About batteries work

A battery is a device that converts chemical energy directly to electrical energy. It consists of one or more voltaic cells. Each voltaic cell consists of two half cells connected in series by a conductive electrolyte. One half-cell is the positive electrode and the other is the negative electrode. The electrodes do not touch each other but are electrically connected by the electrolyte, which can be either solid or liquid.In many cells, the materials are enclosed in a container, and a separator, which is porous to the electrolyte, which prevents the electrodes from coming into contact.

Each half cell has an electromotive force (or emf), determined by its ability to drive electric current from the interior to the exterior of the cell. The net emf of the battery is the difference between the emfs of its half-cells, as first recognized by Volta. Thus, if the electrodes have emfs \mathcal{E}_1 and \mathcal{E}_2, then the net emf is \mathcal{E}_{2}-\mathcal{E}_{1}. (Hence, two identical electrodes and a common electrolyte give a zero net emf.)

The electrical potential difference, or \displaystyle{\Delta V_{bat}} across the terminals of a battery is known as terminal voltage and is measured in volts. The terminal voltage of a battery that is neither charging nor discharging is called the open-circuit voltage and equals the emf of the battery. Because of internal resistance, the terminal voltage of a battery that is discharging is smaller in magnitude than the open-circuit voltage and the terminal voltage of a battery that is charging exceeds the open-circuit voltage. An ideal battery has negligible internal resistance, so it would maintain a constant terminal voltage of \mathcal{E} until exhausted, then dropping to zero. If such a battery maintained 1.5 volts and stored a charge of one Coulomb than it would perform 1.5 Joule of work. In practical batteries the internal resistance will increase as it is discharged, and the open circuit voltage will also decrease as the cell is discharged. If the voltage and resistance are plotted against time the resulting graphs will typically not be a straight line, and the shape of the curve will vary with the chemistry and internal arrangement employed.

The voltage developed across a cell's terminals depends on the chemicals used in it and their respective concentrations. For example, alkaline and carbon-zinc cells both measure approximately 1.5 volts, due to the energy release of the associated chemical reactions. Because of the high electrochemical potential changes in the reactions of lithium compounds, lithium cells can provide as much as 3 volts or more.

From http://en.wikipedia.org/wiki/Battery_(electricity)#How_batteries_work