321 Cell Discharge Operation

In the cell discharge operation (Figure 3.2), electrons are consumed at the positive electrode, the supply of which comes from the negative electrode. The current flow is, therefore, out of the positive electrode into the motor-load, with the battery acting as the source.

FIGURE 3.2 Lead-acid battery: cell discharge operation.

The positive electrode equation is given by:

]>hO is:. ■■■ 411-:x|i + SO.' cup -i > > ly:A(), t- :i|.():|.

A highly porous structure is used for the positive electrode to increase the PbO2(s)/electrolyte contact area, which is about 50 to 150 m2 per Ah of battery capacity. This results in higher current densities, as PbO2 is converted to PbSO4(s). As discharge proceeds, the internal resistance of the cell rises due to PbSO4 formation and decreases the electrolyte conductivity as H2SO4 is consumed. PbSO4(s) deposited on either electrode in a dense, fine-grain form can lead to sulfatation. The discharge reaction is largely inhibited by the buildup of PbSO4, which reduces cell capacity significantly from the theoretical capacity.

The negative electrode equation during cell discharge is:

The electrons are released at the negative electrode during discharge operation. The production of Pb SO4(s) can degrade battery performance by making the negative electrode more passive. The overall cell discharge chemical reaction is:

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