Archimedes-Based Glycerol Displacement for Electrode Porosity Measurement in Lead-Acid Batteries

The performance of lead-acid batteries is highly dependent on the structural and surface properties of the electrodes' active materials. Besides the importance of the material's phase composition at any given time in application, its surface area and porosity contribute to the overall performance of the battery. Mercury porosimetry, which uses a representative sample of the active material for its analysis, has traditionally been used to measure the porosity of battery plates. Apart from the possibility of inducing cracks and cavities in the material while taking the sample, many laboratories have moved away from the use of mercury due to its toxicity. The Archimedes-based glycerol displacement technique has the advantage of being a simple method that considers the porosity of the entire battery plate. Even though the method might not give detailed information on the material's pore volume distribution, reproducibility is achieved for individual plates as well as groups of plates taken from either a single cell or six cells from a battery. The method calculates porosity based on the material's absolute and envelope density, which makes use of absorbed glycerol volume to measure the average porosity over the entire plate. Lead-acid batteries are usually subjected to aging mechanisms that affect the active material's structural integrity and surface properties. This study describes the use of the glycerol displacement technique for the measurement of battery plate porosities, combined with powder X-ray diffraction material characterization to understand the degradation mechanisms of batteries that were subjected to capacity life cycling and calendar aging or shelf life, respectively. The results showed that the failure mechanisms of batteries are significantly different and that both the chemical and physical properties of the active material are interdependent; therefore, one cannot rely on a single analytical tool only to interpret a failure mechanism.