The separator paper is a key component within a valve-regulated lead/acid (VRLA) battery. Here we describe an experimental program aimed at elucidating how separator papers influence battery performance. Identical batteries were manufactured using three different types of separator paper and subsequently cycled to failure. The failed batteries were then subjected to a series of tests, which included measurements of electrode potentials, compositional analysis of active materials, separator paper saturation and stratification measurements. Results indicated that separator paper type significantly influences battery performance. It is believed that subtle changes in the separator structure caused changes in the rate at which imbalances between the cells in each test battery developed, which led to subsequent changes in failure.