Page 332 - Handbook of Battery Materials

302 11 Separators

unwinder splicer calender pinhole detector slitting rewinder

Figure 11.9 Polyethylene separator production process (III) slitting.

Figure 11.10 Starter battery with pocketed plates. Reprinted
from W. B¨ohnstedt, Automotive lead/acid battery separators:
a global overview, J. Power Sources, 1996, 59, 45–50, with
kind permission from Elsevier Science S.A., Lausanne [3].

It is flexible and weldable into three-sided closed pockets, making the previously
usual mud room at the bottom of a starter battery redundant; the consequent
increase in grid size of 8% can crank up performance and energy density results
(cf. Figures 11.10 and 11.11) [3]. It is microporous, that is, its pore diameters are
significantly below 1 µm, which durably prevents penetration by lead particles.
Only in this way has the use of lead–calcium alloys in electrodes, with their
increased tendency to shedding, become possible, together with a reduction in
water consumption over the life of the battery, allowing today’s batteries to be
properly called maintenance-free.
The thin backweb, typically 0.2 mm thick with a porosity of 60%, yields excellent
2
electrical resistance values of ∼50 m cm , permitting further optimization of
high-performance battery constructions. These require very thin electrodes due to
the overproportionally increasing polarization effects at higher current densities
and consequently also low distances: most modern versions have separators only
0.6 mm thick. Such narrow spacings enforce microporous separation!
Practical experience has shown polyethylene pocket separators only in very
exceptional cases to be considered as a cause of failure in starter batteries