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 a nd rates of electrode reactions. The considered reactions are: hydroge n evolution, oxygen evolution, oxygen reduction, grid corrosion, and d ischarge of active material that may occur under unfavorable condition s. This model, combined with selected experiments, is a very effective tool for surveying the complex situation during float charging. It ca n also be applied to vented batteries. With model simulations, some fu ndamental relationships have been shown: when the oxygen reduction eff iciency is near 100%, the kinetics of hydrogen evolution and grid corr osion govern electrode polarization at specified float conditions. To achieve a long service life, the rates of waterless and grid disintegr ation have to be small, and simultaneously a satisfactory state of cha rge of the battery is required. Hence, the optimum design of a valve-r egulated battery requires a high and balanced hindrance of hydrogen ev olution and grid corrosion. Furthermore, small rates of oxygen evoluti on are favorable. Oxygen intake from the surroundings by a leakage may cause discharge of the negative electrodes. The model helps to estima te the maximum size of such a leakage that can be tolerated. Temperatu re has not only a marked effect on all the reaction rates, but also in fluences electrode polarization and the delicate balance of currents, because the activation energies of the various processes differ.