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A battery consists of one or more electrochemical cells. Although the terms battery and cell are often used interchangeably cells are the building blocks of which batteries are constructed. Batteries consist of one or more cells that are electrically connected.

Cells
A cell normally consists of the four principal components shown in Figure 1. These are:

a positive electrode that receives electrons from the external circuit when the cell is discharged,a negative electrode that donates electrons to the external circuit as the cell discharges,electrolyte which provides a mechanism for charge to flow between positive and negative electrodes, and
a separator which electrically isolates the positive and negative electrodes

In some designs, physical distance between the electrodes provides the electrical isolation and the separator is not needed.

In addition to the critical elements listed above, cells intended for commercial batteries normally require a variety of packaging and current collection apparatus to be complete.

How a Cell Works
When a battery or cell is inserted into a circuit, it completes a loop which allows charge to flow uniformly around the circuit. In the external part of the circuit, the charge flow is electrons resulting in electrical current. Within the cell, the charge flows in the form of ions that are transported from one electrode to the other. As mentioned above, the positive electrode receives electrons from the external circuit on discharge. These electrons then react with the active materials of the positive electrode in "reduction" reactions that continue the flow of charge through the electrolyte to the negative electrode. At the negative electrode, "oxidation" reactions between the active materials of the negative electrode and the charge flowing through the electrolyte results in surplus electrons that can be donated to the external circuit.

It is important to remember that the system is closed. For every electron generated in an oxidation reaction at the negative electrode, there is an electron consumed in a reduction reaction at the positive.

As the process continues, the active materials become depleted and the reactions slow down until the battery is no longer capable of supplying electrons. At this point the battery is discharged.

Recharging
The world of batteries divides into two major classes: primary and secondary batteries. Primary batteries such as the common flashlight battery are used once and replaced. The chemical reactions that supply current in them are irreversible. Secondary batteries (for example, car batteries) can be recharged and reused. They use reversible chemical reactions. By reversing the flow of electricity i.e. putting current in rather than taking it out, the chemical reactions are reversed to restore active material that had been depleted. Secondary batteries are also known as rechargeable batteries, storage batteries or accumulators.