Nickel Metal Hydride (NiMH) Battery
Contents
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The principles in which NiMH cells operate are based on their ability to absorb, release, and transport (move) hydrogen between the electrodes within the cell.
Negative Electrode: Metal Hydride such as AB2 (A=titanium and/or Vanadium, B= Zirconium or Nickel, modified with chromium, Cobalt, iron and/or manganese) or AB5 (A = rare earth mixture of lanthanum, cerium, neodymium, praseodymium, B = Nickel, Cobalt, manganese, and/ or aluminium).
Positive Electrode: Nickel Oxyhydroxide (NiO(OH))
Electrolyte: Potassium Hydroxide (KOH)
Reactions at Electrodes
Features
- Higher energy density (40%) than NiCd
- Non-toxic
- Reduced life, discharge rate (0.2-0.5C)
- More expensive (20%) than NiCd.
Advantages
- 30%+ higher capacity compared to standard NiCd battery.
- Less prone to memory than NiCd — fewer exercise cycles are required.
- Simple storage and transportation; not subject to regulatory control.
- Nickel content makes recycling profitable.
- Non toxic. Hence, Environmentally friendly.
- Wide temperature range.
Limitations
- More complex charge algorithm needed — the NiMH generates more heat during charge and requires a longer charge time than the NiCd.
- High self-discharge — typically 50% higher than NiCd. New chemical additives improve self-discharge but at the expense of lower.
- Coulombic efficiency only about 65% (99% with Li-ion)
- Performance degrades if stored at elevated temperatures.
- Requires complex charge algorithm. Sensitive to overcharge.
- Does not absorb overcharge well; trickle charge must be kept low.
