The relationship between the porosity of recombinant battery separator material (RBSM) and separator/battery performance is examined. The porosity is inversely related to the sheet solidity and bulk density of the separator. The connection between high porosity and high acid absorption in the separator is well understood. The ability of RBSM to provide a very high porosity is a major reason why this material functions well in valve-regulated lead-acid (VRLA) batteries. Investigations have shown that the separator, besides providing space for the acid, must also provide a force on the plate active-material to eliminate the effects of premature capacity loss. This study examines the effect of changes in separator porosity/density on the ability to retain the wet force between the plates. It is found that retention of the force in the plate group is enhanced by an increase in the bulk density of the separator. (C) 2004 Elsevier B.V. All rights reserved.