Alkaline Batteries

There are several types of alkaline batteries, so to differentiate the
types, the metal used in the manufacture of the plates usually gives
them their name, for example the most common in use on aircraft is
the Nickel/Cadmium battery as it is these 2 metals that are used for
the plates. However in all alkaline batteries the electrolyte solution
used is Potassium Hydroxide which has a specific gravity of 1.24 to
1.30, dependant on the size and material used for the metal plates
this will determine the value of EMF of the cell, with some cells being
as low as 1.2volts.
Semi Sealed
The cells in these batteries are arranged in steel containers and fitted
with safety valves, this is due to the fact that the cells can be charged
at a high rate – and so can be easily overcharged, this overcharge will
make the electrolyte “bubble” gas and create a large amount of heat,
which can if not dealt with cause the battery to go into a state of
thermal meltdown, Under normal conditions these batteries require
very little maintenance and only a periodic capacity check.
Semi Open
These type of batteries are constructed similar to the semi sealed type,
but when on charge they are deliberately allowed to “gas”, this gassing
is allowed to vent to atmosphere, this aids the temperature control of
the cells and indeed is used to monitor the battery charge by switching
the battery charger on or off dependant on the temperature reached by
the cell.
Due to the cells being allowed to gas during charge the recharge time
is considerably shorter than the semi sealed type however this has the
disadvantage that the electrolyte requires “topping up” more often, the
frequency of this topping up will be determined by the aircraft manual.
The cells are fitted with the same type of vent as the lead acid battery
in that they allow the electrolyte gasses to escape but not the liquid.

Nickel Cadmium Alkaline
This is a common type of alkaline battery; the plates are formed on
a woven wire mesh by heating, the cathode plate is formed by using
Nickel salts and the anode by using Cadmium salts, a separator of
nylon cloth and a gas barrier of cellophane is used between them,
the electrolyte has a specific gravity of 1.3 and consists of
Potassium Hydroxide and distilled water.

Chemical Reaction
The chemical reaction of alkaline batteries works on the same principle
as the lead acid battery:However as can be seen, the difference is that the electrolyte does not
change its specific gravity or chemical structure during
charge/discharge, it is used purely as a medium in which the electrons
can flow through between the plates. As the charge of the battery
cannot be determined from the electrolyte specific gravity as in the lead
acid type, the temperature of the cell has to be monitored to determine
the charge state, for this reason, switching on or off the charge supply
to maintain the correct ampere/hour rating

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