Challenges for automotive battery separator development

Increased energy content, higher cold-cranking performance and improved quality level have driven the transition of automotive battery construction to the pocketing technology. Emphasis of future automotive battery development will be the adaptation to elevated ambient temperatures and continuously more demanding productivity requirements. Translated into a profile of separator requirements, this means improved oxidation resistance (especially at higher temperature), further increased puncture strength for unproblematical use in combination with expanded metal grids, combined with a backweb thickness as low as possible for reasons of cost efficiency. Improved oxidation resistance at higher temperatures can be achieved by the use of even better qualities of polyethylene and increased oil content (in general, or only at especially exposed locations), as well as by special gentle production processes. Results of development work aimed at the improvement of oxidation stability are presented and discussed. Ultra-high molecular polyethylene grades with further increased molecular weight approach manufacturing limitations; in general, increased oil content detrimentally affects important separator properties, whereas locally increased oil content in especially stressed areas, such as the creased edge of the separator, opens interesting possibilities for improving the oxidation stability at higher temperatures. Through modifications of the manufacturing process of polyethylene separators, the oxidation stability can also be enhanced significantly with even a simultaneous improvement in puncture strength. The latter facilitates a less problematic use in combination with expanded metal grids and leads, in turn, to productivity advantages. Furthermore, such improvement in separator properties opens the possibility of using lower backweb thickness and, thus, lower separator cost. The evaluation of important characteristics and the performance of recently developed products in batteries are presented. These will form the basis for an optimized separator design. (C) 1997 Elsevier Science S.A.