New developments in separators for valve-regulated lead–acid batteries

Abstract In valve-regulated lead–acid (VRLA) batteries the electrolyte solution has to be immobilized to ensure tiny channels left open for the transfer of oxygen from the positive to the negative electrode. So far microfibre glassmats have predominantly been used, which based on their high porosity and good wettability of the glass fibres are able to retain durably large electrolyte volumes. The tensile strength of such microfibre glassmats remains unsatisfactory. Developments to produce absorbing mats from organic fibres have recently succeeded due to advanced developments in polymers and in fibre production processes as well as in achieving permanent hydrophilisation. Such polypropylene-microfibremats have excellent tensile and puncture strength and—as pockets—can be well integrated into highly automated assembly processes. Test data for polypropylene-microfibremats are presented and compared to microfibre glassmats. Another approach to hamper the electrolyte in its free mobility is to gel it: batteries with gelled electrolyte have been shown to require conventional microporous separators—both for secure fixing of plate spacing as well as for preventing electronic shorts. Despite their complex filling process gel batteries are well accepted for cycling applications, when simultaneously freedom from maintenance is required. Due to the high power requirements for EV batteries there is a trend towards thinner plates and thinner separation; also substantial pressure on the positive electrode and thus also on the separator is desirable to improve the cycling life decisively. A new separator development is presented, which in spite of high porosity (>80%), suffers only little deformation even under very high pressure. It effectively prevents acid stratification, forms no filling profile and permits oxygen transfer.