Before you read this article it may be worth reading How a battery is made so you can get used to the terms used in describing how a battery works.

How does a dry cell work?

This is a basic guide to how a standard (dry cell) battery works, for more information on the different battery technologies, see the articles below; Fuel cells, Wet cells.

When you connect a wire between the positive and negative terminals of a battery (the same happens when you put them into your walkman and press play), it starts two simultaneous chemical reactions; one at the positive electrode (called the cathode), and another at the negative electrode (the anode). During this discharge, the chemical reactions cause negatively charged electrons to collect at one electrode (the cathode), and positively charged ions collecting at the other (the anode). Once properly connected to a circuit, electrons flow from the cathode, to the anode. These electrons then pass through the circuit and then back to the cathode and re-enter the electrolyte. This may seem strange as we generally regard current flow from positive to negative.

The reason that batteries 'run down' is that the anode and cathode both get converted during this reaction, one is 'eaten away', and the other has a build-up of material on it, meaning that the chemical reaction going on inside runs out of raw materials and cannot produce any further electrical power.

Rechargeable Batteries

With rechargeable batteries the chemicals used in the batteries allow for this process to be reversed. I.e. when you put the battery on charge the current flowing back into the battery restores the anode and cathode to their initial state. However over time, they still deteriorate and there becomes a point where they cannot be restored and the battery will cease to function.

How do Fuel Cells work?

The hydrogen fuel cell operates similar to standard battery. It has two electrodes, an anode and a cathode, separated by a membrane. Oxygen passes over one electrode and hydrogen over the other. The hydrogen reacts to a catalyst on the electrode anode that converts the hydrogen gas into negatively charged electrons and positively charged ions. The electrons flow out of the cell to be used as electrical energy. The hydrogen ions move through the electrolyte membrane to the cathode electrode where they combine with oxygen and the electrons to produce water. Fuel cells never run out, as long as they have a supply of oxygen and hydrogen they will continue to function indefinitely. Limitations in power and safety issues are the main concerns in the research and development of fuel cells, however we may see a large increase in the number of Fuel cell powered vehicles in the next 10 - 20 years.

See also The History of the Fuel Cell

How do Wet Cells work?

The most common wet cell today is the Lead-Acid battery used mainly in motor vehicles. Contained in a resilient Plastic container is an electrolyte, a dilute solution of sulphuric acid with lead plates submerged in the electrolyte solution. The positive plates are formed from lead dioxide and the negative electrodes are made of sponge metallic lead separated by a porous synthetic material. The terminals are also lead providing the connection between the battery and whatever it is to power.

Lead acid batteries are in a constant process of charge or discharge; during the discharge process, the lead plates react with the electrolyte to form lead sulphate and water, this chemical reaction allows for electrons to flow through the electrolyte and provide an electrical current to power the device. When the battery gets recharged, the lead sulfate on both electrodes converts back to lead dioxide and the sulfate ions are driven back into the electrolyte solution to form sulphuric acid. The reason these batteries have a finite life is because over time, sulfate residue builds up on the lead plates and begin to crystallise. As this builds up, the sulfate becomes locked in the crystallised residue increasing the batteries internal resistance and dramatically reducing its capacity causing the battery to fail.

See also The History of the Wet Cell