To elucidate the mechanism by which bismuth enhances the capacity of valve-regulated lead-acid (VRLA) batteries, model experiments are performed on pulverized positive electrodes produced either from leady oxide, which contains virtually no bismuth (termed ＇Bi-free oxide＇), or from Pasminco VRLA Refined(TM) oxide, which is of high purity and contains a specified amount (0.05 wt.%) of bismuth. The electrodes are compressed under a range of pressures (1.4 to 60 kPa). Below 40 kPa, the presence of bismuth increases the initial capacity. At all pressures, bismuth enhances the rate at which the capacity develops during cycling. Reconnection of the separated agglomerates of lead dioxide is the key factor in restoring the capacity of the pulverized electrode. Electron micrographs reveal that there are two essential types of contact in the positive material: (i) ＇micro-contact＇ between individual irregular-shaped or individual needle-like crystals, to form the agglomerates; (ii) ＇macro-contact＇ between individual agglomerates, to form the skeleton of the positive mass. Bismuth encourages the growth of fine needle-like crystals on the surface of the agglomerates. These crystals spread out and inter-weld to form ＇bridges＇ between the agglomerates and, thereby, consolidate the porous mass of the electrode. This influence of bismuth on morphology is considered to be responsible for the demonstrated improvements in capacity performance. (C) 2000 Elsevier Science S.A. All rights reserved.