Effects of compression on recombinant battery separator mats in valve-regulated lead–acid batteries

The spring characteristics of a typical recombinant battery separator mat (RBSM) material used in valve-regulated lead–acid (VRLA) batteries have been monitored at several stages during repetitive deep-discharge cycling service (C3/3, 100% DoD). Through the controlled application of a range of compressive loads, accurate plots of separator thickness vs. compressive force have been recorded for both dry and acid-saturated material. From subsequent analysis, several important properties have been established. Dry RBSM accepts some `crush' during the first application of pressure. At high levels of force (≥60 kPa), RBSM suffers a second stage of crush that can be permanent. Saturation with sulphuric acid solution produces a discernible shrinkage of the material. Thus, for a given thickness of RBSM, an acid-saturated separator will apply less force to the plates than a dry equivalent. This has important consequences for the manufacture of VRLA batteries. In particular, with present separator technology, it would appear that even the use of relatively high levels of compression during the assembly of RBSM VRLA cells is not an effective strategy for attaining the levels of compression required to prevent significant expansion of the positive plates. The difficulties of setting, and then maintaining, adequate levels of plate group compression are further underlined by the promising performance of a constant-compression RBSM VRLA cell. By means of a feedback-controlled system that applies a pressure of close to 40 kPa, this first-of-its-kind cell has displayed relatively stable cycling behaviour.