NiMH Nickel Metal Hydride Battery Technology Overview

- 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.

This overview or tutorial about battery technology is split into several pages, each addressing a particular type of aspect of battery technology:

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 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, namely the positive electrode, the electrolyte and the negative electrode. These will be addressed separately below:

• 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.

NiMH charge / discharge characteristics

In operation the NiMH cell has many similar characteristics to the more familiar NiCd. It follows a very similar discharge curve to that of the NiCad allowing for the extra charge it can take. However it is very intolerant of overcharging, suffering a reduced capacity if this occurs. This presents a significant challenge to battery charger designers.

Many intelligent chargers for NiCds sense a small but distinct "bump" in the output voltage when a NiCad is fully charged. However for NiMH cells this increase is very much smaller, making it more difficult to detect. As a result the temperature of the cells is also detected as well, because once fully charged the cell dissipates much of the additional charge as heat. A further complication is that the characteristics of NiMH cells vary significantly from one manufacturer to the next making charge performance more difficult to detect.

Self discharge characteristics

One of the problems with NiMH cells is that they self-discharge over a relatively short period of time. All cells will loose their charge over time, even if they are not used, but this is a particular problem for NiMH cells.

Typically it might be expected that a fully charged cell might self discharge over a period of a few weeks. NiCds are better than NiMH cells but in turn they not as good as normal primary cells but NiCds will typically retain charge over several months dependent upon the type of battery or cell.

There are several factors that contribute to the self discharge of an NiMH cell dependent upon the state of charge. These can broadly be described as an oxygen cycle that occurs at high states of charge, and then ion movement that contributes to the self discharge over longer periods of time.

One important factor in the rate of self discharge is the temperature at which the cell is held. It is found that at higher temperatures, the rate of discharge significantly increases. Therefore cells should be kept cool if it is necessary for them to hold their charge over longer periods.

NiMH summary

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.

Further pages from this tutorial
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