Characteristics of NiCd/NiMH Cells
NiCd or NiMH batteries are able store large amounts of energy in a very short time and give it back also within a short time which is a benefit. But they have disadvantages too. Especially NiCd batteries: if they are not fully discharged from time to time they tend to "remember" that they have been only partially discharged and the amount of energy they are able to store decreases more and more. At last the battery becomes unusable. This effect is known as "memory effect" and is due to a slow growing of crystals which comes along with a decrease of active surface in the cells. In the worst case coarse crystal structures can even lead to a short circuit: the cell is useless.
Another disadvantage is the high self-discharge rate. After a few weeks stored at room temperature without any load the cells are completely discharged and have to be recharged prior to further use. Newer NiMH-cell-types from differtent manufacturers have a dramatically reduced self-discharge rate. They are called "ready to use" and can be stored for at least one year without a total loss of charge. These cell-types are well suited for use in remote controls or even watches eg. However, the high self-discharge rate can also be quite useful (see Leveling NiCd/NiMH cells).
Summary: NiCd and NiMH batteries can be charged and discharged quickly. Due to their extreme low internal resistance they are able to deliver large currents. Therefor they are the appropriate choice for use in handheld tools for professionals (eg in the handicraft business) with high energy consumption. NiMH batteries have a higher (gravimetric and volumetric) energy density and a lifetime of roughly 300 charge-discharge cycles whereas high quality NiCd cells can be recharged up to 1000 times. However they contain the toxic heavy metal cadmium.
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