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discharged condition, are cadmium hydroxide in the negative plates and nickel hydroxide in the positive
plates. With the application of a charging current, these active materials undergo a chemical change. The
negative material (Cadmium Hydroxide) gradually gains electrons and is converted to metallic cadmium
(Cd); the positive material is gradually brought to a higher state of oxidation (loses electrons). As long as
the charging current continues to flow through the battery, these changes will take place until the active
materials in both electrodes are completely converted, at which point, overcharge commences.
Toward the end of the process (as the materials approach a full charge condition), and during overcharge,
gas will be evolved and released through the cell vent. This gas results from the electrolysis of the water
component of the electrolyte. The gas evolved at the negative plates is hydrogen and at the positive plates
is oxygen. The amount of gas evolved depends upon the charge rate during the period in which the cells
are being overcharged. After complete conversion of the active materials has occurred, the further
application of charge current will only cause further electrolysis of the water and I2R heating.
Discharge
Discharging results in the conversion of the chemical energy stored in the cell to electrical energy. During
discharge, the chemical reactions which occurred in charging are reversed. The active material (Cd) in the
negative plates gradually loses electrons and changes to cadmium hydroxide. The active material in the
positive plates gains electrons and changes to nickel hydroxide. No gassing occurs during a normal
discharge. The insolubility of the active materials and the fact that the potassium hydroxide does not
participate in the cell reaction results in the very flat Ni-Cd discharge voltage curve.
The rate at which the conversions take place is primarily determined by the external resistance (load)
introduced into the circuit in which the cell is connected. Due to its construction, the MarathonNorco cell
has an extremely low internal resistance, and its ability to deliver high currents is due to this factor.
MarathonNorco Aerospace, Inc.
OPERATING and MAINTENANCE MANUAL
NICKEL-CADMIUM AIRCRAFT BATTERIES
24-34-00 Page 3
APR 15/06
Charge, Discharge and overcharge equations:
Positive plate
Charge →
Discharge ←
2Ni(OH)2 + 2(OH)- → 2Ni OOH + 2H2O + 2e-
(Nickel Hydroxide) ← (Nickel Oxy Hydroxide)
Overcharge
4(OH)- → O2 + 2H2O + 4e-
Negative Plate
Charge →
Discharge ←
Cd(OH)2 + 2e- → Cd° + 2(OH)-
(Cadmium Hydroxide) (Cadmium)
Overcharge
4H2O + 4e- → 2H2 + 4(OH)
Overcharge (Net Cell Reaction)
4e-
2H2O → 2H2 + O2
Figure 2
TYPICAL CONSTANT CURRENT DISCHARGE CURVES
Constant Current
Discharge For
Typical 20 Cell
Battery: Rate a
parameter
MarathonNorco Aerospace, Inc.
OPERATING and MAINTENANCE MANUAL
NICKEL-CADMIUM AIRCRAFT BATTERIES
24-34-00 Page 4
APR 15/06
Capacity
Capacity is measured quantitatively in ampere-hours delivered at a specified discharge rate to a specified
cut-off voltage at room temperature. The cut-off voltage is 1.0 volt per cell.
Battery available capacity depends upon several factors including such items as:
1. Cell design (cell geometry, plate thickness, hardware, and terminal design govern performance under
specific usage conditions of temperature, discharge rate, etc.).
2. Discharge rate (high current rates yield less capacity than low rates).
3. Temperature (capacity and voltage levels decrease as battery temperature moves away from the 60°F
(16°C) to 90°F (32°C) range toward the high and low extremes).
4. Charge rate (higher charge rates generally yield greater capacity).
MarathonNorco Aerospace, Inc.
OPERATING and MAINTENANCE MANUAL
NICKEL-CADMIUM AIRCRAFT BATTERIES
24-34-00 Page 101
APR 15/06
1.0 INSPECTION
1.1 Delivery Inspection
When the battery is unpacked, a thorough inspection should be made to ensure that no damage
occurred during shipment. Inspect the shipping container as well as the battery. Before putting the
battery into service, check the following points carefully.
1.1.1 Damage
See if any liquid has spilled into the shipping container. This may be a sign of a damaged cell.
Check for dented battery container. Check for cracked cell cases or covers. Do not place a
damaged battery into service. Report any signs of improper handling to the shipping
company.
1.1.2 Shorting straps
Some batteries are shipped with shorting devices across the main power receptacle output
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Operating and Maintenance Manual For Nickel-Cadmium Aircraft(5)
