Abstract
Leadcalcium standby batteries contain free acid, thick grids of low calcium content, and are discharged infrequently. Valve-regulated lead/acid batteries (VRLAs) are deeply discharged, contain immobilized electrolyte, and contain much thinner grids of leadcalciumtin alloys. Tin is added to enhance rechargeability from deep discharge and to enhance the mechanical properties of the alloys, particularly creep resistance. Until 1980, leadcalciumtin alloys suffered from wide variations in grain structure, caused mainly by poor control of the calcium content. The latter is due to calcium oxidation and the inclusion of suspended oxide into the grids. The introduction of aluminium into leadcalcium and leadcalciumtinaluminium alloys prevents calcium loss from the melt. Leadcalciumtinaluminium alloys can be produced to the desired specification without fear of either calcium loss or poor control of grain structure. Aluminium also serves both as a nucleant to significantly reduce the initial cast grain-structure and as a means to enhance the precipitation of calcium in leadcalciumtin alloys. Grain-structure control is now possible even in very high tin-content alloys. Leadcalciumtin alloys with aluminium are not susceptible to penetrating grain boundary corrosion and will become the favored alloys for deep-cycling VRLAs of the future.
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