During charging a nickel-cadmium battery, the cells emit gas most noticeably when?

• A. Toward the end of the charging cycle.
• B. Throughout the charging cycle.
• C. Especially if the electrolyte level is high.
• D. Only when discharging begins.

Answer: (A)

Gas formation in nickel‑cadmium cells happens when charging continues after the cell has reached its capacity. Early in charging, current mainly drives the reversible chemical reactions that store energy. As the cell nears full charge, the charging efficiency drops and the remaining current drives the electrolysis of water in the alkaline electrolyte, splitting water into hydrogen at one electrode and oxygen at the other. This gas buildup increases the cell’s internal pressure, so you notice it most toward the end of the charging cycle, especially if charging continues past full charge. The gas isn’t caused by the electrolyte level; it’s the result of overcharge and the associated electrolysis reaction.