Battery separators electrical resistance

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. 

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. 

Further improvements in lowering separator electrical resistance have come with the use of wetting agents to ensure complete wetting of the pores, and modifications to the silica concentration or silica type in the separator [41]. Besides lowering the measured separator resistance, a proposal has been made to also lower the functional electrical resistance. The functional resistance of the separator is the measured resistance compensated for area occupied by gas bubbles. Therefore, any work to lower the entrapped gas of the separator will also serve to improve the measured power output of a battery [8]. 

Figure 11.5 Cold crank voltage as a function of separator electrical resistance. Reprinted from W. Bohnstedt, Automotive lead/acid battery separators a global overview. J. Power Sources, 1995, 59, 45-50, with kind permission from Elsevier Science S.A., Lausanne [3].

In the second half of the 1960s, at the same time but independently, three basically different plastic separators were developed. One was the polyethylene separator [16] already referred to in starter batteries, used only rarely in stationary batteries, but successful in traction batteries. The others were the microporous phenolic resin separator (DARAK) [18] and a microporous PVC separator [19], both of which became accepted as the standard separation for stationary batteries. They distinguish themselves by high porosity (about 70 percent) and thus very low electrical resistance and very low acid displacement, both important criteria for stationary batteries. 

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 ... 

Polyethylene separators offer the best balanced property spectrum excellent mechanical and chemical stability as well as good values for acid availability and electrical resistance have established their breakthrough to be the leading traction battery separator. Rubber separators, phenolic resin-resorcinol separators, and mi-croporous PVC separators are more difficult to handle than polyethylene separators their lack of flexibility does not allow folding into sleeves or use in a meandering assembly in addition they are more expensive. 

Stationary batteries serve predominantly as an emergency power supply, i.e., they are on continuous standby in order to be discharged for brief periods and sometimes deeply, up to 100 percent of nominal capacity, in the rare case of need. The following profile of requirements for the separator thus arises very low electrical resistance, low acid displacement, no leaching of substances harmful to float-... 

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... 

The prime requirements for the separators in alkaline storage batteries are on the one hand to maintain durably the distance between the electrodes, and on the other to permit the ionic current flow in as unhindered a manner as possible. Since the electrolyte participates only indirectly in the electrochemical reactions, and serves mainly as ion-transport medium, no excess of electrolyte is required, i.e., the electrodes can be spaced closely together in order not to suffer unnecessary power loss through additional electrolyte resistance. The separator is generally flat, without ribs. It has to be sufficiently absorbent and it also has to retain the electrolyte by capillary forces. The porosity should be at a maximum to keep the electrical resistance low (see Sec. 9.1.2.3) the pore size is governed by the risk of electronic shorts. For systems where the electrode substance... 

Once in an operational battery, the separator should be physically and chemically stable to the electrochemical environment inside the cell. The separator should prevent migration of particles between electrodes, so the effective pore size should be less than 1pm. Typically, a Li-ion battery might be used at a C rate, which corresponds to 1-3 mAcm2, depending on electrode area the electrical resistivity of the separator should not limit battery performance under any conditions. 

Separators are characterized by structural and functional properties the former describes what they are and the latter how they perform. The structural properties include chemical (molecular) and microcrystalline nature, thickness, pore size, pore size distribution, porosity, and various chemical and physical properties such as chemical stability, and electrolyte uptake. The functional properties of interest are electrical resistivity, permeability, and transport number. It is useful to characterize separator materials in terms of their structural and functional properties and to establish a correlation of these properties with their performance in batteries. A variety of techniques are used to evaluate separators. Some of these techniques are discussed in this section. 

Electrical Resistance. The measurement of separator resistance is very important to the art of battery manufacture because of the influence the... 

Battery separator films, as shown in Fig. 14.18, are used in vehicle batteries to produce a defined resistance for electrical insulation between the individual lead plates. At the same time, the defined porosity of these films guarantees the necessary electron exchange. The films normally consist of ultra-high molecular weight polyethylenes (PE-UHMW) and... 

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