The electrolyte of a nickel-cadmium battery is highest when the battery is in which state?

• A. Discharged
• B. Resting
• C. Fully charged
• D. On charge

Answer: (C)

In nickel-cadmium cells the electrolyte is a potassium hydroxide solution, and its ion concentration—and thus density and conductivity—changes as the battery charges and discharges. As charging proceeds, the positive plate material changes from Ni(OH)2 to NiOOH and the negative plate chemistry advances as well, which drives more ions into the electrolyte. This increase in dissolved species makes the electrolyte more concentrated, so its concentration is highest when the battery is fully charged. When the battery is discharged, the reverse reactions reduce the ion content in the solution, lowering its concentration. Resting may allow slight redistribution, but it doesn’t raise the concentration beyond what is reached at full charge. On-charge charging moves the concentration upward toward full charge, but the maximum is reached at the fully charged state.