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NiMH battery

About

A nickel-metal hydride cell, abbreviated NiMH uses a hydrogen-absorbing alloy for the negative electrode instead of cadmium. As in NiCd cells, the positive electrode is nickel oxyhydroxide (NiOOH). A NiMH cell can have two to three times the capacity of an equivalent size Nickel-cadmium battery. However, compared to the lithium-ion cell, the volumetric energy density is lower and self-discharge is higher.[1]

Trickle charging

Some equipment manufacturers consider that NiMH cells can be safely charged in simple fixed, low-current chargers with or without timers, and that permanent overcharging is permissible with currents up to 0.1 C (where C is the current equivalent to the capacity of the battery divided by one hour). According to the Panasonic NiMH charging manual, extensive trickle charging can cause battery deterioration due to overcharging, and it is the least preferred charging method concerning battery performance. If it is used, the trickle charge rate should be limited to between 0.033 C and 0.05 C for a maximum of 20 hours to avoid damaging the batteries.

For a slow charge, or "trickle charge" process, Duracell recommends "a maintenance charge of indefinite duration at 0.0033 C".[2] Some chargers do this after the charge cycle, to offset the natural self-discharge rate of the battery. To maximize battery life, the preferred charge method of NiMH cells uses low duty cycle pulses of high current rather than continuous low current.

Safety

A good safety feature of a custom-built charger is to use a resettable fuse in series with the cell, particularly of the bimetallic strip type. This fuse will open if either the current or the temperature goes too high.

Modern NiMH cells contain catalysts to immediately deal with gases developed as a result of over-charging without being harmed (2 H2 + O2 ---catalyst → 2 H2O). However, this only works with overcharging currents of up to 0.1C (nominal capacity divided by 10 hours). As a result of this reaction, the batteries will heat up considerably, marking the end of the charging process.Some quick chargers have a fan to keep the batteries cool. [3]

A method for very rapid charging called in-cell charge control involves an internal pressure switch in the cell, which disconnects the charging current in the event of overpressure.

There is an inherent risk with NiMH chemistry that overcharging will cause a buildup of hydrogen, causing the cell to rupture. Therefore, cells have a vent. Hydrogen will be emitted from the vent in the event of serious overcharging.[4]

Environmental impact

Improper disposal of NiMH batteries poses less environmental hazard than that of NiCd because of the absence of toxic cadmium. However, mining and processing the various alternate metals that form the negative electrode may be expected to pose other types of environmental impact, depending on the metal, mining method, and environmental practices of the mine.

Most industrial nickel is recycled and not discarded in the trash can, due to the relatively easy retrieval of the metal from scrap, and due to its high value.

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References

  1. http://en.wikipedia.org/wiki/Nickel_metal_hydride_battery
  2. [1] |Procter & Gamble, Duracell NiMH charging methods, Ch. 6, 2009
  3. [2] |Radio Control Wiki, Nickel metal hydride battery, 2010
  4. Mukund R. Patel (2005), "Spacecraft Power Systems" CRC Press ISBN 9780849327865 Page 209

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