Influence of grid alloy and fast charge on battery cycle life and structure of the positive active mass of lead acid batteries

It has been found experimentally that during cycling the pore volume in PAM increases. The fraction of macropores (r>10 μm) may reach up to 30–40% of the pore volume, whereas that of micropores (r<0.1 μm) decreases. The BET surface area is reduced. The size of the crystallites in the crystal zones of lead dioxide particles increases. If the grid alloy contains Sb its concentration in PAM increases on cycling, which leads to a decrease in particle and crystallite size. The degree of these changes in PAM structure is determined by the charging current density as well as by the amount of Sb and Sn in the grid alloy and in PAM. When the charge is conducted with high currents (I≥1.5 C) the role of Sb and Sn in PAM decreases substantially. On fast charge and at Sb content higher than 0.2 wt.%, PbO2 crystallites sized between 40 and 60 nm are formed. On slow charge of plates with pure Pb grids, these values are 90–100 nm. The capacity is influenced by the structure of the aggregates and particles in PAM as well as by their degree of crystallinity. On fast charge, more crystallites are formed in the particles, which improves the contact between the latter’s crystal zones thus interconnecting them into an integral network of crystal zones with uniform electron conductivity throughout the volume of PAM. Consequently, the capacity of the plates is maintained high for a longer period of time. The evolution of this structure on cycling has been followed as well as its effect on the capacity of the positive plates. The phenomena that limit the life of the positive plates are: (a) breaking of the skeleton, (b) formation of crystal particles with poor contact between them, (c) formation of a membrane layer on the surface of the aggregates, and (d) deterioration of the agglomerate structure. It was also found that neither the charge mode nor the presence of additives affect the loss of PAM density on cycling. In order to improve the cycle life of the positive plates they should be charged with high current (I1≥1.5 C). This requirement is not obligatory if the grid alloy contains sufficient amounts of Sb and Sn. The influence of charging current on PAM structure and hence on plate life is interpreted on grounds of a sol–gel–crystal mechanism of the processes that take place on battery charge.