Secondary Cells

Secondary cells are cells that can be recharged by passing current through them in the opposite direction. Accumulators are good examples of secondary cells and they have low internal resistance. Hence, they supply large currents.

There are two types of accumulators:

• Lead-Acid Accumulator
• Alkaline or Nickel-Iron (Ni-Fe) Accumulator

Lead-Acid Accumulator:

The Lead-Acid Accumulator cell consists of:

• lead peroxide as a positive pole
• lead plate as a negative pole
• dilute sulphuric acid (tetraoxosulphate(VI) acid) as an electrolyteAn electrolyte is a substance that dissociates in water into charged particles called ions. Positively charged ions are called cations. Negatively charged ions are called anions. Simply, an electrolyte is a... More

The positive and negative plates are made of grids of lead antimony alloy, and the holes are filled with lead or lead peroxide. The plates are arranged alternatively and are separated by insulators. All positive plates are connected to form one terminal and negative plates are also connected to form a negative terminal. The entire framework is put in a plastic container containing sulphuric acid.

The e.m.f. of the cell is 2.2 volts when fully charged and the relative densityDensity is the measurement of how tightly a material is packed together i.e. how closely the particles are packed in the material. The tighter the material is packed the more its... More of dilute sulphuric acid is 1.25. The cell is regarded as fully discharged when the relative density, (also called specific gravity) of the acid has fallen to 1.15.

A hydrometer