Why is a hydrometer not used to measure the state of charge of a nickel-cadmium battery?

• A. The specific gravity does not change appreciably.
• B. The electrolyte does not enter into the chemical changes that occur when charging or discharging.
• C. The electrolyte is inert.
• D. The hydrometer is not suitable for this battery technology.

Answer: (A)

In nickel-cadmium cells, the electrolyte is an alkaline solution (typically potassium hydroxide), and the chemistry that reflects charge state happens mainly at the electrode materials. The concentration of the electrolyte—and thus its density—changes very little during charging or discharging, so the specific gravity remains almost constant. Because a hydrometer relies on measuring that density, it provides little to no information about how charged the cell is.

This differs from lead-acid batteries, where the electrolyte density changes noticeably with state of charge, making a hydrometer a useful indicator there. The electrolyte in NiCd is not inert and does participate in conducting the cell, but those reactions do not produce a meaningful change in electrolyte density to track SOC. Hence, a hydrometer isn’t a practical tool for determining the state of charge in nickel-cadmium batteries.