Float charging of valve-regulated lead-acid batteries: A balancing act between secondary reactions

The behavior of valve-regulated lead-acid batteries on float charging is influenced by many interacting parameters. A mathematical model has been developed that describes the effects of kinetic cell parameters, float voltage (or current), and temperature on electrode potentials and rates of electrode reactions. The considered reactions are: hydrogen evolution, oxygen evolution, oxygen reduction, grid corrosion, and discharge of active material that may occur under unfavorable conditions. This model, combined with selected experiments, is a very effective tool for surveying the complex situation during float charging. It can also be applied to vented batteries. With model simulations, some fundamental relationships have been shown: when the oxygen reduction efficiency is near 100%, the kinetics of hydrogen evolution and grid corrosion govern electrode polarization at specified float conditions. To achieve a long service life, the rates of waterless and grid disintegration have to be small, and simultaneously a satisfactory state of charge of the battery is required. Hence, the optimum design of a valve-regulated battery requires a high and balanced hindrance of hydrogen evolution and grid corrosion. Furthermore, small rates of oxygen evolution are favorable. Oxygen intake from the surroundings by a leakage may cause discharge of the negative electrodes. The model helps to estimate the maximum size of such a leakage that can be tolerated. Temperature has not only a marked effect on all the reaction rates, but also influences electrode polarization and the delicate balance of currents, because the activation energies of the various processes differ.