CHAPTER 5. THE CHEMICAL INDUSTRY 5.5
- Chemical reactions
The chemical reactionsthat 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, andalkaline and carbon-zinc cells
both measure about 1.5V. However, in other cells such as Lithium cells, the
changes in electrochemical potential are much higher because of the reactions of
lithium compounds, andso lithium cells can produce as much as 3 volts or more.
The concentration of the chemicals that are involved will also affect abattery’s
capacity. The higher theconcentration of the chemicals, the greater the capacity
of the battery. - Quantity of electrolyteand electrode materialin cell
The greater the amountof electrolyte in the cell, the greater its capacity.In other
words, even if the chemistry in two cells is thesame, a larger cell willhave
a greater capacity thana small one. Also, thegreater the surface areaof the
electrodes, the greater will be the capacity of thecell. - 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 hoursat room
temperature. The capacity of the battery will depend on the rate at which it is
discharged or used. If a100 Ah battery is discharged at 50 A (instead of 5 A),
the capacity will be lower than expected and thebattery will run out before the
expected 2 hours.
The relationship between the current, dischargetime and capacity of a battery is
expressed by Peukert’s law:Cp= IktIn the equation, ’Cp’ represents the battery’s capacity (Ah), I is the discharge
current (A), k is the Peukert constant and t is thetime of discharge (hours).Lead-acid batteries ESCCT
In a lead-acid battery, each cell consists of electrodes of lead (Pb) and lead (IV) oxide
(PbO 2 ) in an electrolyte of sulfuric acid (H 2 SO 4 ). When the battery discharges, both
electrodes turn into lead(II) sulphate (PbSO 4 ) and the electrolyte loses sulfuric acid to
become mostly water.
The chemical half reactions that take place at the anode and cathode when the battery
is discharging are as follows: