Absorptive glass mat AGM separators

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. 

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Two VRLA battery technologies are currently predominant, i.e., absorptive glass mat (AGM) and gel designs. In the former, the AGM immobilizes the electrolyte and simultaneously functions as a separator. In gel batteries, the acid is immobilized by means of fumed silica, and an additional separator is required to fix the plate distance and to prevent electronic shorts. 

Absorptive glass mat (AGM) Very high porosity of separator High purity and stability of separator Low internal electrical resistance Relatively large separator pores acid stratification -electrolyte drainage risk of internal shorts -risk of thermal runaway -risk of PCL-3... 

The original wooden sheet separators evolved through sintered PVC to polyethylene or polypropylene and, in some battery types, to absorptive glass mat (AGM). 

Separators are used to electrically insulate between positive and negative plates to prevent a short circuit. Separators are porous to allow acid transport into or ont of the plates. The common separators are microporous polyethylene (PE) or absorptive glass mat (AGM). AGM is a nonwoven fabric made with glass microfiber. It is used widely for VRLAs because it is highly porous (90% to 95%) and can absorb more acid and has small pore size to maintain electrolyte levels in battery. Table 1.11 shows the properties of battery separator materials. 

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