How batteries supply portable electricity to laptops
Experiments in anatomy in the 1780s led to the invention of the battery. Luigi Galvani, a professor of anatomy at Bologna University, noticed that the legs of dead frogs twitched when they were hung from hooks on a rail. He thought (wrongly) that it was because of some kind of animal electricity
Another Italian professor, Allessandro Volta of Pavia University, realised that the electricity resulted from the contact between the copper hooks and iron rail the frogs were hung on - their legs were merely part of the circuit. As a result he produced a Voltaic cell in 1800, the forerunner of all modern batteries.
It is called a battery because some types - such as a car battery - are in fact batteries of single cells. Although it is convenient, a battery wastes a lot of the Earth's energy: the amount of energy needed to make one is up to 50 times greater than the amount it produces.
Electric current is produced in a battery cell by the reactions of two electrodes (electrical conductors) with an electrolyte (a liquid or paste that conducts electricity). Each electrode is linked to one of the battery's metal terminals - the parts where it clips into a circuit. Once the battery is linked into the circuit, there will be a continual flow of electrons from one terminal (known as the negative) through the circuit to the other (known as the positive).
This is because the material of one of the electrodes starts to partly dissolve in the electrolyte - that is, its atoms start to break up and it releases positive ions into the electrolyte and electrons into the wire of the circuit at the negative terminal.
The other electrode is generally made of different material and does not dissolve in the electrolyte to the same extent. But it does lose electrons to positive ions in the electrolyte, and becomes deficient in them.
So the continual flow of electrons from one electrode through the circuit to make up the deficiency at the other forms the current.
The cylindrical, single-cell battery of the type used in a torch is called a dry cell battery because the electrolyte is self-contained and needs no topping up. A common type is the acidic zinc-carbon battery.
The battery's metal casing is a zinc container that forms one electrode of the cell. It holds a mixture of ammonium chloride, which is the electrolyte, and manganese dioxide. The manganese is, in effect, the other electrode, because it loses electrons to the ammonium chloride. A central carbon rod acts as a current collector, transferring electrons from the positive terminal to the manganese.
Such a dry cell battery has an output of 1.5 volts when new, but this decreases with use, as hydrogen bubbles form on the carbon rod, reducing the electrode surface area. The manganese dioxide partially removes the bubbles, so minimises their effect.
Car batteries are storage batteries, also called accumulators, because they can be recharged - that is, the chemical reactions can be revived. Most have six linked cells, each with an output of about 2 volts.
Each cell has several electrodes, or plates, with negative and positive plates alternating. The plates are separated by insulating sheets to prevent short circuits, and are suspended in an electrolyte of sulphuric acid. All the plates are lead grids, the negative ones filled with spongy lead, the positive ones with lead dioxide.
The chemical reactions that create electricity result in both negative and positive plates turning gradually into lead sulphate, and the electrolyte into water. If this happens completely, the battery is flat. But once the car engine is running well, the current flow from its generator charges the battery by reversing the chemical reactions. So the lead plates are converted back to their original material and the strength of the sulphuric acid is restored.
While the car is running, its battery builds up a sufficient store of current to restart the engine next time it is used.
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