Absorptive glass mat (AGM) is commonly used as a separator in valve-regulated lead-acid (VRLA) batteries because of its high porosity (~ 92%), accommodating for oxygen transport and recombination, providing low impedance, excellent oxidation resistance, and ability to immobilise sulfuric acid. The latter results in non-spillable batteries that can be positioned in many different areas of an ICE or electric vehicle. This paper briefly reviews the influence of fibre type, fibre diameter, surface area, and process conditions on key AGM characteristics. We examine all-glass and organic fibre-glass hybrid compositions to understand better differences in porosity, wicking rate, and compression-recovery in both a “wet” and “dry” state. Finally, we investigate X-ray tomography as a tool to evaluate the homogeneity of AGM. Initial results show differences in the wire-side and top-side of the product and evidence of trapped air and other gradients within the wetted bulk structure.