OXIDE FOR VALVE-REGULATED LEAD-ACID-BATTERIES

In order to meet the increasing demand for valve-regulated lead-acid ( VRLA) batteries, anew soft lead has been produced by Pasminco Metals. In this material, bismuth is increased to a level that produces a sign ificant improvement in battery cycle life. By contrast, other common i mpurities, such as arsenic, cobalt. chromium, nickel, antimony and tel lurium, that are known to be harmful to VRLA batteries are controlled to very law levels. A bismuth (Bi)-bearing oxide has been manufactured (Barron-pot method) from this soft lead and is characterized in terms of phase composition, pal-tide size distribution, BET surface area, a nd reactivity. An investigation is also made of the rates of oxygen an d hydrogen evolution on pasted electrodes prepared from the Bi-bearing oxide. Far comparison, the characteristics and performance of a Bi-fr ee (Barton-pot) oxide, which is manufactured in the USA, are also exam ined. Increasing the level of bismuth and lowering those of the other impurities in soft lead produces no unusual changes in either the phys ical or the chemical properties of the resulting Bi-bearing oxide comp ared with Bi-free oxide. This is very important because there is no ne ed for battery manufacturers to change their paste formulae and paste- mixing procedures on switching to the new Bi-bearing oxide. There is l ittle difference in the rates of oxygen and hydrogen evolution on past ed electrodes prepared from Bi-bearing or Di-free oxides. On the other hand, these rates increase on the former electrodes when the levels o f all die other impurities are made to exceed (by deliberately adding the impurities as oxide powders) the corresponding, specified values f or the Bi-bearing oxide. The latter behaviour is particularly noticeab le for hydrogen evolution, which is enhanced even further when a negat ive electrode prepared from Bi-bearing oxide is contaminated through t he deposition of impurities added to the sulfuric acid solution. The e ffects of impurities in the positive and negative plates on the perfor mance of both flooded-electrolyte and VRLA batteries are assessed in t erms of water loss, charge efficiency, grid corrosion, and self-discha rge. Finally, the causes of negative-plate discharge in VRLA batteries under float conditions are addressed. (C) 1998 Elsevier Science S.A, All rights reserved.