The nickel metal hydride battery (NiMH) is a type of rechargeable battery using nickel hydroxide and a hydrogen-absorbing alloy as the active chemicals.

With portable electronics becoming more sophisticated and compact, there has been an increasing demand for batteries to deliver higher levels of energy. Research on NiMH began in the 1970s to meet this requirement. By the end of the 1980s the battery technology was developed and has been since been steadily improving.

Nickel metal hydride batteries have a similar operating voltage as NiCd batteries, but have about twice the energy density. These batteries are less prone to memory as compared to NiCd batteries. The biggest advantage NiMH batteries have to offer over NiCd batteries is that they are environment friendly. NiMH batteries, however, have a limited service life and have a high self-discharge. Their performance starts to deteriorate after 200-300 deep cycles and self-discharges around 50 percent more than NiCd batteries.

Examples of NiMH batteries include two way radio batteries, barcode scanner batteries, and barcode printer batteries.

Applications
Like NiCd dry cells, NiMH cells can also be used either individually or assembled into battery packs.

NiMH batteries find their use as rechargeable batteries in portable equipments like two-way radios, barcode scanners, barcode printers, and specialty phones. They are also used in hybrid vehicles, handheld televisions and prototype robots. These batteries perform better with moderate drain devices like digital cameras and flashlights.

How NiMH Battery Works
Like all batteries, a NiMH battery contains an anode, cathode and an electrolyte. In this case, the cathode is made of nickel hydroxide, the anode is a hydrogen-absorbing alloy and the electrolyte is potassium hydroxide.

Hydrogen is one of the best anode materials for battery cells, but is problematic as a material for batteries. The discovery of hydrides, metal alloys with the ability to store atomic hydrogen 1000 times their own volume, solved the problem.

Hydrogen moves from the cathode to the anode during charge and in the opposite direction during discharge. The electrolyte takes no part in the chemical reaction. Its sole purpose is to serve as a medium for transfer of hydrogen between the two electrodes.

Voltage
The operating voltage of NiMH batteries is similar to that of NiCd batteries, i.e. 1.2 volts. The voltage, however, remains relatively flat throughout the discharge cycle.

Nickel metal hydride cells can be assembled in a series, parallel or a mixture of both to obtain a higher terminal voltage, power density or capacity.

For example, a 12 V, NiMH battery pack would be made up of 10 battery cells connected in a series.

Charging Method
Nickel metal hydride batteries should be rapid charged at 1C rate rather than slow charged. It is advisable that NiMH batteries are charged with chargers especially designed for them to avoid overcharging, which can damage the batteries. Most new NiMH chargers use a combination of voltage plateau, NVD, rate-of-temperature-increase, temperature threshold and timeout timers to terminate the fast charge. The amount of trickle charge applied to maintain full charge is critical and care should be taken not to overcharge the batteries.

New NiMH batteries should be slow trickle charged for around 24 hours in the first charge. When in storage, the cells within a battery pack self-discharge to different capacity levels. The initial slow trickle charge helps to bring all the cells to an equal charge level.

Like NiCd batteries, NiMH batteries also develop “memory” when charged without fully being discharged or when overcharged. The development of memory can reduce the charge carrying capacity of these batteries. Special care needs to be taken while recharging these batteries.

Nickel metal hydride batteries should be charged in room temperatures between 120°F and 40°F. At temperatures above or below this range, their charge accepting capacity gets highly reduced.

Problems with NiMH
NiMH batteries have a limited service life. Their performance starts to deteriorate after around 300 service cycles.

These batteries also have a relatively high self-discharge rate. They have around 50 percent higher self-discharge than NiCd batteries.

As with other nickel-based batteries, nickel metal hydride batteries are also susceptible to “memory effect” and needs periodic maintenance for optimal performance.

Advantages
Nickel metal hydride batteries have a higher charge carrying capacity as compared to NiCd batteries. These batteries are ideal for high drain devices like two way radios and laptops.

Even though NiMH batteries are prone to memory, their susceptibility to this battery condition is less as compared to NiCd batteries and require less frequent maintenance.

One of the major advantages of NiMH batteries is that they are environment friendly. The electrolyte inside these batteries is a mild toxin and poses a threat only in large quantities. It is also profitable to recycle NiMH batteries.

Maintenance Tips for NiMH Batteries:

• Never leave a NiMH battery in the charger for more than 48 hours.


• Use high quality battery chargers.


• Slow trickle charge a new battery in its first charge.


• Charge in room temperatures between 120°F and 40°F.


• Do not recharge if not fully discharged.


• Never leave the battery in the charger after being fully charged.


• Store in cool and dry locations.


• Discharge to about 50% of their capacity before storage.