Survey of soaking and formation phenomena

Generally, formation is carried out under constant-current conditions. The changes in temperature, electrolyte concentration, and cell voltage during plate soaking and formation are presented in Fig. 3.2 [2]. Chemical reactions commence between PbO, basic lead sulfates, and H2SO4 when the plates come into contact with the solution. 

Since the reactions during the soaking procedure are exothermic, the temperature in the tank or battery starts to rise. The electrochemical reactions involved in the formation process also contribute to the overall thermal balance (in the battery or in the tank). These reactions are endothermic and can be represented by the following equations  

In addition, when electric current flows through the electrolyte. Joule heat is released. Heat exchange occurs between the formation tank (or battery) and the surrounding atmosphere. The reactions during the different formation stages all provide individual contributions to the overall thermal balance of the system. Consequently, the temperature curves feature a maximum, and with attenuation of the exothermic chemical reactions of sulfation, the temperature of the tank (battery) decreases slowly. Battery manufacturing practice has shown that the optimum temperature range for the formation process is between 25 and 55°C. 

The data in Fig. 3.2 show that, initially, the H2SO4 concentration decreases, not only during the soaking period but also during the early stages of formation. A decrease in the formation tank (battery) voltage is also observed during the initial period of formation. Fig. 3.2(c). 

Later in the formation process, the electrochemical reactions cause H2SO4 to be extracted from the plates and, consequently, the add concentration increases. At the end of the formation process, the add concentration is higher than that at the beginning of formation. This increase is due to the extraction from the plates of the acid used in the production of basic lead sulfates during paste preparation and in sulfation of the paste during plate soaking. 

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