ELECTROLYTE STRATIFICATION IN LEAD-ACID-BATTERIES - EFFECT OF GRID ANTIMONY AND RELATIONSHIP TO CAPACITY LOSS

Electrolyte stratification is known to cause reduced efficiency in the operation of lead/acid batteries. While this phenomenon can be clearl y related to a short-term loss of performance, little is known about t he effects of stratification on long-term battery usage. This issue mu st be considered in the more demanding types of duty, such as electric vehicle (EV) propulsion, where efficient operation for the duration o f service is essential. The goal of this study has been to monitor the extent of electrolyte stratification throughout cycle life, under a r epetitive deep-discharge cycling regime that typifies EV battery servi ce. It has also been of interest to investigate any relationship betwe en stratification and the incidence of premature capacity loss (PCL), a known cause of failure under cycling duty. We have found that cells with positive plates based on Pb-Ca and Pb-Sb positive grids both unde rgo an initial loss of capacity, due to the development of stratificat ion. (Stratification disappears during the later stages of cycle life due to the extra gassing associated with the loss of capacity.) The fa ll in performance continues, however, through to the end of service (d ischarge capacity at 50% of the initial value). The relatively short c ycle lives and the apparently healthy condition of plates at the end o f cycling suggest that PCL, as opposed to the traditional degradation processes of grid corrosion and active-material softening/ shedding, i s the dominant cause of cell failure. We suggest that stratification, under our experimental regime, exerts an overall mitigating effect in the demise of the cells. The reduction in sulfuric acid concentration throughout most of the plate volume ensures that the bulk of the plate cycles in relatively weak acid: such conditions are known to reduce t he severity of PCL.