Which of the following best describes the operating principal in a nickel-cadmium battery installed in an aircraft?

• A. At full charge, the electrolyte will be at its lowest level and should be filled.
• B. To completely charge a nickel-cadmium battery, some gassing must take place; thus, some water will be used.
• C. When positive plates slowly give up oxygen, which is regained by the negative plates, the battery is charging.
• D. A nickel-cadmium battery does not require any gas during charging.

Answer: (B)

Nickel-cadmium cells in aircraft use a potassium hydroxide electrolyte, and during charging the water in that electrolyte is split by the applied current. Oxygen forms at the positive nickel electrode and hydrogen forms at the negative cadmium electrode and is vented away. Because water is being decomposed, the electrolyte can lose water, so to complete a full charge you need some gas to evolve and you must replenish the water that’s been consumed. This is why the correct description is that some gassing must occur during charging and water will be used. The other statements don’t match how NiCd charging works: they either imply no gas is produced, or oversimplify the process as if oxygen were simply transferred between plates, or rely on a level-maintenance idea more typical of flooded lead-acid cells.