Premature capacity-loss

Sealed batteries have made little entry into this market with heavy cycling service, since the lead-calcium alloys required for these versions tend towards premature capacity loss, a phenomenon intensively investigated in recent years and possibly close to a solution. 

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... 

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. 

Fig. 9.20. Three categories of premature capacity loss (PCL) in deep-cycling applications.

A.F. Hollenkamp, K.K. Constant , M.I. Koop, K. McGregor, ALABC Project AMC-003. Advanced Lead Acid Batteries for Electric Vehicles Examination of Premature Capacity Loss. Final Report April 1993-March 1995, Advanced Lead-Acid Battery Consortium, Research Triangle Park, NC,... 

An important result of these studies was the discovery that a lack of antimony has not only an effect on the grid/positive-material interface, but also on the whole crystalline structure of the positive active mass [43] and on the so-called mass softening process. Nowadays deleterious effects that are promoted by using grids without antimony are collectively termed premature capacity loss (PCL). There has been much research effort worldwide to overcome the PCL problem [47]. 

With the introduction of lead—calcium alloys for the plate grids, however, the life of the battery on deep discharge cycling declined dramatically to 20—25 cycles. This phenomenon was first called Sb-free effect and later premature capacity loss (PCL effect) [20]. It was established that the PCL effect was a result of certain processes that proceed at the positive battery plate, more precisely at the interface grid/positive active mass (PCL-1 effect) [21] and/or in the positive active mass volume (PCL-2 effect) [22]. 

Capacity of batteries with lead—calcium or high-antimony lead grids on cycling a demonstration of the early capacity decline of batteries PCL = premature capacity loss. 

USABC has set the goal so high that lead-acid batteries have been put out of the question for this application [29]. This led to an initiative by the lead-acid battery industry and their suppliers to set up the Advanced Lead-Acid Battery Consortium (ALABC) with the goal of fostering development of the lead-acid battery for use in electric vehicles, at least for an interim period until more powerful batteries with higher energy density will become available. Here a series of complex technical problems have to be solved [30]. Of course, such electric vehicle batteries have to be maintenance-free, that is, of sealed construction the resulting use of lead-calcium alloys and thus the premature capacity loss have already been touched on. 

---