Acid stratification

For the separation of such batteries, gel construction and microfiber glass fleece separators again compete because of the deep discharge cycles, the gel construction with its lower tendency to acid stratification and to penetration shorts has advantages for the required power peaks, microfiber glass fleece construction would be the preferred solution. The work on reduction of premature capacity loss with lead-calcium alloys has shown that considerable pressure (e.g., 1 bar) on the positive electrode is able to achieve a significantly better cycle life [31-36], Pressure on the electrodes produces counter pressure on the separators, which is not unproblematic for both separation systems. New separator developments have been presented with... 

FM3. Acid stratification. On recharge, sulfuric acid is produced in and between the plates and there is a tendency for acid of higher concentration, which has a greater relative density, to collect at the bottom of the cell, see Fig. 1.3 [7]. 

Fig. 1.3. Schematic representation of processes and electrolyte flow that contribute to acid stratification during (a) discharge and (b) charge [7],...

FM3. Acid stratification. Acid concentration gradients can be removed through agitation of the acid by setting the battery to deliberate gassing during extended overcharge. 

FM3. Any acid stratification which occurs cannot be overcome by overcharging because there is little free electrolyte and, hence, ineffective mixing. Moreover, water lost cannot be replaced because the battery is expected to remain sealed for life. 

Surface area. This can be determined using BET adsorption [11]. The significance of this parameter is that the performance of a VRLA battery is partly dependent on the surface area of the mat between the plates. For example, surface area has an influence on acid stratification. 

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... 

Gel Little acid stratification Difficult gel-filling processes... 

As a result of such considerations, the surface area, particularly fine-fibre content of AGM separators has been the subject of two ALABC projects on EV batteries. In the first project [16], acid stratification was found to decrease as the surface area of the separator was increased, see Table 7.5. Nevertheless, any advantage in terms of battery cycle-life was less clear-cut, see Table 7.6. 

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. 

Buoyant force (depth of dip) C Local, at the top electrolyte gassing acceleration forces inclination acid level acid stratification No High... 

Refractive index (fibre optic) B Local, at the top electrolyte Gassing dirt acid stratification Possible Medium... 

The two technologies, gel and AGM, have many similarities, but there are also some differences. For example, it is well known that for tall cells in a vertical position, only gel cells can be used otherwise, acid stratification caimot be avoided. There are some fairly tall AGM cells on the market, but these are intended for use only in a horizontal position in order to avoid the acid stratification problem. For short cells, both gel and AGM designs are possible without restriction on cell orientation. Often, AGM is used when high power is requested whereas gel is especially useful to achieve a long lifetime. 

Schematic representation of processes and electrolyte flow that contribute to acid stratification...

Acid stratification in flooded batteries used in cycling application, when there is no overcharging with gassing and no electrolyte circulation [54]. 

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