Porosity measurements of electrodes used in lead-acid batteries

A method is presented that determines the porosity of a complete electrode plate used in lead-acid batteries. It requires only elementary equipment and is simple to operate, so that laboratory workers can use it as a routine method during manufacturing to determine the complete electrode's average porosity over a range of electrode sizes and types of both flat plate and tubular configuration. The method makes use of Archimedes' principle and uses glycerol as displacement medium. This allows for the porosity determination of both cured and formed positive and negative electrodes, without the detrimental effect of lead oxidation, which is common when using water as a displacement medium. The study showed that the method of using glycerol as a displacement medium gave on average, good repeatable results for both cured and formed positive and negative electrode plates used in the manufacture of automotive lead-acid batteries. The porosity results of the method were compared to the results obtained using Hg porosimetry, where a statistical paired t-test showed the two techniques to produce comparable results for all types of plates analyzed. The porosity of various plates was compared to the surface area of the respective active material of both positive and negative electrodes. These results showed unusual trends in that, depending on the manufacturing conditions, the surface area of formed positive electrodes could vary significantly from sample to sample of different batches without little change in its respective porosity. The surface area of different formed negative electrodes, however, would only vary slightly with significant changes in their corresponding porosity. The glycerol displacement method was also shown to be suitable to determine the effective porosity of cured and formed positive tubular electrodes.