Valve regulated lead acid (VRLA) battery constitutes towards the largest part of the worldwide secondary battery market share. Indisputably, absorptive glass mat (AGM) is a key component in a VRLA battery that is often engineered utilizing the synergy that exists between fiber and structural parameters. Over the past few decades, a considerable progress has been made in designing and tailoring the AGM separators but often limited by ‘trial-and-error’ approach. Here in, we present a comprehensive review that specifically targets the design of AGM separators by bridging a gap between fiber-structure-property relationships. The critical review on various experimental and theoretical studies aims to present a template for ‘designer’ AGM separator to fulfill the demands of any application of VRLA batteries. The review surveys important functions and various properties of AGM separators, which have been related with a key set of fiber and structural parameters. Several challenges presented in the form of failure modes of the VRLA battery can be overcome by leveraging the synergy between key set of fiber and structural parameters. This review also highlights the structural commonalities between AGM separators and the other nonwoven materials. This opens up an opportunity to utilize the existing theoretical models of nonwovens to the AGM separators.