Valve regulated lead acid (VRLA) batteries are traditionally classified on the basis of gel and absorptive glass mat (AGM) separators. To fulfill the desired functions of AGM batteries, a key design feature of the separator relies on the uptake of the electrolyte in shortest transport time. Herein, we present a three-dimensional (3D) analytical model to predict the fastest electrolyte uptake in AGM separators based upon the optimal set of fiber and structural parameters. The predictive model has utilized 3D data of fiber orientation in AGM separators, obtained via X-ray micro-computed tomography analysis. Such realistic structural information of AGM has assisted in simulating the separators made up of cheaper coarser glass fibers, which was subsequently benchmarked with the experimental samples consisting of finer fibers for attaining the fastest electrolyte uptake. Through theoretical modeling, a design criterion has successfully evolved for the fastest electrolyte uptake by mapping the key effects of the fiber diameter, 3D fiber orientation distribution and porosity of AGM separators. In general, high-density AGM separators comprising of preferentially aligned coarser fibers tend to attain the fastest electrolyte uptake.