Quantifying secondary reactions in VRLA batteries. What purity standards and other factors are necessary to prevent negative plate discharge?
In a VRLA cell with a fully developed oxygen cycle, the negative plates will suffer discharge during float service if their self-discharge rate is too high to be balanced by the corrosion rate of the positive grids. Positive grids have been developed with very low corrosion rates. However, there is little quantitative data available on negative plate self-discharge rates that are required to balance such low corrosion rates. This paper presents values measured from several different battery brands, American and European. The data shows clearly that some production batteries have high negative self-discharge rates that account for the observed tendency for self-discharge in service. Surprisingly, the data also shows that other brands have achieved extremely low self-discharge rates-low enough to be compatible with the best positive grids that the authors have tested. These results are very encouraging for VRLA technology and confirm that the principle can be realized. The focus now shifts to the practical issues: finding the source of the impurities that increase hydrogen evolution and weighing the cost and practicality of reducing them.
The gassing behaviour of lead-acid negative active material and development of a quality control test for lead oxide purity
Abstract Maintenance-free (M.F.) batteries are widely applied, particularly in the areas of automotive SLI and stand-by power. The M.F. characteristics are due to low evolution of hydrogen and oxygen from the plates on open circuit and during constant potential overcharge, thus obviating the need for ‘topping up’. M.F. batteries use a non-antimonial alloy, typically Pb/Ca/Sn, and care is required in the selection of lead feedstock for oxide production to ensure low levels of undesirable impurities. Feedstock lead was analysed for impurities in an attempt to define their safe limits and to see if secondary lead could be used in M.F. batteries. The large number of impurities present, however, together with uncertainties in analytical accuracy led us to a more pragmatic test which measured the gassing rate directly. This served as a rapid method for evaluating lead supplies, a suitable quality control technique for factory use, and a research tool for evaluation of individual impurities and synergistic effects.
Float behavior of VRLA cells: theory vs. reality
Production VRLA cells, made by different manufacturers and designed for a 20 year life, were tested on float. Their actual water consumption rates were measured and compared with target rates based on the amount of water available in the cells. Although the tests are continuing, the data so far suggests that while some cells will achieve their 20 year goal, others may be marginal. Tafel curves and conductivity profiles are presented which show that VRLA cells of the AGM type have characteristics that are extremely variable, even in cells of the same design and batch.