NiMH Nickel Metal Hydride Battery Technology

- information, tutorial about the basics of nickel metal hydride, NiMH battery technology giving an overview of how NiMH cells work, how to use these batteries to their best and a view of their applications.

Nickel Metal Hydride, NiMH batteries and cells have come into widespread use in recent years as a viable form of rechargeable battery. These NiMH cells offer almost identical characteristics to those provided by the older NiCad technology, but with the advantage that the NiMH cells do not have the same adverse environmental effects, and they are also able to provide a slightly higher level of energy density and therefore overall charge capacity. As a result, NiMH cells are now widely used, offering high levels of performance.

NiMH battery overview

NiMH batteries and cells are now widely used. They have a number of highlight characteristics that have enabled them to be used, although they do have some disadvantages:

  • Environmental impact:   NiMH battery technology has overtaken that of its NiCd brother because of their lower environmental impact. The use of cadmium is of particular concern, and therefore the use. In the European Union, EU, legislation referred to as the Battery Directive has required the use of NiCd batteries to be terminated for portable use by consumers.

    While the use of toxic cadmium is removed from the NiMH cells the mining and processing of the other metals used poses some environmental threats. Fortunately when the NiMH batteries reach end of life, most of the nickel can be recovered with relative ease.
  • Specific energy density:   NiMH cells and batteries have a higher level of specific energy, i.e. the amount of energy that can be contained within a certain volume of weight. It can be between 60 - 100 W h / kg against 40 - 60 for a NiCd.
  • Output voltage:   One particularly fortunate parameter is that they provide a cell voltage of 1.2 volts which is very similar to that of a NiCd, making them an almost direct replacement.
  • Self discharge :   On disadvantage of the NiMH cell is that it has a high rate of self-discharge. They can lose up to 3% of its charge per week of storage.

NiMH cell basics

The NiMH cell bears many similarities to the older NiCd cell technology, using many similar constituents. The NiMH cell consists of three main elements:

  • Positive electrode:   The positive electrode of the NiMH battery is nickel hydroxide having the same composition as the positive electrode in a NiCd cell. The nickel oxide - hydroxide electrode only exchanges a proton in the charge-discharge reaction, and this results in a very small change in size, resulting in a high level of mechanical stability, and this in turn results in a longer cycle life.
  • Electrolyte:   The electrolyte in the NiMH cell is an aqueous solution of potassium hydroxide (KOH) which has a very high conductivity. The solution does not enter into the NiMH cell reaction to any significant extent. It is found that the electrolyte concentration remains almost constant over the charge / discharge cycle. This is important because the concentration of the electrolyte is the main contributor to the cell resistance. This means that the performance of the cell remains almost the same over the entire charge range.
  • Negative electrode:   The active material for the negative electrode is actually hydrogen. However it is not physically possible to use hydrogen directly and therefore the hydrogen is stored in the NiMH cell as a metal hydride which also serves as the negative electrode. As a point of interest, the metal hydrides used in NiMH cells can normally hold between 1% and 2% hydrogen by weight.

With many new battery and cell technologies being developed, NiMH technology is looked upon as an interim solution by many. NiMH cells have the advantage that they offer very similar characteristics to NiCads and in that way they can be used as a direct replacement. For the longer term, Li-ion batteries are finding widespread use, but despite this the use of NiMH batteries and cells is still widespread.

By Ian Poole

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