PURE LEAD AND THE TIN EFFECT IN DEEP-CYCLING LEAD-ACID-BATTERY APPLICATIONS

The use of lead-calcium or pure lead grids in valve-regulated lead/acid (VRLA) batteries has been generally satisfactory, but one drawback of these materials is the unpredictable build-up of a passivation layer on the surface of the positive grid. In deep-cycling applications, this passivation layer can result in a rapid loss of discharge capacity that is reversible with proper treatment. This phenomenon is reminiscent of the 'memory effect' in nickel/cadmium cells and although it is reversible its unpredictability renders products not entirely suitable for deep-cycling applications. The use of small amounts of tin, alloyed into the positive grid or as a surface coating, has been promoted as a cure for this shortcoming, and to a large extent is effective. This paper presents a careful review of the 'tin effect' and provides information on how best to take advantage of minimal tin levels so that optimal cycling performance is realized. For example, it is noted that the nominal grid tin content may be less important than the way in which the metal is distributed. In VRAL products designed for deep-cycling applications, incorporation of tin into or on the surface of the positive grid in the proper way is of substantial benefit, if not crucial.