Why is a hydrometer not suitable for assessing the charge of a nickel-cadmium battery?

A hydrometer is designed to measure the specific gravity of the electrolyte in a battery, which helps determine the state of charge in lead-acid batteries. In the case of nickel-cadmium (NiCad) batteries, the composition of the electrolyte does not undergo significant changes in density or specific gravity during the charging and discharging cycles. As a result, relying on a hydrometer to assess the charge state would yield minimal to no variation in readings, rendering it ineffective for accurately determining the battery's charge status.

This distinction is crucial because it implies that different battery chemistries require specific methods for charge assessment. Thus, in the context of nickel-cadmium batteries, the fundamental property that a hydrometer relies upon—changes in specific gravity—is not applicable, which is the core reason why it is unsuitable for this type of battery.