Valve-regulated lead/acid (VRLA) batteries provide the very high levels of reliability required for standby service. Various types are available, in particular: cells with lead-calcium-tin alloy grids and absorptive glass-mat (AGM) separators; cells with pure lead grids and AGM separators; cells with tubular plates and gelled electrolyte. These cell types are subject to a number of factors that affect durability in float service. These factors are reviewed and it is shown that grid corrosion is the usual failure mode. As a result, practical cell and battery designs need to ensure all other potential causes of failure are either eliminated or occur at a slower rate. Test results based on thermal acceleration are presented and have been correlated with real-time tests. The attainment of satisfactory product life under practical conditions is fully demonstrated. Techniques for battery monitoring and surveillance also have a strong impact on reliability and can be used to define the best strategy for replacement. The overall result is better levels of protection which, together with precise specification and careful consideration of the service conditions, enable user requirements to be met in full.