different processes that occur at the two types of plates as well as of the resistance that the ion flows of H2SO4 and H2O have to overcome when passing through the separator. In VRLA batteries, their movement is impeded by the AGM separator, which reduces the available capacity of the battery. The effect of the MAGM separator on the concentration of H2SO4 at the two separator interfaces during charge, discharge, open circuit (OC) and on cycling is investigated. MAGM influences the processes in the cells in such a way that the differences in H2SO4 concentration between the two types of plates are eliminated. These effects of the MAGM separator result in an increase of the available capacity of the battery on cycling and float service, which leads to a substantial increase in VRLA battery life. Moreover, MAGM improves the efficiency of the oxygen cycle in the cells and thus the water loss is reduced. Besides, MAGM separators reduce the differences in cell voltage when numerous cells are connected in long strings.