Page 238 - Battery Reference Book

19/4 Nickel batteries

On discharge (Figure 19.1), not immersed in a pool of electrolyte as they are in
a flooded cell.
discharge Oxygen gas must be free to pass between the
ZNiO(0H) + Cd + 2H20 T--- 2Ni(OH), + Cd(OH), electrodes. There must be sufficient open area in
charge
Positive Negative Positive Negative and around the separator and electrolyte for oxygen
electrode electrode electrode electrode to be able to diffuse from the positive to the
(19.1) negative electrode.
Two parts of the charged positive mass of nickel(II1) charging a sealed cell the positive electrode will
hydroxide change to nickel(I1) hydroxide, while the reach full charge before the negative electrode. At this
negative mass of cadmium oxidizes to cadmium stage additional charging causes the positive electrode
hydroxide, and two molecules of water are taken potential to rise until all the incoming current is oxi-
from the electrolyte. The above equation holds dizing hydroxyl ions and generating oxygen gas at the
good on charge or discharge only so long as positive electrode:
convertible material is still available. If, for instance,
on charge the positive mass of nickel(I1) hydroxide 20H- + $02 + H20 + 2e
in the positive electrode is completely converted to
nickel(II1) hydroxide, then the reaction is modified The oxygen generated at the positive electrode diffuses
and oxygen is evolved; that is, the plate is in an rapidly to the negative electrode where it is reduced
overcharged state: back to hydroxyl ions:
overcharge $02 + H20 + 2e- + 20H-
Cd(OH), 402 + Cd + HzO (19.2)
overdischarge The hydroxyl ions complete the circuit by migrating
If the same condition is applied to the negative plate, back to the positive electrode. Thus, in overcharge all
hydrogen gas will be evolved. With an open battery, the current generates oxygen which is subsequently
both conditions will occur at the end of every charge recombined. The oxygen pressure initially increases
and water will be decomposed. but then stabilizes at a low equilibrium pressure deter-
Note that the electrolyte potassium hydroxide is not mined by cell design, the ambient temperature and
mentioned in Equations 19.1 and 19.2. the charge rate. In this equilibrium state the generated
Water is produced when the battery is charging and oxygen is recombined at the negative electrode, the
taken up when it is discharging; when the electrolyte negative electrode never becomes fully charged and
is in the correct proportion, this water has very little no hydrogen gas is generated.
effect on the operation of the cell. In comparison, on The efficient recombination of oxygen enables the
discharge, if one of the electrodes becomes exhausted sealed cell to be continuously overcharged at specific
and the discharge current is maintained by the voltage rates without developing excessive internal pressure.
of the other cells, then hydrogen is given off by the It can thus be kept on trickle charge, maintaining the
positive plate and oxygen by the negative plate. Also, cell in a fully charged condition for long periods.
in a discharged condition, the battery polarity can be It is important to remember that during charge the
reversed. conversion of active material is an exothermic reaction.
When the cell is in overcharge, essentially all of the
19.1.1 Sealed nickel-cadmium batteries energy in the current coming in is converted to heat.
Sealed nickel-cadmium cells normally operate at With proper matching of the charger to the battery
internal pressures well below the vent pressure because and attention to battery location and heat dissipation,
gas evolved during charging is reasily recombined. the battery system can be designed to reach a thermal
Compliance with three essential design criteria is steady state that will permit the battery to remain in
necessary to accomplish this: overcharge indefinitely.
Figure 19.2 shows the electrode design of a typical
1. The capacity of the negative electrode must be sealed nickel-cadmium cell. When the cell is fully
greater than that of the positive electrode, i.e. the charged, the positive electrode active material has
negative electrode must have more active material been converted to the high valence form, NiOOH,
available than the positive. As a result, the posit- but the negative electrode still contains uncharged
ive electrode achieves full charge and emits oxygen active material Cd(OH),. In overcharge, oxygen gas
before the negative electrode is fully charged and is evolved from the positive electrode.
emits hydrogen which cannot be readily ‘recombined. In the modern sealed nickel-cadmium battery the
2. The electrode must be uniformly distributed by the active mass is modified so that the battery is ‘safe from
separator as a thin film across the surface of the two overcharge’ and protected against ‘overdischarge’ due
electrodes. The quantity of electrolyte used in the to oxygen only being evolved. In the case of over-
cell must be only enough to wet the plate surface charge this is achieved by the negative plate having
or be absorbed in the separator. The electrodes are a surplus amount of cadmium hydroxide incorporated