Primary and Secondary Cells

Cells split into two types:

Primary cells produce current as a result of irreversible chemical changes taking place within the cell. When all the zinc in the negative electrode has been used up the cell cannot be restored to its original condition. Cheaper cells are of this type. Once used they must be disposed of.

Secondary cells can be recharged once discharged by passing a current through them in the opposite direction – from the negative terminal to the positive terminal. This can be done several hundred times, making them much cheaper in the long term than primary cells. Important types are the nickel cadmium and nickel metal hydride cells, and in mobile phones and portable computers lithium ion cells are widely used as they are light and have a high energy per unit mass or volume.

A primary cell must be changed when it is discharged but by fitting electrical devices with charging connectors and secondary or rechargeable batteries made up of several cells, a phone or portable computer may not have to be partially dismantled for several years. Anybody who owns one of them knows that the opportunity to cause damage to them or lose bits is multiplied every time they need to have a part replaced.

Lead acid are secondary cells and are widely used to start motor vehicles. They have no practical alternative among other secondary cells. They have a very low internal resistance and can deliver a large internal resistance with comparatively little drop in terminal potential difference. They are connected to a charging circuit inside the vehicle to keep them fully charged constantly, which extends their useful life considerably.

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