In some embodiments, a battery separator described herein may comprise a layer having a relatively low apparent density; for example, the density that includes any unoccupied space within the outermost boundaries of the layer may be relatively low. The low apparent density may be attributed to, at least in part, the geometry of the layer. For instance, in some embodiments, the layer may include undulations and/or have at least one non-planar surface (e.g., a corrugated layer; an embossed layer). In some embodiments, a battery comprising a layer having a relatively low apparent density may have desirable properties, including relatively low electrical resistance and/or relatively high capacity. The battery separators described herein may be well suited for a variety of battery types, including lead acid batteries.