Polyethylene backweb

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 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. 

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... 

For problem-free processing, high tensile and puncture strengths are desirable. Especially when using expanded metal electrodes, sharp edges or points which may puncture the backweb and lead to shorts have to be taken into account Even though the polyethylene separator is unique in these properties compared with conventional separators, these are considerable differences between the products of various suppliers. 

The microporosity is also important for this apphcation, in order not to allow shorts through the backweb during battery hfe. Bottom shorts are avoided by a mud room of sufficient dimensions, and side shorts by plastic edge protectors on the frames of the negative electrode. Some manufacturers have switched to using sleeves of polyethylene separator material, rendering an edge protection superfluous. The use of three-side-sealed separator pocket in traction batteries should be avoided, because experience has shown this can lead to increased acid stratification, subsequent sulfation, and thus capacity loss. 

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