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Nickel Metal Hydride (NiMH)

Characteristics

These cells use nickel hydroxide Ni(OH)2 for the positive electrode (cathode). Hydrogen is used as the active element in a hydrogen-absorbing negative electrode (anode). This electrode is made from a metal hydride, usually alloys of lanthanum and rare earths that serve as a solid source of reduced hydrogen that can be oxidized to form protons. The electrolyte is alkaline, usually potassium hydroxide.

Performance data

  • Voltage: 1.2 volts
  • Charge cycles: 500 to 1,000
  • Temperature range: -15°C to +40°C
  • Typical energy density: 50 - 70 Wh/kg
  • Typical power density: 100 - 200 W/kg
  • Typical spontaneous discharge: approx. 20-50% within 1 month

Applications

  • Mobile phones
  • Hearing screening
  • Measurement systems
  • Games
  • Lighting
  • Robotic
  • Electric knife
  • Power tools
  • Medical
  • e-bicycle

Advantages

  • High energy density
  • Can be deep cycled.
  • Using NiMH batteries, up to 3000 cycles at 100 % Depth of Discharge (DOD) have been demonstrated. At lower depths of discharge, for example at 4 % DOD, more than 350,000 cycles can be expected.
  • Robust - NiMH batteries also tolerate over-charge and over-discharge conditions and this simplifies the battery management requirements.
  • Low internal impedance
  • Flat discharge characteristic (but falls off rapidly at the end of the cycle)
  • Wide operating temperature range
  • Rapid charge possible in 1 hour
  • Trickle charging cannot normally be used with NiMH batteries since overcharging can cause deterioration of the battery. Chargers should therefore incorporate a timer to prevent overcharging.
  • Reconditioning is possible.
  • More environmentally-friendly than other cell chemistries: no cadmium, mercury or lead and the nickel can be recycled.

Drawbacks

  • Very high self discharge rate, nearly ten times worse than lead acid or Lithium batteries.
  • Suffers from memory effect, though not as pronounced as with NiCd batteries
  • Battery deteriorates during long time storage. This problem can be solved by charging and discharging the battery several times before reuse. This reconditioning also serves to overcome the problems of the "memory" effect.
  • High rate discharge not as good as NiCds
  • Less tolerant of overcharging than NiCds
  • As with NiCds the cells must incorporate safety vents to protect the cell in case of gas generation.
  • The coulombic efficiency of nickel metal hydride batteries is typically only about 66% and diminishes the faster the charge.
  • While the battery may have a high capacity it is not necessarily all available since it may only deliver full power down to 50% DOD depending on the application.
  • Cell voltage is only 1.2 Volts which means that many cells are required to make up high voltage batteries.
  • Lower capacity and cell voltage than alkaline primary cells.

Charging

  • Run down fully once per month to avoid memory effect.
  • Do not leave battery in charger.
  • Slow charging method: Constant current followed by trickle charge.
  • Rapid charging method uses dT/dt charge termination.
  • Use timer cut off to avoid prolonged trickle charge.
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