How do Batteries Work?
This is the number one question we receive from people in the battery industry. We will try to keep it simple for now and ellaborate on the subject a little more down the road. So let us start of with a basic understanding of batteries. A battery is a device that converts chemical energy directly to electrical energy.
Batteries 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, 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 zero net emf.)
The electrical potential difference, or across the terminals of a battery is known as its 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 do 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 concentrations. For example, alkaline and carbon-zinc cells both measure about 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.