Why can't the state of charge of a nickel-cadmium battery be determined by measuring specific gravity?

The reason the state of charge of a nickel-cadmium (NiCad) battery cannot be determined by measuring specific gravity is that specific gravity remains relatively constant regardless of the charge level. In traditional lead-acid batteries, specific gravity changes significantly with the state of charge because it is affected by the concentration of sulfuric acid in the electrolyte. However, in nickel-cadmium batteries, the electrolyte used is a potassium hydroxide solution, and its specific gravity does not vary much during discharge or charge cycles. This inherent property makes specific gravity an unreliable metric for assessing the charge state of NiCad batteries.

The other options touch on different aspects of battery technology but do not accurately represent the reasons behind the behavior of specific gravity in relation to nickel-cadmium batteries.