Electrochemistry and batteries
You will remember from chapter
[link] that a
galvanic cell (also known as a
voltaic cell) is a type of electrochemical cell where a chemical reaction produces electrical energy. The
electromotive force (emf) of a galvanic cell is the difference in voltage between the two half cells that make it up. Galvanic cells have a number of applications, but one of the most important is their use in
batteries . You will know from your own experience that we use batteries in a number of ways, including cars, torches, sound systems and cellphones to name just a few.
How batteries work
A battery is a device in which
chemical energy is directly converted to
electrical energy . It consists of one or more voltaic cells, each of which is made up of two half cells that are connected in series by a conductive electrolyte. The voltaic cells are connected in series in a battery. Each cell has a positive electrode (cathode), and a negative electrode (anode). These do not touch each other but are immersed in a solid or liquid electrolyte.
Each half cell has a net electromotive force (emf) or voltage. The voltage of the battery is the difference between the voltages of the half-cells. This potential difference between the two half cells is what causes an electric current to flow.
Batteries are usually divided into two broad classes:
-
Primary batteries irreversibly transform chemical energy to electrical energy. Once the supply of reactants has been used up, the battery can't be used any more.
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Secondary batteries can be recharged, in other words, their chemical reactions can be reversed if electrical energy is supplied to the cell. Through this process, the cell returns to its original state. Secondary batteries can't be recharged forever because there is a gradual loss of the active materials and electrolyte. Internal corrosion can also take place.
Battery capacity and energy
The
capacity of a battery, in other words its ability to produce an electric charge, depends on a number of factors. These include:
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Chemical reactions The chemical reactions that take place in each of a battery's half cells will affect the voltage across the cell, and therefore also its capacity. For example, nickel-cadmium (NiCd) cells measure about 1.2 V, and alkaline and carbon-zinc cells both measure about 1.5 V. However, in other cells such as Lithium cells, the changes in electrochemical potential are much higher because of the reactions of lithium compounds, and so lithium cells can produce as much as 3 volts or more. The concentration of the chemicals that are involved will also affect a battery's capacity. The higher the concentration of the chemicals, the greater the capacity of the battery.
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Quantity of electrolyte and electrode material in cell The greater the amount of electrolyte in the cell, the greater its capacity. In other words, even if the chemistry in two cells is the same, a larger cell will have a greater capacity than a small one. Also, the greater the surface area of the electrodes, the greater will be the capacity of the cell.
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Discharge conditions A unit called an
Ampere hour (Ah) is used to describe how long a battery will last. An ampere hour (more commonly known as an
amp hour ) is the amount of electric charge that is transferred by a current of one ampere for one hour. Battery manufacturers use a standard method to rate their batteries. So, for example, a 100 Ah battery will provide a current of 5 A for a period of 20 hours at room temperature. The capacity of the battery will depend on the rate at which it is discharged or used. If a 100 Ah battery is discharged at 50 A (instead of 5 A), the capacity will be
lower than expected and the battery will run out
before the expected 2 hours.
The relationship between the current, discharge time and capacity of a battery is expressed by
Peukert's law :
In the equation, 'C
' represents the battery's capacity (Ah), I is the discharge current (A), k is the Peukert constant and t is the time of discharge (hours).
