Nickel-cadmium batteries have been available for some time, but they did not come into extensive use in aviation until the increase in the number of commercial and executive jet aircraft made them economically practicable. The many advantages of the nickel-cadmium battery were well known, but its initial cost was several times that of the lead-acid battery. The increasing use of the nickel-cadmium battery (often referred to as "ni-cad") stems largely from the low maintenance cost derived from the long service life of the battery. Additionally, the nickel-cadmium battery has a short recharge time, excellent reliability, and good starting capability.
Nickel-Cadmium Cell Construction
As in the lead-acid type, the cell is the basic unit of the nickel-cadmium battery. It consists of positive and negative plates, separators, electrolyte, cell vent, and cell container. The positive plates are made from a porous plaque on which nickel-hydroxide has been deposited. The negative plates are made from similar plaques on which cadmium-hydroxide is deposited. In both cases the porous plaque is obtained by sintering nickel powder to a fine mesh wire screen. Sintering is a process which fuses together extremely small granules of powder at a high temperature. After the active positive and negative materials are deposited on the plaque, it is formed and cut into the proper plate size. A nickel tab is then welded to a corner of each plate and the plates are assembled with the tabs welded to the proper terminals. The plates are separated from each other by a continuous strip of porous plastic.
The electrolyte used in the nickel-cadmium battery is a 30 percent solution (by weight) of potassium hydroxide (KOH) in distilled water. The specific gravity of the electrolyte remains between 1.240 and 1.300 at room temperature.
No appreciable changes occur in the electrolyte during charge or discharge. As a result, the battery charge cannot be determined by a specific gravity check of the electrolyte. The electrolyte level should be maintained just above the tops of the plates.
Operation of Nickel-Cadmium Cells
When a charging current is applied to a nickel-cadmium battery, the negative plates lose oxygen and begin forming metallic cadmium. The active material of the positive plates, nickel-hydroxide, becomes more highly oxidized. This process continues while the charging current is applied or until all the oxygen is removed from the negative plates and only cadmium remains.
Toward the end of the charging cycle the cells emit gas. This will also occur if the cells are overcharged. This gas is caused by decomposition of the water in the electrolyte into hydrogen at the negative plates and oxygen at the positive plates. The voltage used during charging, as well as the temperature, determines when gassing will occur. To completely charge a nickel-cadmium battery, some gassing, however slight, must take place; thus, some water will be used.
The chemical action is reversed during discharge. The positive plates slowly give up oxygen, which is regained by the negative plates. This process results in the conversion of the chemical energy into electrical energy. During discharge the plates absorb a quantity of the electrolyte. On recharge, the level of the electrolyte rises and, at full charge, the electrolyte will be at its highest level. Therefore, water should be added only when the battery is fully charged.
The nickel-cadmium battery is usually interchangeable with the lead-acid type. When replacing a lead-acid battery with a nickel-cadmium battery, the battery compartment must be clean, dry, and free of all traces of acid from the old battery. The compartment must be washed out and neutralized with ammonia or boric acid solution, allowed to dry thoroughly, and then painted with an alkali resisting varnish.
The pad in the battery sump jar should be saturated with a three percent (by weight) solution of boric acid and water before connecting the battery vent system.
Servicing Nickel-Cadmium Batteries
There are significant differences in the servicing methods required for the nickel-cadmium batteries and those of the lead-acid batteries. The most important points to be observed are as follows:
(1) A separate storage and maintenance area should be provided for nickel-cadmium batteries. The electrolyte is chemically opposite to the sulfuric acid used in a lead-acid battery. Fumes from a lead-acid battery can contaminate the electrolyte in a nickel-cadmium battery. This precaution should include equipment, such as hand tools and syringes, used with lead-acid batteries. Indeed, every possible precaution must be taken to keep anything containing acid away from the nickel-cadmium battery shop.
(2) The potassium hydroxide electrolyte used in nickel-cadmium batteries is extremely corrosive. Protective goggles, rubber gloves, and rubber aprons should be used to handle and service batteries. Suitable washing facilities should be provided in case electrolyte is spilled on clothing or the skin. Such exposures should be rinsed immediately with water or vinegar, lemon juice, or a boric acid solution. When potassium hydroxide and distilled water are mixed to make electrolyte, the potassium hydroxide should be added slowly to the water, not vice versa.
(3) Severe arcing may result if a wire brush is used to clean a battery. The vent plugs should be closed during the cleaning process and the battery should never be cleaned with acids, solvents, or any chemical solution. Spilled electrolyte can react with carbon dioxide to form crystals of potassium carbonate. These, which are nontoxic and noncorrosive, can be loosened with a fiber brush and wiped off with a damp cloth. When potassium carbonate forms on a properly serviced battery, it may indicate the battery is overcharging because the voltage regulator is out of adjustment.
(4) Additional water should never be added to the battery earlier than three or four hours after it has been fully charged. Should it be necessary to add water, use only distilled or demineralized water.
(5) Since the electrolyte does not react chemically with the cell plates, the specific gravity of the electrolyte does not change appreciably.
Thus, it is not possible to determine the state of charge of the battery with a hydrometer; nor can the charge be determined by a voltage test because the voltage of a nickel-cadmium battery remains constant during 90 percent of the discharge cycle.
(6) Nickel-cadmium batteries should be serviced at regular intervals based on experience since water consumption varies with ambient temperature and operating methods. At greater intervals the battery should be removed from the aircraft and given a bench check in the shop. If a battery is completely discharged, some cells may reach zero potential and charge in the reverse direction, affecting the battery in such a manner that it will not retain a full capacity charge. In such cases, the battery should be discharged and each cell short circuited to obtain a zero potential cell balance before recharging the battery. This process is called "equalization."
(7) Charging can be accomplished by either the constant voltage or the constant current method. For the constant potential charging, maintain the charging voltage constant until the charging current decays to 3 amperes or less assuring that the battery cell temperature does not exceed 100° F. For constant current charging start the charge and continue until the voltage reaches the desired potential, then reduce the current level to 4 amperes continuing the charging until its desired voltage or until the battery temperature exceeds 100° F and the voltage begins to decline.
The troubleshooting chart outlined in figure 8-111 can be used as a guide in troubleshooting battery malfunctions.