Lead dioxide discharge performance

Since grid material is converted into lead dioxide, a slight increase in the actual capacity is often observed with lead-acid batteries. The reduced cross-section in Fig. 9 does not affect the performance of batteries that are used for discharge durations in the order of one hour or more. Attention must, however, be paid to batteries that are loaded with high currents, because the conductivity of the grid gains importance with increased current flow. 

The type of basic lead sulfate in the paste affects the structure of the active materials formed, especially of the lead dioxide active mass. And the active mass structure in turn determines the charge/discharge cycling performance of the battery. Thus, control of the a-PbO p-PbO ratio... 

The active material comprises the substances that constitute the charge-discharge reaction. In the positive electrode of lead-acid batteries, the active material in the charged state is lead dioxide (PbOj), which is converted into lead sulfate (PbS04) when the electrode is discharged. The active material is the most essential part of a battery, and battery technology has to aim at optimum constitution and performance for the expected application. This does not only concern the chemical composition but also the physical structure and its stability. Specialized methods have been developed to fulfill these requirements, and the primary products as well as the manufacturing process are usually specified by the individual battery manufacturer. 

Differences in the electrochemical activity of lead dioxides can be characterized by their discharge performances. Normally, P-Pb02 has a 1.5 to 3 times higher capacity than does a-Pb02, as shown in Figure 3.10. 

Figure 18.22 shows that reducing the separator thickness improves cell voltage under high-rate discharge conditions. Although decreasing the separator thickness has a beneficial effect on cold cranking performance of the battery, it does have a deleterious effect on reserve capacity. This is because, with thinner separators, the volume of sulphuric acid adjacent to the plates is diminished, thereby decreasing the utilization of the lead and lead dioxide active material in the plates. This is illustrated in Figure 18.23. Thus there has to be a trade-off in selection of separator thickness to achieve a desired compromise between acceptable cold cranking performance on the one hand and acceptable reserve capacity on the other.

Table 30l1 High-rate discharge performance of Mallory lithium-sulphur dioxide hermetic D-size cell test performed on resistive leads after 6 months storage at 20°C...

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