Internal oxygen cycle

Fig. 1.4. Conceptual view of internal oxygen cycle in a valve-regulated lead-acid cell (Nelson, 1999) [11].

When transport by diffusion of reacting neutral particles (like that of O2 in the internal oxygen cycle (Fig. 1.25)) precedes the transfer reaction, the actual concentration is reduced with increasing current. If Cj reaches zero, a further increase of the current is not possible. Such a situation is called a (diffusion) limiting current, which according to Eq. (28) is given by... 

When the internal oxygen cycle is established, almost all the overcharging current is consumed by the internal oxygen cycle (center bar in the graph). The bar on the right corresponds to a vented battery. Internal resistance assumed as 0.8 mQ per 100 Ah of nominal... 

The situation is different for sealed nickel/cadmium batteries, due to the internal oxygen cycle. Figure 1.15 illustrates the heat evolution of a sealed nickel/ cadmium battery during constant-current charging with a charge factor of 1.4 (such an amount of overcharge is usual for conventional charging methods but can only be applied to comparably small batteries < 10 Ah). 

The middle figure shows the (constant) current and its distribution between charging process and internal oxygen cycle. 

When the charging process approaches completion, nearly all the current is used for the internal oxygen cycle, which causes much heat generation. 

Middle current distribution between charging and internal oxygen cycle. 

This indicates the strong influence of overcharging on heat generation in sealed or valve-regulated batteries caused by the internal oxygen cycle. 

In batteries with the internal oxygen cycle, like sealed nickel/cadmium, nickel/ metal hydrid, or VRLA batteries, the felt not only separates the electrodes, but also stores the electrolyte while the large pores stay open for fast oxygen transport through the gaseous phase (Sections 1.8.1.5 and 1.8.2.2). 

In sealed nickel/cadmium batteries, oxygen evolution and oxygen reduction that form the internal oxygen cycle are the only reactions during overcharging (cf. Section 1.8.2.2). 

The internal oxygen cycle requires fast oxygen transport that only can be achieved by diffusion in the gaseous phase, as indicated by the ratio shown in Eq. (67). To provide void volume for such a fast transport, the electrolyte must be immobilized . This can be achieved by two methods ... 

Figure 1.27 The float charging situation of a VRLA battery. Current voltage curves as in Fig. 1.23, but at 100% of recombination efficiency of the internal oxygen cycle.

The float current is determined by the sum of the internal oxygen cycle and grid corrosion and has no direct relation to water loss. 

Strictly speaking these relations are valid only at 100% of efficiency of the internal oxygen cycle. But they can be transferred to most VRLA batteries, since usually such a high efficiency is closely approached. 

Figure 1.30 Effect of a catalyst. Characteristic data as in Fig. 1.28. Efficiency of the catalyst is 4.0 mA or 10% of the internal oxygen cycle. The efficiency of the internal oxygen cycle is thereby reduced to 90%.

During charging, water decomposition can only be critical as a heat source, when the cell voltage considerably exceeds 1.48 V (= Ucai for water decomposition). As a result, vented nickel/cadmium batteries can be charged at quite high rates without suffering heat problems. But in the sealed version, the internal oxygen cycle can cause serious thermal problems (cf. Fig. 1.15). 

The internal oxygen cycle, formed by oxygen evolution at the nickel-hydroxide electrode and its subsequent reduction at the cadmium electrode, was already detected in the 1940s as a possibility to avoid gas escape during overcharging, and the sealed nickel/cadmium battery appeared on the market in the 1950s. Immobilization of the alkaline electrolyte is achieved by absorption in mats of fibers of polyamide or polypropylene. Formation of a gel, as described in Section... 

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