Physical properties of AGM materials

The AGM materials used in VRLA batteries must sustain the required plate-group pressure, must hold the electrolyte in place, and must allow sufficient (but not excessive) transport of oxygen from the positive plate to the negative. Some of the properties of typical all-glass AGM separators are shown above in Table 6.2 [7]. The following important features are noted  

The characteristics of the key physical parameters of AGM separators are discussed below. Reference should also be made to the BCI test methods for these materials [10]. 

Thickness. Because of the delicate and very compressible nature of AGM, it is necessary to measure the thickness at a controlled, standard pressure. The pressure chosen is lOkPa (1.5 psi). Thus, calculation of the percentage compression is based on the initial thickness of the separator as measured at 10 kPa and, ideally, the BCI test method is employed [10]. It should be noted that the relative humidity of the test conditions and prior handling of the material may affect the result. 

Basis weight (or grammage ). This is the weight per unit area and is usually expressed in terms of g m . Since AGM is normally sold by weight but used by area, this is a critical specification. The weight is measured on a moisture-free sample, and then corrected for the moisture content of the test specimen (AGM may pick up moisture between manufacture and testing/use). 

Volume porosity. This is the ratio of the total void volume to the total apparent volume, expressed as a percentage. The apparent volume is obtained by multiplying the sample area by the thickness measured at a specified pressure. The void volume is that portion of the apparent volume which is not occupied by the solids used to make the separator, and is usually determined by subtracting the volume of the solids from the apparent volume. The volume porosity of the AGM in the compressed state limits the maximum amount of acid between the plates. Determination of the volume porosity requires knowledge of the thickness, basis weight, and average density of the solids in the sample. 

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