How 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 
and 
, then the net emf is 
. (Hence, two identical electrodes and a common electrolyte give a zero net emf.)
The electrical potential difference, or 
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 always have a terminal voltage of 
. This means that to produce a potential difference of 1.5 V, chemical reactions inside would perform 1.5 J of work for a charge of 1 C.
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.
