Effect of Discharge Current Density on Structure of the Lead Negative Plate

Abstract

To help understand how the high current demands of an electric vehicle might affect the electrode microstructure and performance of lead‐acid batteries, experiments were run wherein commercial negative plates were given 26 cycles of deep discharge at current densities of either 18, 125, 500, or 2000 A/m2. Sections of the plates were removed after cycles 1, 6, 16, and 26 and carefully examined by light and electron microscopy, image analysis, and surface area analysis to determine how the electrode active material composition and structure changed with cycling at the different current densities. Electrochemical performance was monitored by measuring plate capacity and electrode potential as a function of current density and number of cycles. The size of the crystals in the discharged plates decreased roughly in correspondence to the decrease in plate capacity produced by the increasing current densities. This suggests passivation as the cause for loss of capacity, since passivation is accepted as becoming more critical as crystal size decreases. The shapes of the electrode polarization curves indicate, however, that limited electrolyte diffusion rates and lessened contact of active material crystals, the result of the porosity that develops with cycling, may also be acting to reduce plate capacity.