How batteries work
A battery is a device in which chemical energy is directly converted to electrical energy. It consists of one or more voltaic cells, each of which is composed of two half cells connected in series by the conductive electrolyte. A battery consists of one or more voltaic cells in series. Each cell has a positive electrode (cathode), and a negative electrode (anode). These do not touch each other but are immersed in a solid or liquid electrolyte. In a practical cell the materials are enclosed in a container, and a separator between the electrodes prevents them from touching.
Each half cell has a net electromotive force (or voltage), with the net voltage of the battery being the difference between the voltages of the half-cells. The electrical potential across the terminals of a battery is known as its terminal voltage, measured in volts. The terminal voltage of a battery that is neither charging nor discharging (the open-circuit voltage) equals its electromotive force (EMF). The terminal voltage of a battery that is discharging is less than the EMF, and that of a battery that is charging is greater than the EMF.
The voltage produced by a cell depends on the chemicals used in it, which have different electrochemical potentials. For example, alkaline and carbon-zinc cells both have EMFs of about 1.5 volts, due to the energy release of the associated chemical reactions. Because of the high electrochemical potentials of lithium compounds, lithium cells can provide as much as 3 or more volts.