Today batteries are used for every purpose - in cars, PCs, laptops, portable MP3 players and cell phones. A battery is basically a device consisting of one or more cells that can produce a direct current by converting chemical energy to electrical energy or a container full of chemicals that generate a chemical reaction producing electrons, this is called an electrochemical reactions. Every battery has two terminals, one end of the battery is marked (+) or positive and the opposite end is marked (-), or negative. In an AA, C or D cell (flashlight batteries), the ends of the battery are the terminals. In a large car battery, there are two heavy lead posts that act as the terminals.
Electrons polarize on the negative terminal of the battery. If a wire is connected between the negative and positive terminals, the electrons will flow from the negative to the positive terminal as fast as they can (wearing out the battery very quickly - this also tends to be dangerous, especially with larger batteries, so it is not recommended). Normally, you connect some type of load to the battery using the wire. The load might be a light bulb, a motor, or an electronic circuit like a radio.
Inside the battery itself, a chemical reaction produces the electrons. The speed of electron production by this chemical reaction (the battery's internal resistance) controls how many electrons can flow between the terminals. Electrons flow from the battery into a wire, and must travel from the negative to the positive terminal for the chemical reaction to take place. That is why a battery can sit on a shelf for a year and still have plenty of power - unless electrons are flowing from the negative to the positive terminal, the chemical reaction does not occur. However, once you connect a wire, the reaction begins.
There are two types of batteries- disposable and rechargeable. Both of them contain two different kinds of cells. Primary cells compose the power in ordinary, disposable batteries. Electricity is produced by slowly using the chemicals which makeup the electrode and electrolyte material.
Rechargeable batteries use secondary cells. They are known as secondary cells because their electrochemical reactions are electrically reversible You can find them in the big lead-acid batteries that start cars and the nickel-cadmium (NiCd or "nicad") batteries that power cellular phones. By passing current in the reverse direction to normal, secondary cells can be recharged. Cell Phones use this secondary battery where the chemical reactions take place in reverse order.