Which of the following is a key indicator of the state of a nickel-cadmium battery?

The state of a nickel-cadmium battery is best indicated by its voltage level. This measurement provides crucial information about the battery's charge status, health, and overall performance. A fully charged nickel-cadmium battery typically has a voltage reading of about 1.2 volts per cell, and changes in this voltage can signify varying states of charge and discharge, as well as potential issues like cell imbalance or capacity fade.

Monitoring the voltage level enables users to determine whether the battery is ready for use, needs charging, or may require maintenance or replacement. This aspect of battery management is critical to ensure reliability, especially in applications where consistent power is essential, such as in aviation operations.

While factors like battery weight, temperature regulation, and physical size can be important in specific contexts, they do not provide the direct measure of the electrochemical state of a nickel-cadmium battery that voltage does. Thus, focusing on the voltage level is essential for assessing the battery's condition and performance comprehensively.