Valve-regulated lead-acid (VRLA) batteries with absorptive glass mat (AGM) separators have been in use for over 20 years in different standby applications. These applications are increasingly varied, especially regarding environmental conditions. Standby batteries are not only for use in applications where conditions are strictly defined and controlled (air conditioning) and it is therefore necessary to review and clarify the key parameters for the use of VRLA batteries with respect to the optimum conditions. Several series of chemical and electrochemical reactions occur in VRLA batteries particularly when in a charge or float charge condition. These reactions give specific properties such as minimal water loss (low maintenance) but also create specific precautions for use. VRLA battery functioning is limited by four main phenomena that are positive grid corrosion, irreversible active mass sulfation, active mass degradation by cycling and dry-out by loss of water. Positive grid corrosion is the usual failure mode in float operation or due to persistent overcharge. Irreversible active mass sulfation occurs due to lack of charge. In cycling, dependent upon the frequency and depth of discharge, the active mass undergoes numerous structural changes that cause degradation. These four limiting phenomena define a framework inside which several parameters determine the service life of VRLA batteries. These parameters are commissioning, temperature, and frequency and depth of discharge. Commissioning is necessary to equalise and fully charge the cells before use. Temperature, and temperature dispersion, is the main factor determining the rate of corrosion. The frequency and depth of discharge determine how the active mass is utilised. This paper, by considering these parameters both qualitatively and quantitatively, attempts to indicate how and why to adjust the charge and float voltages to optimise the use of AGM VRLA batteries according to the environmental conditions. (c) 2004 Elsevier B.V. All rights reserved.