discharge
2 NiOOH + 2H
2
O + Cd
2 Ni(OH)
2
+ Cd(OH)
2
charge
The nickel-cadmium battery uses
nickel hydroxide as the active
material for the positive plate, and
cadmium hydroxide for the negative
plate.
The electrolyte is an aqueous solution
of potassium hydroxide containing
small quantities of lithium hydroxide
to improve cycle life and high
temperature operation.
The electrolyte is only used for ion
transfer; it is not chemically changed
or degraded during the charge/
discharge cycle. In the case of the
lead acid battery, the positive and
negative active materials chemically
react with the sulphuric acid
electrolyte resulting in an ageing
process.
The support structure of both plates
is steel. This is unaffected by the
electrochemistry, and retains its
characteristics throughout the life of
the cell. In the case of the lead acid
battery, the basic structure of both
plates are lead and lead oxide which
play a part in the electrochemistry
of the process and are naturally
corroded during the life of the battery.
The charge/discharge reaction is as
follows:
During discharge the trivalent nickel
hydroxide is reduced to divalent
nickel hydroxide, and the cadmium
at the negative plate forms cadmium
hydroxide.
On charge, the reverse reaction takes
place until the cell potential rises to
a level where hydrogen is evolved at
the negative plate and oxygen at the
positive plate which results in water
loss.
Unlike the lead acid battery, there is
little change in the electrolyte density
during charge and discharge. This
allows large reserves of electrolyte
to be used without inconvenience
to the electrochemistry of the couple.
Thus, through its electrochemistry, the
nickel-cadmium battery has a more
stable behavior than the lead acid
battery, giving it a longer life,
superior characteristics and a greater
resistance against abusive conditions.
Nickel-cadmium cells have a nominal
voltage of 1.2 volts.
5
3.
Electrochemistry of nickel-cadmium
batteries
BlockBat 3/11/98 10:11 Page 8
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