TABLE OF CONTENTS Alvin J. Salkind, Ronald O. Hammel, Anthony G. Cannone, and Forrest A. Trumbore
Lead-acid battery designs for many small portable, and some larger fixed applications, have often been referred to as sealed and/or maintenance free. This has been an accurate description to the extent that there was no need or opportunity to replace electrolyte. However, virtually no design included a true hermetic seal, and only a pressure release valve which limited inflow or outflow of gas to the cell. The valves released internal gasses at pressures ranging from a few tenths of an atmosphere to a few atmospheres. In addition, most lead-acid battery cases are molded of plastics, which are hydrogen permeable.
The newer designation for these designs is as a Valve Regulated Lead-Acid Battery or VRLA. They differ from the conventional flooded lead-acid battery designs by containing only a limited amount of electrolyte ( starved electrolyte) absorbed in a separator or immobilized in a gel. In most designs, the cell capacity is limited by the amount of positive active material. The starved electrolyte and excess of negative active material facilitate the recombination of oxygen produced during overcharge or float charge with the negative active material. The resealable valves are normally closed to prevent the entrance of oxygen from the outside air. The vent pressure design depends on the manufacturer and predominantly by the case shape and material. VRLA designs have two usual shapes, one with spirally-wound electrodes (jelly-roll construction) in a cylindrical container, and the second with...
