Backweb

Separator backweb refers to the porous separating membrane. It is of uniform thickness and has a macroscopically uni-... 

The dependence of separator electrical resistance on porosity for the selected SLI battery separator (0.25 mm backweb thickness) and the practical approximation T P = 1 can be seen in Fig. 6. 

The thin backweb, typically 0.2 mm thick with a porosity of 60 percent yields excellent electrical resistance values of 50 rafl cm2, 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 ... 

The term leaf separator characterizes the customary stiff version of a starter battery separator that can be inserted individually between the electrodes on automatic stackers, in contrast to pocket separators. This processing requires considerably higher bending stiffness than for pocket separators, calling for thicker backwebs, typically 0.4-0.6 mm (Fig. 18 and 19). 

Brand name Backweb thickness (mm) Electrical resistance ( mficnr) Oil content (%) Porosity (%) Puncture strength (N) Supplier... 

Other backweb thicknesses upon request n.a. not available... 

Whereas a maximum number of contact points between PVC grains is desired to achieve mechanical stability, this prevents higher porosities. Typical values for porosity are 30 - 35 percent therefore the electrical resistance is rather high, i.e., 170 mQcm2, despite thin 0.3 mm backwebs for top qualities. As mentioned, the range is very wide — even considerably higher electrical resistances are sometimes acceptable, e.g., in areas where cold crank performance is of no significant importance. 

The development of the starter battery in Japan has taken an independent course (see Sec. 9.2.1.2), visibly expressed by the separator s thick glass mat and its lack of spacing ribs (cf. Fig. 19). The cellulosic backweb impregnated with phenolic resin, generally in use until around 1980 and largely identical to the separator of the same type already mentioned has been completely replaced by thin ( 0.3 mm) fleece materials made of organic fibers. 

Brand name Backweb Electrical Porosity Pore size Supplier... 

Since the glass mat supplies sufficient stiffness, high backweb thickness was no longer needed These fleeces are made of organic fibers (polyester and polypropylene, as well as so-called synthetic pulp , i.e., fibrillated polypropylene) on paper machines. 

The electrical resistance, at 60 - 90 mQ cm", is astonishingly low, because the backweb is only 0.25 - 0.30 mm thick and the glass mat with its porosity in excess of 90 percent also contributes only little. In some types of construction the low electrical resistance cannot be fully utilized, however, due to a tendency for gas to be trapped within the glass mat. 

The microfiber glass separators have to fill the space between the electrodes completely the backweb thickness, is thus identical to the total thickness. Due to the high compressibility of such porous glass mats, a standard measuring pressure of 2 kPa or 10 kPa (BCI method) is generally used during assembly they are compressed... 

However it has to be conceded that after battery life cycle tests at such temperatures polyethylene separators also reach their limits, although this fact does not yet reflect in failure-mode studies [49], even in locations with extreme ambient temperatures. The tendency towards using ever-thinner backwebs cannot be continued, however, without seeking protective measures. Suitable provisions have to be made espc-... 

The microporosity is also important for this application, in order not to allow shorts through the backweb during battery life. Bottom shorts are avoided by a mud room of sufficient dimensions, and side shorts by plastic edge protectors on the... 

The production process for polyethylene separators (Sec. 9.2.2.1) as well as the characteristic properties (see Sec. 9.2.2.1 and 9.2.3.1) have already been described in detail above. Deviating therefrom, the desire for low acid displacement has to be added for separators in open stationary batteries. This can be met either by decreasing the backweb thickness or by increasing the porosity the latter, however, is at the expense of separator stability. 

The production process and the principal properties of this system have been described in detail in the section on traction battery separators (see Sec. 9.2.3.1). The outstanding properties, such as excellent porosity (70 percent) and resulting very low acid displacement and electrical resistance, come into full effect when applied in open stationary batteries. Due to the good inherent stiffness the backweb may even be reduced to 0.4 mm, reducing acid displacement and electrical... 

Table 12 shows the physicochemical data of separators used in open stationary batteries. Since the emphasis is on low acid displacement, low electrical resistance, and high chemical stability, the phenolic resin-resorcinol separator is understandably the preferred system, even though polyethylene separators, especially at low backweb, are frequently used. For large electrode spacing and consequently high separation thickness, microporous as well as sintered... 

Gel batteries require an additional separator to fix the plate distance and to prevent electronic shorts. The most effective protection against shorts is achieved by means of separators with low pore size ideally, microporous materials should be used (pore size less than 1 pm). Additionally, the separator should have a low acid-displacement since the fumed silica and the cracks in the gel already reduce the volume available for electrolyte. To minimize the internal resistance of the battery, the electrical resistance of the separator should be as low as possible. These two requirements, viz., low acid-displacement and low electrical resistance, translate into a need for separators with good wettability, high porosity, and low geometrical volume, i.e., rib configuration and backweb thickness should both be optimized. 

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