The lifetime of a battery may be specified in several different ways depending on the application and hence on which mechanism asre most significant. For applications in which the battery is regularly charged and discharged (such as in photovoltaic systems), the most appropriate measure of lifetime is the number of charge/discharge cycles over which the battery maintains a given fraction of its capacity.
Since batteries inherently involve chemical reactions that are reactive, the materials used in batteries are susceptible to alternate reactions that degrade battery performance. While certain catastrophic battery failure mechanisms are possible, battery lifetime is typically controlled by the gradual degradation in battery capacity which accompanies charge/discharge cycles. Consequently, battery lifetime is typically given as the number of charge/discharge cycles which it can undergo and still maintain its original capacity. However, in systems which do not frequently experience charge/discharge cycles (such as in uninterruptable power supplies), battery lifetime is more appropriately specified in years. Improper use of the battery can greatly accelerate battery aging and further decrease the number of cycles over which a battery can be used.
Battery life is defined either in years (if it remains fully charged or in # of cycles under a given set of conditions (including temperature and DOD).
The type of battery used will also have an important impact on the maintenance requirements of the battery. Some types of battery reactions evolve gasses and other products which change the volume of the components in the battery. In cases in which the volume of a battery changes, it is more difficult to seal the battery, and the battery will need to have certain chemical components (usually simply water) added to compensate for the evolution of gasses. A hermetically sealed battery does not exchange any materials with its surrounding environment. Such a battery will have lower maintenance requirements than a battery in which the various battery elements interact with the surroundings. Nearly all small common primary batteries are hermetically sealed and require no maintenance, but many secondary batteries, particularly lead acid batteries, require a strict maintenance schedule.
A battery can degrade or can fail catastrophically. Modes are: shorts, degradation of electrode material, freezing, increases in resistance.
Battery Safety and Disposal
Most battery systems, including those used in renewable energy systems, contain corrosive or dangerous chemicals and the safety regulations for each type of battery should be carefully checked. Additional safety concerns relate to their ability to produce large current. Finally, for lead-acid battery systems, the evolution of hydrogen is a potential issue.
Batteries should not be thrown away as most batteries contain toxic and/or corrosive material.