Two practical power sources

21 TWO PRACTICAL POWER SOURCES 17.21.1 The Lead Storage Cell 

Consider first the lead-acid storage cell. As we draw current from the cell, at the positive plate, the cathode, Pb02 is reduced to PbS04  

The potential of the cell is 2.0 volts. As current is drawn from the cell, the cell potential does not drop very much so the power, SI, is near the reversible value, S I. Rather large currents—hundreds of amperes—can be drawn from the fully charged device without dropping the potential excessively. 

The voltage efficiency of the lead-acid cell is about 80 %. This near reversibility is a consequence of the rapidity of the chemical reactions in the cell. As we have seen, the ability to supply large currents at potentials near the open-circuit potential means that the chemical reactions at the electrodes are fast as the charge is drained away by the current, the potential should drop, but the chemical reaction occurs rapidly enough to rebuild the potential. 

If we compare the quantity of charge obtained from the lead-acid cell to the quantity that must be passed in to charge the cell, we obtain values of 90 to 95 %, or even higher in special circumstances. This means that very little of the charging current is dissipated in side reactions (such as electrolysis of water). Overall, the lead storage cell is an extra- 

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