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Secondary Cells

In secondary cells, chemical reactions are reversible. That is, the active materials used up when the cell delivers current can be reproduced by passing a current through the cell in opposite direction. The chemical process involved in obtaining current from a secondary cell is called discharge. The process by which the chemicals are reformed is called charging.

Lead-Acid Accumulator

In this accumulator, the electrode are lead dioxide (+ve electrode) and spongy lead (-ve terminals. To prevent plates of opposite polarity from coming into contact, suitable insulting separators are also used. The electrolyte is a solution of sulphuric acid. The container is a glass or Bakelite case. The reactions during the process of charging are given below:
At positive plate,
PbSO4 + SO4 + SH2O→2H2O→PbO2 + 2H2 SO4

At the negative plate,
PbSO4 + 2H→Pb + H2SO4

The total reaction during charging can be written as
2PbSO4 + 2H2O→Pb + PbO2 + 2H2 SO4                             … (i)

The chemical reactions taking place during the discharging process may be represented by the following equations:

At the positive plate,
    PbO2 + 2H + H2SO4 →PbSO4 + 2H2O
At the negative plate,
    Pb + O + H2SO4 → PbSO4 + H2O
The total reaction in the cell during discharge is
    Pb +PbO2 + 2H2SO4 → 2PbSO4 + 2H2O                        … (ii)
Reactions (i) and (ii) can be represented together as

2PbSO4 + 2H2O ------------------ Pb + PbO2 + 2H2SO4                     … (iii)

The during the discharging process, whitish PbSO4 is formed on both the electrodes. During charging, however, the positive plate gets its dark coating of lead peroxide while the negative plate is reduces to gray metallic lead.

For proper maintenance of the cell, the density of sulphuric acid is maintained between 1220 to 1250kg m-3. When the density becomes about 1700kg m-3, drawing of current from the cell is stopped and the cell is recharged.

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