Characteristics
Nickel-metal-hydride batteries are related to sealed nickel-cadmium batteries and only differ from them in that instead of cadmium, hydrogen is used as the active element at 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 potassium hydroxide. Cell voltage is 1.2 Volts
The NiMH battery was patented in 1986 by Stanford Ovshinsky, founder of Ovonics.
The basic concept of the nickel-metal hydride cell negative electrode emanated from research on the storage of hydrogen for use as an alternative energy source in the 1970s. Certain metallic alloys were observed to form hydrides that could capture (and release) Hydrogen in volumes up to nearly a thousand times their own volume. By careful selection of the alloy constituents and proportions, the thermodynamics could be balanced to permit the absorption and release process to proceed at room temperatures and pressures.
Now that the technology is reasonably mature, NiMH batteries have begun to find use in high voltage automotive applications. The energy density is more than double that of Lead acid and 40% higher than that of NiCads
They accept both higher charge and discharge rates and micro-cycles thus enabling applications which were previously not practical.
The components of NiMH batteries include a cathode of Nickel-hydroxide, an anode of Hydrogen absorbing alloys and a Potassium-hydroxide (KOH) electrolyte which are collectively more benign than the active chemicals used in rival Lithium batteries.
Like NiCd batteries, Nickel-metal Hydride batteries are susceptible to a "memory effect" although to a lesser extent. They are more expensive than Lead-acid and NiCd batteries, but they are considered better for the environment.
Recent Developments
NiMH cell chemistry has had a bad press ever since the introduction of Lithium based cell chemistries. NiMH technology has not been standing still however. Unlike the consumer applications where NiMH has been almost completely replaced by Lithium ion, NiMH chemistry is still finding use in automotive applications where it is the technology of choice for powering HEVs and where it has accumulated over 10 years of trouble free service and can thus last for the lifetime of the car. The operating temperature range for NiMH cells has been extended to over 100 °C (-30 °C to + 75 °C) which far exceeds the temperature range currently achievable by Lithium cells making NiMH technology ideal for automotive use. NiMH can handle the high power levels typical in EV applications, the active chemicals are inherently safer than Lithium based cells and NiMH batteries don't need the complex battery management systems (BMS) essential with Lithium batteries.Early cells were susceptible to memory effect and also suffered from high self discharge, nearly ten times worse than Lead acid or Lithium batteries, but both of these weaknesses have been overcome and it is claimed that the most recent cells can hold their charge for a year.
Advantages
High energy density (W/kg), about 50% better than Nicads, but only about 60% of Lithium ion.Low internal impedance though not as low as NiCads
Typical cycle life is 3000 cycles.
Can be deep cycled. (80% to 100% DOD)
Using NiMH batteries, more than 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.
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 can not normally be used with NiMH batteries since overcharging can cause deterioration of the battery. Chargers should therefore incorporate a timer to prevent overcharging.
Because of potential pressure build up due to gassing they usually incorporate a re-sealable vent valve
Reconditioning is possible.
Environmentally friendly (No Cadmium, Mercury or Lead)
Much safer than Lithium based cells in case of an accident or abuse due to the use of more benign active chemicals, a particularly important property in high power and automotive applications.
Shortcomings
High self discharge rate.Can be stored indefinitely either fully charged or fully discharged.
Suffers from memory effect though not as pronounced as with NiCad 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 NiCads
Less tolerant of overcharging than NiCads
As with NiCads the cells must incorporate safety vents to protect the cell in case of gas generation.
The coulombic efficiency of nickel metal hydride batteries could be up to 85% but is typically only around 65% and diminishes the faster the charge although this is projected to improve.
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. The competing Lithium cells typically have 3 times the cell voltage (3.2 Volts to 3.7 Volts) and a much higher energy density.
Lower capacity and cell voltage than alkaline primary cells.
Limited supplies of rare earth element Lanthanum. Mostly in China.
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.
Applications
Low cost consumer applications, however Lithium cells are taking over this market.Electric razors
Toothbrushes
Cameras
Camcorders
Mobile phones
Pagers
Medical instruments and equipment
Automotive batteries
High power static applications (Telecoms, UPS and Smart grid).
Costs
Originally more expensive than NiCad cells but prices are now more in line as NiMH volumes increase and the use of toxic Cadmium based cells is deprecated.About half the cost of Lithium ion batteries.
