Battery

A battery turns chemical reactions at two electrodes, separated by an electrolyte that conducts ions, into a steady electric current. Alessandro Volta's pile of zinc, copper and brine was the first in 1800, and every cell since, from the telegraph jar to the lithium phone battery, works on the same principle.

What it is

A battery stores energy as chemistry and gives it back as electric current when a circuit with a lamp or other load joins its two terminals. The basic unit is a cell. In Volta’s time a battery meant several cells joined together, like the column he stacked in 1800; today the word covers single cells too.

Before the pile, electricity came only as brief sparks; the pile gave a current that kept flowing for hours.

How it works

Each cell of the pile has a zinc disc, a copper disc, and between them a disc of cardboard or cloth soaked in salt water. Zinc gives up electrons more easily than copper. On the zinc side, metal atoms pass into the brine as ions and leave their electrons in the disc. On the copper side, a reaction takes electrons up again, using oxygen dissolved in the brine or releasing hydrogen. When an external circuit joins the two ends, electrons flow through it from zinc to copper, and ions moving through the wet layer close the circuit.

In the Daniell cell, copper is deposited on the vessel instead of hydrogen, so the voltage stays steady.

One zinc-copper pair in brine gives a little under one volt. Stack twenty pairs in series and the voltages add up, enough to feel a sharp tingle through wet fingers, which is how Volta tested his piles. The current depends on the area of the discs and on how fast the reaction can run.

As the pile works, the dissolved oxygen near the copper runs out and hydrogen bubbles begin to cover it, so the current falls. The weight of the column also squeezes brine out of the lower layers, and the zinc corrodes. In 1836 John Frederic Daniell gave each metal its own liquid: zinc in one solution, copper in copper sulfate, with a porous barrier between them. Copper is plated out instead of hydrogen, and the cell holds about 1.1 volts for a long time.

In a rechargeable cell, current pushed in the opposite direction drives the reaction backwards.

How to build it

A small pile gives a low voltage and a weak current, but it shows the principle clearly.

You need fifteen to twenty copper discs (old copper coins will do), as many zinc discs (zinc sheet or galvanised washers), blotting paper or cardboard cut slightly smaller, salt, water, two insulated wires and, ideally, a voltmeter.

Clean the metal. Rub each copper disc with fine sandpaper until it is bright.

Soak the separators. Dissolve two heaped spoonfuls of salt in a glass of water and soak the cardboard. Press each disc until it is damp but no longer dripping. Brine running down the column short-circuits the cells.

Stack in one order. Lay a copper disc, a wet separator and a zinc disc, then repeat in the same order to the top. Never reverse the order halfway, and never let the two metals of one cell touch around the edge of the separator.

Hold the column. Three wooden or glass rods set in a base keep the stack straight without pressing on it.

Connect and test. Fix one wire to the bottom copper disc and one to the top zinc disc. Expect a little under one volt per cell. A red LED needs about two volts and very little current: check that its long leg goes to the copper end and its short leg to the zinc end, and test it on four or five cells only, where it should glow faintly. Connected backwards across more cells, an LED can be damaged. When the separators dry out after a few hours, take the pile apart, then wash and dry the discs.

Safety. The pile itself is low voltage, but a short circuit across a strong battery heats wires enough to burn skin and melt insulation, so never join the terminals directly. Never connect any battery to mains electricity or try to charge it from a wall socket. Do not open commercial batteries: lead-acid batteries hold sulfuric acid, which burns skin and eyes, and give off explosive hydrogen while charging; lithium cells can catch fire if punctured, crushed or shorted. Copper sulfate and zinc salts are poisonous: wear gloves and goggles, wash your hands, and keep the materials away from children.

History and evolution

Galvani’s frog legs twitched whenever an arc of two metals joined nerve and muscle.

In the 1780s the Bologna anatomist Luigi Galvani saw dissected frog legs twitch when they touched two different metals at once. In 1791 he published his theory of “animal electricity”. Alessandro Volta, professor of physics at Pavia, held that the metals produced the electricity and the frog only detected it, and he set out to show it with no animal at all.

He first linked cups of brine with metal strips, then built the more compact pile. On 20 March 1800 he described both in a letter, written in French, to Sir Joseph Banks, president of the Royal Society in London. The Society printed it that year with an engraving of the pile, the first published picture of a battery.

Within weeks William Nicholson and Anthony Carlisle used a pile to split water into hydrogen and oxygen. A few years later Humphry Davy used large batteries to isolate sodium and potassium. Napoleon watched Volta demonstrate the pile in Paris in 1801 and rewarded him, and the unit of electrical potential, the volt, is named after him.

Planté’s spiral of lead sheets in dilute acid could be charged and discharged again and again.

Telegraph lines needed a steady current, and Daniell’s cell supplied it. In 1859 Gaston Planté rolled two lead sheets into a spiral with a separator between them and immersed them in dilute sulfuric acid. Johann Ritter had shown an experimental storage pile in 1803, but Planté’s was the first practical rechargeable battery. Improved by Camille Faure in 1881, the lead-acid battery still starts most cars.

In 1866 Georges Leclanché built a cell from a zinc rod, a carbon rod packed in manganese dioxide, and a solution of ammonium chloride; by 1868 twenty thousand were working on telegraph systems. Later inventors thickened the liquid into a paste, and this dry cell powered early pocket torches and the radios of the 1920s. The first commercial cylindrical alkaline cells followed in 1959, and lithium-ion cells reached the market in the early 1990s.