Valve-regulated lead-acid batteries

Valve-regulated lead-acid (VRLA) batteries with gelled electrolyte appeared as a niche market during the 1950s. During the 1970s, when glass-fiber fel ts became available as a further method to immobilize the electrolyte, the market for VRLA batteries expanded rapidly. The immobilized electrolyte off ers a number of obvious advantages including the internal oxygen cycle whic h accommodates the overcharging current without chemical change within the cell. It also suppresses acid stratification and thus opens new fields of a pplication. VRLA batteries, however, cannot be made completely seated, but require a valve for gas escape, since hydrogen evolution and grid corrosion are unavoidable secondary reactions. These reactions result in water loss, and also must be balanced in order to ensure proper charging of both elect rodes. Both secondary reactions have significant activation energies, and c an reduce the service life of VRLA batteries, operated at elevated temperat ure. This effect can be aggravated by the comparatively high heat generatio n caused by the internal oxygen cycle during overcharging. Temperature cont rol of VRLA batteries, therefore, is important in many applications. (C) 20 01 Elsevier Science B.V. All rights reserved.