Is the electrolyte of a nickel-cadmium battery lowest when the battery is fully charged or fully discharged?

• A. Fully charged
• B. Fully discharged
• C. Neither fully charged nor fully discharged
• D. Both

Answer: (B)

In nickel-cadmium cells with an alkaline electrolyte, the state of charge changes how much dissolved hydroxide (the electrolyte) is present. During discharge, the active materials convert to solid hydroxides, which consumes hydroxide ions from the electrolyte. This reduces the concentration of dissolved ions and lowers the electrolyte’s density (specific gravity). When the cell is charged, the reverse happens: the solid hydroxides are converted back, putting more hydroxide back into the solution and increasing the electrolyte density. So the electrolyte is least concentrated, i.e., lowest, when the cell is fully discharged.