Page 343 - Handbook of Battery Materials

11.2 Separators for Lead–Acid Storage Batteries 313

of microfiber glass separators by their thickness alone has proven to be ambiguous;
therefore the preferred method is by area weight. For a typical separation thickness
of 1 mm, glass fiber mats of 200 g m −2 are used. Resulting from the extremely
high porosity of more than 90% the measured electrical resistance is extremely
low, but the difference in the pore spectrum inside the battery due to compression
during installation has to be taken into account; moreover, not all pores must be
acid-filled, in order not to block the oxygen transfer. The actual electrical resistance
‘experienced’ by the battery is in the order of magnitude of other modern separation
2
systems (50–70 m cm ). An excellent description of these relationships exists in
the literature [23].
Despite all the efforts over many years to establish this ‘sealed’ construction
in starter batteries, field results have been published only sparingly and they
have not always been satisfactory. Cold crank results are very good; despite using
lead–calcium, the cycle life – at least in laboratory tests – has been found to
be very good [74], probably due to the mat support of the positive active mass.
In the day-today practice, other influences, such as insufficient recharging and
microshorts as a result of deep discharges or valve leaks, appear to lead to pre-
mature sulfation of the negative electrode and eventually to capacity deterioration.
Improved constructions are continually being presented and tested on a large
scale. Besides some open technical questions, the cost structure also has prohibited
a wider introduction to date: sturdier containers and more precise electrode
geometry, voluminous separators, and reliable valves, an expensive filling process,
and last but not least temperature-controlled charging management could only be
justified with difficulty in times of cost trimming within the automotive industry.
The range of microfiber glass mat separators offered by the leading producers is
presented in Section 11.2.3.3 with typical data in connection with their predominant
application in sealed stationary batteries.

11.2.2.3 Comparative Evaluation of Starter Battery Separators
The individual starter battery separator systems have been described; here they
are evaluated comparatively. There are no standards for evaluating separators!
Therefore the comparison will be concentrated primarily on the effects on the
performance of the starter battery, with other decisive criteria such as cost structure
and effects on productivity indicated.
Cold crank performance, battery life expectancy, and freedom from maintenance
are generally co-affected by the separators, whereas amp` ere-hour capacity remains
largely unaffected at a given separator thickness. The properties of the different
leaf and pocket separators are compared in Table 11.10. These typical separator
properties (lines 1–4) are reflected in the electrical results of battery tests (lines
5–8). The data presented here are based on the 12 V starter battery standard DIN
43 539–02; tests based on other standards lead to similar results.
The cold crank voltage is directly affected by the separator electrical resistance
(cf. Section 11.1.2.3; Figure 11.5), but to a much smaller extent than is normally
assumed. Nevertheless the effect of low electrical resistance of the separator is not