The electrolyte of a nickel-cadmium battery is highest when the battery is

• A. in a discharged condition.
• B. under a no-load condition.
• C. under a high-temperature condition.
• D. in a fully charged condition.

Answer: (D)

In nickel-cadmium cells, the electrolyte is a concentrated alkaline solution, and its density changes with the state of charge because the electrode reactions involve the hydroxide ions in the solution. When the cell is fully charged, more ions remain free in the solution, so the electrolyte becomes more concentrated and its density is higher. If the cell is discharged, those ions are drawn into forming solid hydroxides on the electrodes, reducing the free-ion concentration and lowering the density. A no-load condition doesn’t drive any new chemical change, so it doesn’t raise the density beyond the charged state. Higher temperature tends to expand the liquid and lowers density rather than increasing it. So the electrolyte density is greatest when the battery is fully charged.