Working Principle
Any secondary cell is a combination
of active materials which can be electrolytic oxidized and reduced repeatedly.
The oxidation of the negative electrode occurring simultaneously with the
reduction of the positive generates electric power. In a rechargeable battery
both electrode reactions are reversible and the input of current in the proper
direction from an outside source will drive the primary or discharge reaction
backwards and in effect recharge the electrodes.
In the uncharged condition the
positive electrode of a nickel-cadmium cell is nickelous hydroxide, the negative
cadmium hydroxide. In the charged condition the positive electrode is nickelic
hydroxide, the negative metallic cadmium. The electrolyte is potassium
hydroxide. The average operating voltage of the cell under normal discharge
conditions is about 1.2 volts. The over-all chemical reaction of the
nickel cadmium system can be considered as:
During
the latter part of a recommended charge cycle and during overcharge,
nickel-cadmium batteries generate gas. Oxygen is generated at the positive
(nickel) electrode after it becomes fully charged and hydrogen is formed at the
negative (cadmium) electrode when it reaches full charge. These gases must be
vented from the conventional nickel-cadmium system. In order for the system to be over chargeable while sealed, the evolution of hydrogen must be prevented and
provisions made for this reaction of oxygen within the cell container. These
things are accomplished by the following:
Construction:
Energizer nickel-cadmium
cells are available in cylindrical configuration and range in capacity up to 5
Amp hours in sizes from AAA to D.
Cylindrical Cells
This cell type incorporates a different electrode arrangement than the
button cell. Sintered plates are used in all cylindrical cells for the positive
electrode. This electrode consists of thin, highly porous nickel plaques
impregnated with active materials. The plaque is made by heating nickel powder
in an inert atmosphere until the particles are welded together. The metallic
phase serves as a highly conductive supporting structure for the electrode. The
structure of the plate is such that a large surface is furnished for reaction
of the active materials. With the sintered electrode it is possible to build
cells of very low internal resistance.
The negative electrode of most Energizer cylindrical cells is a pasted
electrode which consists of blended active materials pressed onto a metal
carrier. It is this electrode that gives Energizer cylindrical nickel-cadmium
cells outstanding cycle life, long term overcharge capability, with essentially
no fade and with little or no memory effect.
Sealed nickel-cadmium cells under certain abuse
conditions such as excessive charge or overcharge rate, deep discharge with subsequent polarity
reversal, may develop high internal gas pressure. Usually the gas is oxygen,
although hydrogen is also evolved in some cases. Either or both hydrogen and
oxygen must be vented. All Energizer high rate cylindrical cells have a
resealing pressure vent. This vent permits the cell to release excess gas
evolved if the cell, for example, is abused. When the internal pressure has
dropped to an acceptable level, the vent will reseal, permitting the cell to be
recycled in the normal manner with little or no further loss of electrolyte or
capacity. Repeated venting will reduce capacity and cycle life.
Contact Material
External electrical connections can be made with any good conductor
having adequate current handling capabilities.
Potting
Nickel-cadmium cells or batteries of any type should not be totally
potted. Energizer cells have resealable vent mechanisms which would be rendered
inoperative by the potting compound.
Capacity
The capacity rating of Energizer nickel-cadmium cells and batteries is
based upon output in discharge at the 1 hour rate to an endpoint of 1.0V/cell
for all cylindrical cells. If current is withdrawn at faster rates than these
standards, capacity is decreased.
Storage
At elevated storage temperatures self-discharge will be considerably
higher than at room temperature. It is recommended that batteries be stored at
21°C (70°F) or lower for this reason.
Application
- Calculators
- Cassette players and recorders
- Dictating machines
- Digital Cameras
- Instruments
- Personal Pagers
- Photoflash
equipment
- Portable communications equipment
- Portable hand tools and appliances
- Portable computers
- Radios
- Radio control models
- Shavers
- Tape recorders
- Television sets
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