Structural changes of active materials and failure mode of a valve-regulated lead-acid battery in rapid-charge and conventional-charge cycling

Spirally wound 12-V valve-regulated lead-acid batteries were subjected to c onventional-charge and rapid-charge cycling tests. The cycle life was 250 c ycles for the conventional-charge regime and 1000 cycles for the rapid-char ge regime. In conventional-charge cycling, the positive active material qui ckly expanded and developed a coralloid structure in association with lower ed utilisation and integrity. In rapid-charge cycling, no coralloid structu re developed and the expansion was smaller and much slower. Correspondingly , the particle size of the negative active material grew in both cycling te sts, but at a much slower rate in rapid-charge cycling. With the expansion of the positive active material, the negative active material was compresse d. In the failed batteries, about one-third of the negative active material in the centre of the electrode was compressed almost into a solid non-poro us mass. This densification process also occurred at a much slower rate in rapid-charge cycling. At the point of failure, the discharge capacity of al l test batteries was limited by the negative electrode, although it was lim ited by the positive electrode at the beginning of the cycling tests. The c ause of failure for most of the batteries, regardless of the charging regim e, was the occurrence of "soak-through" shorts caused by numerous minute le ad dendrites formed in the separator. This might have been encouraged by th e formation of shorter distances between the two electrodes, created by the compression of the separator as a result of the expansion of the positive active material. (C) 2000 Elsevier Science S.A. All rights reserved.