Primary cells, secondary cells and fuel cells are the major sources of electrical energy. In the case of secondary cells, the cell reaction producing electrical energy (discharge process) can be reversed by applying an external source of current (the charging process). In the case of primary cells, the discharge process is irreversible and cannot be reversed. In fuel cells, the chemical energy associated with the oxidation of fuels (reducing agents) is directly converted into electrical energy more efficiently than in other conventional processes. In photovoltaic solar cells, the light energy from sun is converted to electrical energy. Efforts to build rechargeable and reusable cells, environmentally friendly cells and pollution free batteries will always continue.
Very soon , cars running on fuel cells and solar cells on a large scale will become a reality.
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