Nickel Cadmium Batteries   NiCd

During years, the nickel-cadmium battery have proved to be a remarkable device.  Nickel-cadmium batteries may be recharged many times and have a relatively constant potential during discharge. They will stand more electrical and physical abuse than any other cell, have good low temperature performance characteristics, and are more than competitive with other systems in terms of cost per hour of use. They are true storage batteries using one of the very best electrochemical systems.

Sealed Nickel-cadmium Cells

The nickel-cadmium cell has been used in Europe for many years in its original form, as a vented or unsealed cell. Technological advances have made possible the extension of the nickel-cadmium system to small hermetically sealed batteries-rechargeable batteries that are free of the usual routine maintenance, such as the addition of water. These developments have brought the economic advantages of rechargeability to small batteries. Most sealed nickel-cadmium cells can be recharged many times to give long useful life, and are not adversely affected by standing many months, either charged or discharged. These high quality batteries, when used within their recommended ratings and in applications where the use of rechargeable cells is justified, will provide economical, trouble-free service. New portable devices requiring more energy than is economically available from ordinary primary batteries are practical with this complete line of rechargeable batteries.

Applications

Sealed nickel-cadmium batteries are ideally suited for use in many types of battery-operated equipment. Some of the many applications are listed here: calculators, cassette players and recorders, digital cameras, pagers, photoflash equipment, portable communications equipment, portable hand tools, portable computers, radios, shavers, tape recorders, television sets.

Operation of the Sealed Nickel-Cadmium Battery

Any secondary cell is a combination of active materials which can be electrolytically 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. 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 overchargeable 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: the battery is constructed with excess capacity in the cadmium electrode. Starting with both electrodes fully discharged, charging the battery causes the positive electrode to reach full charge first and it starts oxygen generation. Since the negative (cadmium) electrode has not reached full charge hydrogen will not be generated. The cell is designed so that the oxygen formed in the positive electrode can reach the metallic cadmium surface of the negative electrode which it oxidizes directly. Thus, in overcharge, the cadmium electrode is oxidized at a rate just sufficient to offset input energy, keeping the cell in equilibrium indefinitely. At this point of equilibrium the positive electrode is fully charged and the negative is somewhat less than fully charged. Polarity reversal: When cells are connected in series and discharged completely, small cell capacity differences will cause one cell to reach complete discharge sooner than the remainder. The cell which reaches full discharge first will be driven into reverse by the others. When this happens in an ordinary nickel-cadmium sealed cell, oxygen will be evolved at the cadmium electrode and hydrogen at the nickel electrode. Gas pressure will increase as long as current is driven through the cell and eventually it will either vent or burst. This condition is prevented in some sealed nickel-cadmium cells by special construction features. These include the use of a reducible material in the positive in addition to the nickel hydroxide, to suppress hydrogen evolution when the positive expires. If cadmium oxide is used it is possible to prevent hydrogen formation and to react the oxygen formed at the negative by same basic process used to regulate pressure during overcharge. A cell is considered electrochemically protected against reversal of polarity if, after discharge at the 10 hour rate down to 1.1 volts, it may receive an additional 5 hour discharge with the same current without being damaged or otherwise affected.

Capacity

The capacity rating of  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.

Paralleling of Cells

Sealed nickel-cadmium cells should not be charged in parallel unless each cell or series string of the parallel circuit has its own current limiting resistor. Minor differences in internal resistance of the cells may result, after cycling, in extreme variation in their states of charge. This may lead to overcharge at excessive currents in some cells and undercharge in other cells.

Voltage Characteristics

Except in the case of complete discharge, neither cell condition nor state of charge can be determined by open circuit voltage. Within a short while after charging it may be above 1.4 volts. It will fall shortly thereafter to 1.35V and continue to drop as the cell loses charge. During discharge, the average voltage of a sealed nickel-cadmium battery is approximately 1.2 volts per cell. At normal discharge rates the characteristic is very nearly flat until the cell approaches complete discharge. The battery provides most of its energy above 1.0 volt per cell. If the cell is discharged with currents exceeding the rated value, however, the voltage characteristic will have more of a slope, a lower endpoint voltage will be necessary and the ampere hours per cycle will be reduced.

High Current Pulse Discharge

High rate nickel-cadmium cells will deliver exceedingly high currents. If they are discharge continuously under short circuit conditions, self-heating may do irreparable damage. The heat problems vary somewhat from one cell type to another, but in most cases internal metal strip tab connectors overheat or the electrolyte boils. In some instances both events occur. General overheating is normally easy to prevent because the outside temperature of the battery can be used to indicate when rest, for cooling, is required. In terms of cutoff temperature during discharge, it is acceptable practice to keep the battery always below 45oC (113oF). The overheated internal connectors are difficult to detect. This form of overheating takes place in a few seconds or less, and overall cell temperature may hardly be affected. It is thus advisable to withdraw no more ampere seconds per pulse, and to withdraw it at no greater average current per complete discharge, than recommended on the data sheet for the cell in question. In special cases, where cooling of the cell or battery is likely to be poor, or unusually good, special tests should be run to check the important temperatures before any duty cycle adjustment is made. Output capacity is any discharge composed of pulses is difficult to predict accurately because there are infinite combinations of current, "on" time, rest time, and end point voltage. Testing on a specific cycle is the simplest way to get a positive answer.

Self-Discharge

The discharge capabilities of nickel-cadmium batteries are shown as a decline in percent of rated capacity available. Self-discharge is increased by elevated temperatures. Batteries are not harmed even if not used for long periods of time.

Continuous Overcharge

The overcharge capability of cylindrical nickel-cadmium cells is outstanding.

Memory Effect

Memory effect is that characteristic attributed to nickel-cadmium cells wherein the cell retains the characteristics of the previous cycling. That is, after repeated shallow depth discharges the cell will fail to provide a satisfactory full depth discharge. Cylindrical nickel-cadmium cells are particularly excellent with regard to lack of memory effect. You will note that the subsequent full depth discharges yield nearly equal capacity to the initial curve at slightly reduced voltage levels.

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.

Temperature Characteristics

Sealed nickel-cadmium cells experience a relatively small change of output capacity over a wide range of operating temperature. Charging, however, must be done in a much narrower range. Temperature limits applicable to operation of the cells are listed in the specification sheets for each battery. The capacity vs. temperature curves which are on some individual specification sheets represent cells discharged at the temperatures shown after charging at room temperature for 14 hours at the 10 hour rate. Charging nickel cadmium cells below the recommended temperature can cause oxygen pressure build up and activation of the resealable safety vent. Multiple vent activations will reduce cell capacity. Effect of high and low temperatures on storage, discharging and charging of Nickel-Cadmium cells and batteries.

Temperature effect  (all types)

a. High Temperature

Storage

a. at -40°C (-40°F) No detrimental effect. However, cells or batteries should be allowed to return to room temperature prior to   charging.
b. at 60°C (140°F) No detrimental effect. However, self-discharge is more rapid starting at 32°C (90°F) and increases as          temperature is further elevated. 

Discharge 

a. at -4C (-4°F) No detrimental effect but capacity will be reduced.
b. at 45°C (113°F) No detrimental effect.

Charge

a. (7 -10 hour rate) -- Cells or batteries should not be charged below 0°C (32°F)
b. (7 -10 hour rate) -- Cells or batteries show evidence charge acceptance of approximately 50%.
c. (1 to 3 hour rate 60°F) -- Cells or batteries should not be charged below 15°C (60°F) at the 1 hour rate or below 10°C (50°F) at the 3 hour rate.
d. (1 to 3 hour rate (113°F) -- Cells or batteries evidence charge acceptance of approximately 90%.

Impedance and Internal Resistance

Sealed nickel-cadmium cells have a high effective capacitance. Their impedance is so low that cells which, in effect, are being continuously overcharged, make excellent ripple filters. Cell impedance is dependent upon frequency and state of charge of the cell. It is lower for a charged cell than it is for a discharged cell. 

Charging

Constant current charging is recommended for sealed nickel-cadmium cells. The 10 hour rate should not be exceeded unless overcharge is specifically to be prevented. The recharge efficiency of sealed nickel-cadmium cell is dependent on a number of things, but it is most important to remember that charging becomes more difficult as temperature increases and charge rate decreases. It is possible, under certain conditions, to charge at rates much higher than the 10 hour rate, but control devices which prevent high rate over-charge are sometimes required. The nickel-cadmium battery can be trickle charged but floating and constant voltage charging are not recommended. For maximum performance in situations of long term trickle charge current required to keep the battery fully charged is approximately the 30-50 hour rate plus whatever is necessary to compensate for any major withdrawals. 

Warnings

Any treatment which causes a cell to vent is harmful. Frequent or extended venting of even properly valved cells eventually destroys them. In rating cycle life, end of life of the sealed nickel-cadmium cell is considered to be when it no longer provides 80% of its rated capacity. If a cell can be considered to be satisfactory while delivering less than the 80% endpoint figure, cycle life will be greater than that listed. The ratings are for 21°C (70°F) performance.

Nickel Cadmium conversion table

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