Primary and secondary cells
The cells already described in this chapter are known as primary cells. In them the current is produced as a result of non-reversible chemical changes taking place between their various components, When all the zinc has been used up the cell
cannot be restored to its original condition by passing a charging current through it in the reverse direction.
In contrast with primary cells there is another class of cells called secondary cells. These can be recharged after they have run down by passing a current through them from a dynamo or other source of current. They are also known as storage cells or accumulators, and the two most important types are the lead-acid cell and the nickel-cadmium alkaline cell. Lead cells are extensively used for ignition and lighting on motor cars. Their main advantage is that they have a very low internal resistance, and hence can give a large current with very little drop in terminal potential difference (page 413).