How does a battery work?

A flashlight battery is made of zinc can containing a damp chemical paste and a carbon rod. The zinc and carbon are the poles of the battery. Chemical reaction creates an electrical difference between the poles. If the two are connected by a wire, current will flow from the pole with the higher electric charge to the pole with the lower charge. If a light bulb is connected into the circuit, the current will make it glow.

Strictly speaking, the flashlight battery should be called a "cell”. A "battery” is several cells linked together.

A lemon with two strips of different metals (for example – Zinc & Copper) stuck into it is another example of an electric cell.

A flashlight battery is a "dry” cell that has a limited life. The "wet” cell battery used in automobiles can be re-charged and used for years. Wet cell batteries contain liquid, usually acid. The chemical reactions occur more efficiently in liquid. Re-charging of the battery, will restore  the chemicals to their original state, so the battery can be used again.

When an automobile ignition is switched on, the circuit is completed between the terminals of the battery, and the current flows. Once the engine is running, it re-charges the battery.

Wet cell batteries are not suitable for all purposes, however. They would be inconvenient, for instance, in a hearing aid, a portable radio or an artificial limb. Zinc air batteries are used in these appliances. The zinc air battery is a dry cell and is not rechargeable, but it is compact, there is no liquid to spill and its output of electricity is relatively constant throughout the life of the cell.