Nickel-based battery systems, including nickel-cadmium (NiCd) and nickel-metal hydride (NiMH), exhibit distinct thermal runaway mechanisms compared to lithium-ion batteries. Understanding these differences is critical for implementing appropriate safety measures. Thermal runaway in nickel-based systems is primarily triggered by overcharge and internal short circuits, though the progression and severity differ from lithium-ion chemistries.

Overcharge in nickel-based batteries leads to excessive current flow after reaching full charge, causing electrolysis of the aqueous electrolyte. This produces oxygen at the positive electrode and hydrogen at the negative electrode in NiCd systems, while NiMH batteries primarily generate oxygen. The recombination efficiency in NiMH batteries is higher, as oxygen diffuses to the negative electrode and reacts with hydrogen to form water. However, if the recombination rate cannot keep pace with gas generation, internal pressure builds up. In NiCd batteries, hydrogen accumulation poses a greater risk due to its flammability. Pressure vents are critical safety features, designed to open at predetermined thresholds, typically between 10-30 psi, to prevent casing rupture. Unlike lithium-ion systems, where overcharge leads to lithium plating and exothermic reactions with the electrolyte, nickel-based systems face risks from gas venting and potential ignition if hydrogen concentrations reach 4-75% in air.

Short circuits in nickel-based batteries generate heat through Joule heating, similar to lithium-ion systems, but with different failure modes. Internal shorts in NiCd or NiMH batteries cause localized heating, which can melt separators and exacerbate the short. The aqueous electrolyte in these systems has higher thermal conductivity than organic electrolytes in lithium-ion batteries, allowing more efficient heat dissipation initially. However, sustained shorting leads to thermal decomposition of the nickel oxyhydroxide positive electrode, releasing oxygen. This exothermic reaction can reach temperatures of 150-250°C, lower than the 600-900°C seen in lithium-ion thermal runaway but still hazardous. The absence of flammable organic electrolytes reduces the likelihood of violent combustion, though ruptured cells may eject corrosive potassium hydroxide electrolyte.

Safety protocols for nickel-based systems prioritize pressure management and current interruption. Pressure vents are engineered to reseal after activation in NiMH batteries, while NiCd systems often use one-time vent mechanisms. Thermal fuses are employed in both chemistries, typically set to open at 70-100°C, breaking the circuit before temperatures reach critical levels. Some designs incorporate positive temperature coefficient (PTC) devices that increase resistance with temperature, limiting current during faults. Unlike lithium-ion batteries, nickel-based systems rarely require complex battery management systems for overcharge protection, relying instead on simpler charge termination methods like voltage plateau detection or temperature cutoffs.

Contrasting with lithium-ion batteries, nickel-based systems exhibit slower thermal runaway propagation due to their lower energy density and aqueous electrolytes. A lithium-ion cell undergoing thermal runaway can transfer heat to adjacent cells at rates exceeding 50°C per minute, while nickel-based battery packs typically show propagation rates below 20°C per minute. This allows more time for safety systems to intervene in nickel-based configurations. However, the alkaline electrolyte in nickel batteries presents unique hazards, as potassium hydroxide can cause chemical burns and corrode metal components during failure events.

Material choices in nickel-based batteries influence their thermal behavior. NiMH systems using rare-earth-based hydrogen storage alloys exhibit better overcharge tolerance than NiCd batteries due to their superior oxygen recombination capability. The thermal stability of nickel electrodes decreases with state of charge, with fully charged NiOOH decomposing at lower temperatures (around 200°C) than partially charged states. This contrasts with lithium-ion cathodes like NMC or LCO, which become more thermally unstable at higher states of charge.

Manufacturing defects pose similar risks in both battery types, but the consequences differ. Nickel electrode misalignment can create internal shorts, while separator flaws may allow gas pocket formation. These defects are typically screened through formation cycling and high-potential testing during production. Aging effects also differ—nickel-based batteries experience gradual capacity loss rather than the sudden failure modes sometimes seen in aged lithium-ion batteries.

Safety testing protocols for nickel-based systems emphasize different parameters than lithium-ion. Overcharge tests typically apply a 0.1C continuous charge beyond 200% state of charge, monitoring pressure and temperature. Short circuit tests measure peak temperatures and case integrity, with pass criteria requiring no explosion or fire. Nail penetration tests, common for lithium-ion batteries, are less frequently applied to nickel-based systems due to their different failure modes.

Operational environments affect nickel-based battery safety differently than lithium-ion systems. High temperatures accelerate corrosion in nickel electrodes and degrade separators, while low temperatures increase electrolyte resistance without the lithium plating risks seen in lithium-ion batteries. Cycling under extreme temperatures reduces recombination efficiency in NiMH batteries, increasing overcharge risks.

The thermal runaway byproducts differ significantly between the chemistries. Lithium-ion failures produce a mixture of flammable gases including hydrogen, methane, and ethylene, while nickel-based systems primarily release oxygen and hydrogen. This affects ventilation requirements for battery enclosures—lithium-ion systems need explosion-proof ventilation, while nickel-based installations focus on preventing hydrogen accumulation.

Safety standards reflect these differences. IEC 60622 and IEC 61951 govern nickel-based battery safety, emphasizing pressure containment and electrolyte retention, whereas lithium-ion standards like UL 1973 focus on fire prevention and thermal propagation. These standards inform the design of safety systems, with nickel-based batteries prioritizing mechanical integrity during gas generation and lithium-ion batteries emphasizing thermal barriers and flame arrestors.

Maintenance practices also diverge. Nickel-based batteries require periodic equalization charges to prevent capacity imbalance, which must be carefully controlled to avoid overcharge conditions. Lithium-ion systems rely on balancing circuits during normal operation rather than intentional overcharge cycles.

End-of-life failure modes present different safety considerations. Nickel-based batteries often fail through separator dry-out or electrode corrosion, rarely exhibiting sudden thermal events. This makes them more suitable for applications where gradual failure is preferable to catastrophic failure, though the toxic cadmium in NiCd batteries requires careful disposal.

The evolution of nickel-based battery safety continues, with improvements in recombination efficiency and pressure management. Advanced NiMH designs achieve over 99% oxygen recombination efficiency, nearly eliminating electrolyte loss during overcharge. New separator materials with higher thermal stability and lower gas diffusion resistance further enhance safety. These developments maintain nickel-based batteries in applications where their safety profile and tolerance to abuse outweigh their lower energy density compared to lithium-ion systems.

Understanding these fundamental differences in thermal runaway behavior enables proper system design and safety protocol implementation for nickel-based battery applications. The distinct mechanisms require tailored approaches rather than direct adoption of lithium-ion safety strategies, particularly in areas of pressure management and gas recombination.