Nickel-cadmium batteries represent a mature rechargeable battery technology that has been widely used for decades in specialized applications. The electrochemical system relies on the reversible reaction between nickel hydroxide and cadmium, with an alkaline electrolyte facilitating ion transport. This chemistry offers unique advantages in ruggedness and longevity, though environmental concerns and technological advancements have reduced its dominance in many markets.

The fundamental electrochemical reaction in a NiCd battery involves the reduction of nickel oxyhydroxide at the cathode and oxidation of cadmium at the anode during discharge. The cathode consists of nickel hydroxide Ni(OH)₂, which converts to nickel oxyhydroxide NiOOH during charging. The anode comprises metallic cadmium, which transforms to cadmium hydroxide Cd(OH)₂ during discharge. The electrolyte is an aqueous solution of potassium hydroxide KOH, typically in the concentration range of 20-30%, which remains unchanged during the charge-discharge cycle as it only serves as an ionic conductor.

During discharge, the overall reaction proceeds as:
2NiOOH + Cd + 2H₂O → 2Ni(OH)₂ + Cd(OH)₂
The reverse occurs during charging. The nominal cell voltage is 1.2V, with actual operating voltage ranging between 1.0-1.4V depending on state of charge and load conditions. The theoretical energy density of the chemistry is approximately 100 Wh/kg, though practical systems achieve 40-60 Wh/kg due to necessary packaging and excess materials.

NiCd batteries employ several specialized material formulations to enhance performance. The cathode typically contains nickel hydroxide mixed with conductive additives like graphite or cobalt compounds to improve electronic conductivity. The anode consists of finely divided cadmium, often with iron additions to prevent agglomeration and maintain high surface area. Separators are made from porous polypropylene or nylon membranes that resist degradation in the strong alkaline environment.

The charge mechanism follows a characteristic voltage profile. During constant current charging, voltage rises gradually until reaching about 1.45V per cell, then increases sharply when full charge is approached. This sharp rise serves as the basis for charge termination in many NiCd charging systems. Overcharging leads to oxygen evolution at the positive electrode, which then recombines at the negative electrode, creating an internal recombination cycle that prevents pressure buildup in sealed cells.

Discharge behavior shows relatively flat voltage characteristics until near complete discharge, where voltage drops rapidly. This allows for efficient energy extraction over most of the discharge cycle. The batteries can deliver high currents, with some designs capable of 10C continuous discharge rates, making them suitable for power-intensive applications.

Key advantages of NiCd batteries include exceptional cycle life, with properly maintained systems achieving 2000+ cycles. They maintain performance across a wide temperature range from -40°C to 60°C, outperforming many other chemistries in extreme conditions. The robust nature of the chemistry allows for long shelf life with minimal self-discharge, typically 10-20% per month at room temperature, lower than many aqueous systems.

The memory effect represents a notable disadvantage, where repeated partial discharge-charge cycles can cause the battery to "remember" the reduced capacity. This phenomenon results from crystalline formation in the cadmium electrode and can be reversed through proper maintenance charging. Environmental concerns surround cadmium, a toxic heavy metal requiring careful handling and recycling. The relatively low energy density compared to modern alternatives limits applications where weight or volume is critical.

Performance metrics for NiCd batteries show distinct characteristics compared to other systems:
Energy density: 40-60 Wh/kg
Power density: 150-300 W/kg
Cycle life: 500-2000 cycles
Operating temperature: -40°C to 60°C
Self-discharge rate: 10-20%/month

Major applications leverage the robustness and reliability of NiCd chemistry. Aviation remains a significant user, particularly for aircraft starting batteries and emergency systems where reliability is paramount. Emergency lighting systems benefit from the long shelf life and instant readiness. Power tools historically used NiCd extensively for high current capability, though newer technologies have displaced many of these applications. Railway signaling, remote telecommunications equipment, and medical devices continue to employ NiCd where its characteristics outweigh environmental concerns.

Compared to other nickel-based systems, NiCd shows both similarities and differences. Like nickel-metal hydride batteries, it uses the same nickel hydroxide cathode chemistry, but the cadmium anode provides different characteristics. NiCd generally offers better low-temperature performance and higher current capability than NiMH, though with lower energy density. Both systems share the same nominal voltage, allowing some interchangeability in applications.

Manufacturing of NiCd batteries involves specialized processes to handle the cadmium materials safely. Electrode production typically uses sintering or paste methods to create the porous nickel and cadmium structures. Sealed cell designs incorporate recombinant features to prevent electrolyte loss during the battery's life. Venting mechanisms in larger cells prevent dangerous pressure buildup during misuse.

Maintenance requirements for NiCd systems include periodic full discharges to prevent memory effect and proper charging to avoid overcharge damage. Storage in cool conditions minimizes self-discharge, though the batteries tolerate high temperatures better than many alternatives. End-of-life handling requires proper recycling to recover the valuable nickel and contain the toxic cadmium.

The environmental impact of NiCd batteries has led to strict regulations in many jurisdictions. The European Union's Battery Directive restricts cadmium use in consumer applications, though exempts certain professional uses. Recycling systems have been established to recover over 95% of the cadmium from spent batteries, with the recovered materials used to produce new batteries or other nickel and cadmium products.

Future prospects for NiCd technology focus on niche applications where its unique advantages remain valuable. While not competitive for mass-market consumer electronics or electric vehicles, the extreme temperature tolerance, high reliability, and long service life ensure continued use in aerospace, military, and industrial applications where these characteristics outweigh environmental concerns. Research continues on improving specific energy and reducing memory effects, though fundamental limitations of the chemistry prevent dramatic improvements.

In summary, nickel-cadmium battery technology represents a balance of performance characteristics that remain relevant in specific applications despite environmental challenges. The combination of nickel hydroxide and cadmium electrodes with alkaline electrolyte creates a robust electrochemical system capable of withstanding harsh conditions and providing reliable service over thousands of cycles. While newer technologies have displaced NiCd in many areas, its unique advantages ensure continued use in applications where reliability under extreme conditions outweighs other factors.