Processes during soaking and formation of battery plates
Soaking. Before switching on the current for the formation of the active masses cured plates are left in H2SO4 solution on open circuit for a certain period of time. This process is called “soaking”. PbO and basic lead sulphates are unstable in H2SO4 solution and, hence, chemical reactions of sulphation proceed. The chemical processes that take place during soaking of 3BS plates in H2SO4 solutions of three different concentrations (1.05, 1.15 or 1.25 relative density) for 8 hours have been studied.
It has been established that soaking in H2SO4 of 1.05 relative density results in formation of 3BS, 1BS and, after 4 hours, small amounts of PbSO4. In more concentrated H2SO4 solutions, PbSO4 predominates. Capacity, cold-cranking and cycle-life tests have been performed with batteries soaked in the three acid solutions for different periods of time. It has been established that, depending on acid concentration and duration of soaking, the battery capacity may increase by 10% and the time of high-rate discharge at –18oC may be prolonged by about 1 min. Battery life may also be improved.
These results indicate that soaking should be introduced as a separate technological procedure with continuously controlled parameters in order to ensure stable battery performance characteristics.
Formation of positive plates. Zone processes. It has been found that formation of positive active mass (PAM) proceeds in two stages.
- First stage. During the first formation stage, H2SO4 and H2O penetrate from the bulk of the solution into the plate. As a result of chemical and electrochemical reactions PbO and basic lead sulfates are converted into α-PbO2 and PbSO4. These compounds form zones which advance into the paste interior. Part of the hydrogen ions evolved during the reaction of PbO2 formation migrate from the plate so as to keep latter electroneutral.
- Second stage. During the second stage of formation PbSO4 is oxidized to β-PbO2. H2SO4 is formed and diffuses into the volume of electrolyte. Taking into account the specific conditions of chemical and electrochemical reactions in porous electrodes a mechanism is suggested for the formation processes of PAM. Ion diffusion and migration are considered to be the rate-limiting steps of the processes. The α/β PbO2 ratio and the direction of growth of (PbO2 + PbSO4) zones is explained in terms of this mechanism. During the first formation stage the direction of advance of (PbO2 + PbSO4) zones into the paste is determined by the degree of paste sulfation, formation current, temperature and phase composition of cured paste. The above parameters influence the capacity and cycle life performance of the battery. It has been established that the capacity of the positive plate is determined by the structure of PAM and the β/α PbO2 ratio.
Formation of negative plates. It has been established that formation of the negative active mass proceeds in two stages too:
- First stage. During the first stage, electrochemical reduction of PbO and basic lead sulfates occurs and lead skeleton (network) is formed. Beside these processes chemical reactions of PbSO4 formation also proceed. PbSO4 crystals remain incorporated in the lead skeleton. (PbSO4 + Pb) zones are formed on both plate surfaces and advance into the interior of the plate.
- Second stage. During the second stage, reduction of PbSO4 to Pb occurs and the obtained lead crystals are deposited on the lead skeleton surface in strongly acidic solution. The mechanism of the elementary chemical and electrochemical reactions as well as their mutual relationships have been determined. During formation, both the pore radii and the porosity of the active mass increase.
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G. Papazov, Investigation of the formation of the positive lead-acid battery plate
Keywords: soaking of battery plates, chemical processes during soaking, battery performance characteristics, formation of positive plates, zone processes, β/α PbO2 ratio, formation of negative plates, lead skeleton, energetic structure of NAM