Introduction

Nickel-based battery systems, encompassing nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) chemistries, exhibit distinct thermal runaway mechanisms when compared to lithium-ion batteries. Understanding these differences is critical for implementing appropriate safety measures in research and industrial applications. This article presents a quantitative analysis of overcharge, short circuit, and thermal propagation behaviors, drawing on established engineering data.

Overcharge-Induced Thermal Runaway

Overcharge in nickel-based batteries leads to excessive current flow after full charge, causing electrolysis of the aqueous electrolyte. In NiCd systems, oxygen evolves at the positive electrode and hydrogen at the negative electrode; NiMH batteries primarily generate oxygen. The recombination efficiency in NiMH can exceed 99%, but if the recombination rate cannot keep pace with gas generation, internal pressure buildup occurs. Pressure vents are designed to open at predetermined thresholds, typically between 10 and 30 psi, to prevent casing rupture.

Comparative Overcharge Characteristics

Parameter	NiCd	NiMH
Primary gas products	Oxygen + hydrogen	Oxygen (with recombination)
Recombination efficiency	Moderate	High (>99% in advanced designs)
Hydrogen flammability risk	Higher (4-75% in air)	Lower
Vent mechanism	One-time	Resealable
Critical pressure threshold	10-30 psi	10-30 psi

Unlike lithium-ion systems, overcharge in nickel-based batteries does not typically involve lithium plating. Instead, the primary hazards are gas venting and potential ignition of hydrogen.

Short Circuit Failure Modes

Internal shorts in NiCd or NiMH batteries cause localized Joule heating that can melt separators and exacerbate the short. The aqueous electrolyte 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 in an exothermic reaction that reaches temperatures between 150 and 250°C. This is lower than the 600-900°C typical of lithium-ion thermal runaway, but still hazardous. Ruptured cells may eject corrosive potassium hydroxide electrolyte.

• Peak short circuit temperatures: 150-250°C (NiCd/NiMH) vs. 600-900°C (Li-ion)
• Electrolyte hazard: Corrosive KOH vs. flammable organic solvents
• Heat dissipation: More efficient due to aqueous electrolyte
• Separator melting: Contributes to short propagation

Safety Protocols and Design Considerations

Safety systems for nickel-based batteries prioritize pressure management and current interruption. Thermal fuses are designed to open at 70-100°C, breaking the circuit before critical temperatures are reached. Positive temperature coefficient (PTC) devices increase resistance with temperature, limiting current during faults.

1. Pressure management: Vents for NiMH reseal; NiCd vents are one-time.
2. Current interruption: Thermal fuses set at 70-100°C.
3. Overcharge protection: Voltage plateau detection or temperature cutoff; BMS rarely needed.
4. Cell design: Separators with high thermal stability and low gas diffusion resistance.

Thermal Propagation Rates

Nickel-based systems exhibit slower thermal runaway propagation due to 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 provides more time for safety systems to intervene.

Chemistry	Propagation rate (typical)	Peak thermal runaway temperature
Nickel-based (NiCd/NiMH)	<20°C/min	150-250°C
Lithium-ion (NMC/LCO)	>50°C/min	600-900°C

Material and Aging Effects

The thermal stability of nickel electrodes decreases with state of charge. Fully charged NiOOH decomposes at lower temperatures (around 200°C) compared to partially charged states. In contrast, lithium-ion cathodes become more thermally unstable at higher states of charge. Aging in nickel-based batteries manifests as gradual capacity loss rather than sudden failure. Manufacturing defects such as electrode misalignment can create internal shorts, screened through formation cycling and high-potential testing.

Byproducts and Ventilation Requirements

Thermal runaway byproducts differ significantly. Lithium-ion failures produce a mixture of flammable gases including hydrogen, methane, and ethylene. Nickel-based systems primarily release oxygen and hydrogen. This affects ventilation requirements: lithium-ion installations need explosion-proof ventilation, while nickel-based systems focus on preventing hydrogen accumulation, especially given its flammability range of 4-75% in air.

• Li-ion byproducts: Hydrogen, methane, ethylene (flammable mixture)
• Ni-based byproducts: Oxygen and hydrogen
• Ventilation focus: Li-ion requires explosion-proof; Ni-based requires hydrogen monitoring

Testing Standards and Maintenance

Safety testing for nickel-based systems emphasizes different parameters than for lithium-ion. Overcharge tests 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 are less frequently applied to nickel-based systems. Standards IEC 60622 and IEC 61951 govern nickel-based battery safety, focusing on pressure containment and electrolyte retention.

Maintenance practices also diverge: nickel-based batteries require periodic equalization charges to prevent capacity imbalance, which must be controlled to avoid overcharge. Lithium-ion systems rely on balancing circuits during normal operation without intentional overcharge cycles.

Conclusion

Understanding the quantitative differences in thermal runaway behavior between nickel-based and lithium-ion batteries enables proper system design and safety protocol implementation. Nickel-based systems offer slower propagation and lower peak temperatures, but pose unique hazards from hydrogen evolution and alkaline electrolyte. Continued improvements in recombination efficiency and pressure management maintain their relevance in applications where abuse tolerance is prioritized.