Battery lifetime in a photovoltaic (PV) system is important in determining life-cycle costs and servicing requirements. Unfortunately, this is often not calculated with any certainty. We present a simple model for estimating PV battery lifetimes which are application- and battery-specific, using data normally available (or easily estimated) at the time of system design. In a correctly designed and operated PV system, one of two properties will limit the ultimate lifetime of the battery: the cycle life or the battery＇s resistance to internal corrosion. The cycle life is more or less independent of ambient temperature, but the resistance to internal corrosion falls rapidly at higher ambient temperatures. Whether the cycle life or the temperature-dependent corrosion is the limiting factor on battery life depends on the particular details of the photovoltaic system, especially the type of battery used, the daily depth of discharge and the average ambient temperature experienced. Illustrations are given of the particular circumstances for a variety of PV systems with open (vented) lead-acid batteries, ranging from rural lighting systems and vaccine refrigerators to large telecommunications systems. Where possible, the predicted Lifetime is compared to actual field experience. In PV systems using tubular plate vented batteries, it is nearly always the temperature-dependent corrosion process that limits the battery lifetime, and not the cycle life. In larger PV systems in hot climates, active cooling of the battery enclosure (powered by photovoltaics) can sometimes increase battery lifetime, and a suitable battery-cooling system under development is described briefly.