| Secondary Cells |
| The widely used automobile battery is a storage battery capable of delivering either 6 V or 12 V depending on the number of cells connected in series. These are used as stationary power sources in telephone exchanges, switching systems, emergency lighting etc. The cell can be represented as |
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Pb ( grid)  PbSO4 ( s) | H2SO4 ( aq, 38 % by weight ) | PbO2 ( s)  Pb ( grid) |
| Electrode reactions |
| Anode: |
| Pb ( s) + SO2-4 = PbSO4 ( S ) + 2e- |
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| Figure 23.2: Schematic representation of a lead accumulator (Secondary cell) |
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1. Glass or Plastic container . 2. Grill electrodes made of Pb - Sb aIIoy. 3. Spongy lead packed in anode.
4. PbO2 packed in cathode. 5. Porous non-conductive Plastic Separator. 6. Electrolyte (38% by weight of H2SO4 ) sp. gravity = 1.30. |
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Cathode: |
PbO2(s) + 4H+ +
 + 2e = PbSO4 (s) + 2H2O |
Cell reaction for the passage of 2F |
PbO2+ Pb+ 2 H2SO4  2PbSO4 +2H2O |
( 23.1) |
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The normal voltage of a single lead storage cell is about
2.0 V. As the cell discharges electricity, PbSO4 is deposited on both the electrodes and sulphuric acid is consumed, resulting in a decrease in the specific gravity of the electrolyte. With the aid of a hydrometer, the specific gravity can be checked and if it is equal to or below 1.20 (approx. 28% by weight of H2SO4) the battery is (re)charged. The charging operation is performed in such a way that the negative pole of the battery is connected to the negative pole and the positive pole to the positive pole of the external charging device. The charging is done in an automobile by its electrical generator or alternator. During the charging operation, water is converted into sulphuric acid (refer to Eqn.23.1). The charging is done till the specific gravity increases to the required value. |
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