R15 A lead-acid accumulator

An accumulator, or in more general terms a battery, is a classic example of the application of electrochemical reactions. It is an electrochemical cell, which accumulates or stores electric charge. It consists of chemical components, which function as the anode and others which function as the cathode, connected together by an electrolyte solution or an electrolyte paste and a porous barrier or salt bridge between the anode and cathode cells.

If a resistance, e.g. an engine, or a lamp is placed between the anode and cathode, chemical energy will be converted into electrical energy in a self-sustaining electron transfer reaction. An accumulator is a chemical source of electricity.

Batteries and accumulators are very much part of the modern way of life. They vary in size from minute button batteries, such as are found in watches, to large heavy blocks, that function as emergency electricity sources for hospitals, ships and vehicles. They are usually made for a specific use and the lead-acid accumulator is widely used as an electrochemical source of electricity in cars and lorries. The construction and operation of a lead-acid battery is shown in illustration R15.

Construction of a lead-acid battery

It consists of a polypropene container, containing six lead-acid electrochemical cells, each giving an e.m.f. of 2 V (2.05 V), connected in series. While the voltage of a lead-acid battery is always a multiple of 2 V, its power can vary considerably. It can be increased by dividing each cell with microporous polyethene dividers. All the anode and cathode plates within a single cell are linked together, the anode of one cell being linked with the anode of the next cell etc. and the cathodes of the cells being likewise linked together.

The anodes are all made of lead and the cathodes all of solid lead(IV) oxide or lead dioxide, which is mounted on a lead base. Both poles of the battery are also made of lead. The electrolyte is a 80% solution of sulphuric acid saturated with PbSO₄.

Electrochemistry of a lead-acid battery

Discharging :
During discharge the electrode reactions to be taken into account are, at the negative pole (now the anode)

at the positive pole (now th ecathode)

The battery then functions as an electrochemical cell.
Electric current is produced by the self-sustaining redox reaction between the top right substances with the bottom left substances :

Each compartment in the battery corresponds to a cell voltage

as long as concentrations and temperature are standard.

In use, lead-acid batteries gradually become less efficient. The main redox reaction (but also other processes) consume sulphuric acid. Since the mass density of the electrolytic solutions increases with concentration of H₂SO₄, any efficiency loss will result in a decrease in the density of the battery acid. By measuring the mass density (specific gravity) of the electrolyte solutions, the efficiency of the battery can be checked.

Charging:
A lead-acid battery has a reasonable lifetime due to a possible reversion of the redox reactions.

The reversed reactions

can regenerate the substances used during the discharging process. Charging is only possible by continuing applications of a potential from an outside power source, e.g. a car’s alternator or generator, or a charger plugged into the mains.
As long as current is supplied to the battery, Pb²⁺ ions are reduced to lead metal (at the lead electrode) while, at the PbO₂ electrode, Pb²⁺ ions are oxidised to PbO₂. Pb and PbO₂ are deposited on the electrodes.
The half-cell reactions during charging and discharging are exactly the same. The charging process however involves the weaker oxidising agent (top left) and the weaker reducing agent (bottom right) :