Research of the NiMH system started in the seventies as a means for hydrogen storage for a Nickel Hydrogen battery. The metal hydride alloys were unstable in the cell environment and the desired performance characteristics could not be achieved. As a result, the development of the NiMH slowed down. New hydride alloys were developed in the 1980’s that were stable enough for use in a cell. Since the late eighties, the NiMH has steadily improved, mainly in terms of energy density. Design engineers have indicated that the NiMH has a potential of yet higher energy densities.
Some of the distinct advantages of today’s NiMH are:
30% more capacity over a standard NiCd.
Less prone to memory than the NiCd. Periodic exercise cycles need to be done less often.
Fewer toxic metals. The NiMH is currently labeled "environmentally friendly".
Unfortunately, the NiMH also exhibits some negative attributes and in some aspects lags behind the NiCd. For example:
Number of cycles: The NiMH is rated for only 500 charge/discharge cycles. Shallow rather than deep discharge cycles are preferred. The battery’s longevity is directly related to the depth of discharge.
Fast charge: The NiMH generates considerably more heat during charge and requires a more complex algorithm for full-charge detection than the NiCd if temperature sensing is not available. (Most NiMH batteries are equipped with internal temperature sensing to assist full-charge detection). In addition, the NiMH cannot accept as fast a charge as the NiCd; its charge time is typically double that of the NiCd. The trickle charge must be controlled more carefully than on the NiCd.
Discharge current: The recommended discharge current of the NiMH is considerably less than that of the NiCd. For best results, manufacturers recommend a load current of 0.2C to 0.5C (one-fifth to one-half of the rated capacity, see 3.1 C-Rate). This shortcoming may not be critical if the required load current is low. For applications requiring high power or a pulsed load, such as on GSM digital cellular phones, portable transceivers and power tools, the more rugged NiCd is the recommended choice.
Self-discharge: Both NiMH and NiCd are affected by reasonably high self-discharge . The NiCd loses about 10% of its capacity within the first 24 hours, after which the self-discharge settles to about 10% per month. The self-discharge of the NiMH is one-and-a-half to two times higher than that of the NiCd. Selecting hydride materials that improve hydrogen bonding to reduce self-discharge typically also decrease the battery capacity.
Capacity: The NiMH delivers about 30% more capacity than a NiCd of the same size. The comparison is made with the standard, rather than new ultra-high capacity NiCd. Some ultra-high capacity NiCd cells provide a capacity level approaching that of the NiMH. (Ultra-high-capacity NiCd batteries cannot provide as high a load current as standard NiCd batteries. They are also less durable in terms of cycle-count but are longer lasting than NiMH batteries).
Price: The price of the NiMH is about 30% higher than that of the NiCd. Price may not be a major issue if the user requires high capacity and small size. In comparison, ultra-high capacity NiCd cells are only slightly higher priced than standard NiCd cells. Capacity for cost, the ultra-high capacity NiCd is more economical than the NiMH.