Separators compression/resiliency

Other Fiber Evaluation Methods. The extent of fiber separation (fiber openness) is an important evaluation criteria that is commonly measured by several techniques, namely air permeability, adsorbed gas volume, bulk density, and resilience (compression and recovery). The adsorption and retention of kerosene is also used as a measure of fiber openness and fiber adsorption capacity (34). 

The contrasting structure of the plates and the separators is also relevant to the functioning of the battery. For example, the capillary pressures dictate that electrolyte fills the plates preferentially. This preferential filling appears to be the ideal situation since it can best support the electrochemical reaction, i.e., it leaves the separator partially saturated so that movement of electrolyte can provide pathways for gas transport. If, however, the overall saturation is too low or there is excessive loss of water, the separator will dry out and give rise to an increase in the internal resistance of the battery and the possibility of thermal runaway. An increase in internal resistance, and consequent low service-life, can also result if the compression between separators and battery plates relaxes over a period of time. Overcompression may cause fibres to fracture with a loss of resilience, i.e., the separators lose the ability to return to original thickness after a high pressure is applied and... 

Another feature of AGM separators is their compressibility. With compression of the plate and separator stack, this AGM property guarantees good plate-separator contact, even if the plates are not perfectly smooth. Also, battery assembly is facilitated since the stack can be easily inserted into the cell after compression to a thickness lower than the cell dimension. An undesirable result of the compressibility is that the AGM separator does not exert sufficient resistance against expansion of the positive plate during battery cycle-life. This expansion is particularly prevalent in deep-cycle applications and can cause the battery to suffer premature capacity loss (PCL) via reduced inter-particle conductivity — a phenomenon known as PCL-2 [7]. In the literature, two additional characteristics, which are related to the PCL-2 failure mode, are discussed, namely, AGM separators shrink when first wetted with electrolyte and their fibres can be crushed at high pressure levels [8-10]. These features result in a loss of separator resilience, i.e., a lessening of the ability to display a reversible spring effect. 

In recent years, the AGM separator has become the focus of much attention (see Chapters 6 and 7), and attributes such as compressibility and resilience have... 

Particularly, although the repetitive compressive load applied is a haversine and is applied along the vertical diametral plane of a specimen, the resilient Poisson ratio, p, is first calculated using the recoverable vertical and horizontal deformations, and subsequently two separate resilient moduli, Mr, are determined the instantaneous resilient modulus and the total resilient modulus. 

With regard to flooded lead-acid battery separators, the normal materials of construction are relatively noncompressible and they fix the electrode distance, providing a degree of compression on the electrodes, at least where the ribs contact the plate surface. In flooded designs, separators with a laminate layer are regularly utilized to provide more support of the active material that is likely to shed during deep discharge. Because the laminate thickness is normally a small portion, say, 10% to 20%, of the total structure thickness, the amount of compression or resiliency is minimized. 

If the VRLA battery is cycled to any depth of discharge, the electrodes will grow when in the sulfated state and when recharged the electrode thickness will likely shrink. During this expansion or contraction, it is important for the separator material to have sufficient resiliency to maintain intimate contact with the electrode surfaces so that the battery can continue to function. To that end, much work has been done to develop compression and resiliency curves for various AGM materials both in a dry and wet state [33]. 

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