Positive plate expansion

FMl. Positive-plate expansion. This can occur both in the plane of the plate (if the grid is stretched by a growing corrosion layer) and in the direction normal to the plate (through expansion of the active material itself). Repetitive discharge and... 

FMl. Positive-plate expansion. The use of lead-antimony alloy enhances the creep strength of the positive grid and thus retards growth in the plane of the plate. For the flat design of positive plate, expansion normal to the plate can be moderated by applying a compressive force to the plate group. Tubular positive plates have gauntlets that constrain the active material and reduce its tendency to expand, disconnect, and shed. 

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 contrast to most conventional separators for flooded batteries, separators for gel batteries have ribs positioned not only towards the positive plate, but also towards the negative plate in order to facilitate the gel-fllling process. For batteries with pasted positive plates, the separator is usually laminated with a glass fleece, which protects the positive plate against shedding, especially in cycle applications. Although this surface fleece stabilizes the active material, the present design of gel batteries cannot prevent completely the expansion of the positive material and the occurrence of PCL-2. The most important characteristics of separators used in gel batteries are listed in Table 7.2 (adapted from Ref. 12). 

Post-service examination revealed that all positive plates had undergone softening and considerable expansion. In addition, lead dioxide particles had penetrated the separator. The general condition of the positive plates was better with separators of higher fine-fibre content, which was related to the lower degree of acid stratification. 

In a different battery test with a simulated EV load pattern, a SWP-7 cell with an assembly pressure of 60 kPa achieved 450 cycles versus 270 cycles for an AGM cell with 73 kPa. The failure mode was found not to be the expansion of positive plate but, rather, sulfation of the negative plate. This led to the conclusion that the favourable mechanical properties of SWP-type separators suppress degradation of the positive active-material. 

The delivery of too much overcharge can also result in damage to the positive active-material [24]. It has been suggested that the vigorous evolution and movement of gas as a result of this process can disrupt the internal structure of the active material. The related decrease in performance has been explained in terms of a loss of electrical conductivity within the positive plate due to progressive expansion of the active mass. 

The positive plate consists of lead alloy spines surrounded by synthetic fibre tubes filled with a mixture of lead oxides (Figure 32.3). The tubes keep the active material in contact with the conducting spines during expansion and contraction resulting from the charge and discharge cycle, and so contain the stresses that would break up other types of plate. 

Example 2.6 The bobbin shown in Fig. 2.16 has been manufactured by sliding the acetal ring on to the steel inner and then placing the end-plate in position. At 20°C there are no stresses in the acetal and the distance between the metal end-plates is equal to the length of the acetal ring. If the whole assembly is heated to 1(X)°C, calculate the axial stress in the acetal. It may be assumed that there is no friction between the acetal and the steel. The coefficients of thermal expansion for the acetal and the steel are 80 x 10 °C and 11 X 10 °C respectively. The modulus of the acetal at 100°C is 1.5 GN/m. ... 

Fig 1 shows the situation, at some time after the start of detonation, for a 1-D system consisting of a semi-infinite expl slab, initially in contact with a metal plate on one side and vacuum on the other side, with detonation started simultaneously all along the original HE-vacuum boundary As indicated, the plate moves to the right at a velocity V and product gases expand into vacuum (in Appendix A it is shown that expansion into air is negligibly different from expansion into vacuum) at velocity -U. The coordinate system used assigns x = 0 to the product/vacuum boundary and x = 2 to the position of the plate at time t. Then, as taken directly from Ref 14 ... 

A device for foam dispersity determination by measuring the local foam expansion ratio and the pressure in Plateau borders is illustrated in Fig. 4.4. It consists of a glass container equipped with platinum electrodes and a micromanometer. The container bottom is a porous plate (a sintered glass filter). The pressure Ap is measured with a capillary micromanometer and the expansion ratio is determined by the electrical conductivity of the foam. The manometer and the electrodes are positioned so that to ensure a distance of 1.0-1.5 cm between them and the porous plate. When the distance is small the liquid in the porous... 

In this section, we assume that the green body is already dry and the stress is caused by the thermal expansion of the ceramic particles that make up the porous ceramic due to the temperature profile in the green body in either heating or cooling. For an infinite plate of thickness 2xq> the normal stress ofx) at a position x in the green body depends on the temperature difference between that point, T, and the average temperature, T. This gives the strain at that point and fixes the net local stress at [28]... 

Experimentally, TMA consists of an analytical train that allows precise measurement of position and can be calibrated against known standards. A temperature control system of a furnace, heat sink, and temperature-measuring device (most commonly a thermocouple) surrounds the samples. Fixtures to hold the sample during the run are normally made out of quartz because of its low CTE, although ceramics and invar steels may also be used. Fixtures are commercially available for expansion, three-point bending or flexure, parallel plate, and penetration tests (Fig. 4). 

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