3.3 Nickel-metal hydride batteries: principles and performance

Nickel-metal hydride batteries offer higher energy density than nickel-cadmium, with 60-120 Wh/kg. They use metal hydride alloys for the anode and nickel oxyhydroxide for the cathode, allowing for better energy storage and performance.

NiMH batteries are widely used in consumer electronics and hybrid vehicles. They have good overcharge tolerance but are sensitive to temperature. Their self-discharge rate is higher than NiCd, but improved formulations have addressed this issue.

Nickel-Metal Hydride Battery Composition and Characteristics

Anode and Cathode Materials

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• Anode composed of a metal hydride alloy that can absorb and release hydrogen during charging and discharging
• Cathode typically made of nickel oxyhydroxide (NiOOH), similar to nickel-cadmium batteries
• Metal hydride alloys commonly used include AB5 (e.g., LaNi5) and AB2 (e.g., ZrV2) type alloys
• Metal hydride alloys have a high hydrogen storage capacity, enabling higher energy densities compared to nickel-cadmium batteries

Energy Density and Voltage Characteristics

• Nickel-metal hydride (NiMH) batteries have higher energy densities than nickel-cadmium (NiCd) batteries
  • NiMH batteries typically have energy densities of 60-120 Wh/kg, while NiCd batteries have energy densities of 40-60 Wh/kg
• NiMH batteries exhibit a phenomenon called voltage depression or memory effect
  • Repeated shallow discharges can cause a temporary reduction in the battery's voltage and capacity
  • Voltage depression can be reversed by performing a full discharge and recharge cycle

Temperature Sensitivity

• NiMH batteries are more sensitive to temperature compared to NiCd batteries
• Optimal operating temperature range is between 0°C and 40°C (32°F to 104°F)
  • Charging efficiency and capacity decrease at low temperatures
  • High temperatures can accelerate self-discharge and reduce battery life
• Temperature management systems may be required for applications in extreme temperature environments (e.g., electric vehicles)

Nickel-Metal Hydride Battery Performance

Overcharge Tolerance and Charge Efficiency

• NiMH batteries have a higher overcharge tolerance compared to NiCd batteries
  • Can withstand moderate overcharging without significant damage
  • Oxygen recombination reaction helps prevent pressure buildup during overcharge
• Charge efficiency of NiMH batteries is lower than NiCd batteries
  • Typical charge efficiency is around 66%, meaning 50% more charge input is required to fully charge the battery

Self-Discharge Characteristics

• NiMH batteries have a higher self-discharge rate compared to NiCd batteries
  • Self-discharge rate is typically 15-20% per month at room temperature
  • Self-discharge rate increases with higher storage temperatures
• Higher self-discharge rate can be a disadvantage for applications requiring long storage periods (e.g., emergency devices)
• Improved NiMH battery formulations have been developed to reduce self-discharge rates (e.g., low self-discharge or LSD NiMH batteries)

Nickel-Metal Hydride Battery Applications

Consumer Electronics

• NiMH batteries are widely used in portable consumer electronics devices
  • Examples include digital cameras, portable media players, and wireless computer peripherals (e.g., mice and keyboards)
• NiMH batteries offer higher capacities and longer runtimes compared to NiCd batteries in these applications
• Rechargeable NiMH batteries have largely replaced disposable alkaline batteries in many consumer devices

Electric and Hybrid Electric Vehicles

• NiMH batteries have been extensively used in hybrid electric vehicles (HEVs) and electric vehicles (EVs)
  • Examples of HEVs using NiMH batteries include the Toyota Prius and Honda Insight
• NiMH batteries offer good energy density, power density, and cycle life for automotive applications
• Thermal management systems are often employed to maintain optimal battery temperature in vehicle applications
• In recent years, lithium-ion batteries have become more prevalent in EV applications due to their higher energy densities and decreasing costs