How can the state-of-charge of a nickel-cadmium battery be determined?

• A. By a measured discharge
• B. By a visual inspection of the case
• C. By open-circuit voltage alone
• D. By electrolyte temperature

Answer: (A)

Determining state of charge for nickel-cadmium batteries is best done by performing a measured discharge. NiCd cells don’t provide a reliable, single reading of remaining capacity from voltage alone, especially under load or after different temperatures and charge histories. The voltage can look similar at various levels of charge, and resting voltage is influenced by temperature and age, making it an unreliable indicator of how much energy is left.

By discharging the battery at a known, controlled current and monitoring how much capacity is delivered until the specified end voltage is reached, you obtain a direct measure of how much energy has been used. Comparing that discharged capacity to the rated capacity gives a clear idea of what portion remains, i.e., the state of charge. This method accounts for real operating conditions and provides a practical, quantitative assessment of remaining energy.

Visual inspection provides no information about remaining energy, electrolyte temperature is a secondary parameter related to use conditions rather than a direct SOC measure, and relying on open-circuit voltage alone is misleading for NiCd cells because resting voltage can vary with temperature and history.