Study of Charge Kinetics in Valve-Regulated Lead-Acid Cells

Model predictions of half-cell voltages, current, and gassing behavior are compared to experimental results of valve-regulated lead-acid (VRLA) cells to elucidate the charge mechanisms. Good comparisons of experimental Pb half-cell voltages with model predictions confirm the importance of liquid-phase Pb 2+ transport early during charge. In the latter stages of Pb-electrode charge, limitations in the PbSO 4 dissolution rate are more important than those of Pb 2+ transport and control charge behavior. Model predictions with an analogous mechanism for the PbO 2 electrode (i.e., involving dissolution and Pb 2+ transport) were not consistent with the charge polarization behavior, since comparisons of experimental half-cell voltages with model results were poor. Satisfactory comparisons with a model that incorporates a solid-state PbO 2 charge mechanism confirmed the insignificance of dissolution and transport in PbO 2 charging. The good agreement between model and experimental current behavior during the constant-voltage portion of charge support our conclusions regarding the electrode charging mechanisms. Conditions under which to expect charging difficulties as well as improved charge regimes are also discussed. The model also predicted the characteristic double-peak gas-flow behavior seen when charging VRLA cells with constant current to a specific voltage lid. The first peak is due to the onset of significant amounts of O 2 generation even though the internal residual gas is dominantly H 2 . The second peak is due to the onset of H 2 evolution.