Telecommunications
In recent years, NiMH batteries are being deployed to provide reliable backup power to load equipment located in a network environment of a typical telecommunications service provider.
The NiMH batteries are recommended for use in the Central Office (CO), Outside Plant (OSP), and at locations such as Controlled Environmental Vaults (CEVs), Electronic Equipment Enclosures (EEEs), huts, and in uncontrolled structures such as cabinets.
To ensure conformance and product safety, the telecommunications industry has accepted a three-level compliance system, which is described in GR-3168, Generic Requirements for Nickel Metal Hydride (NiMH) Battery Systems for Telecommunications Use. The compliance system provides a common framework for evaluating and qualifying various NiMH battery technologies. The framework intends to alleviate the complexities associated with product introduction and qualification.
The service life of NiMH batteries is extremely important in a telecommunications environment. The main failure modes of lead-acid batteries are very temperature sensitive. Grid corrosion and water loss are sped up as the battery temperature increases. The rate of battery degradation doubles with each 8 °C (15°F) rise in temperature. Thus, a battery designed to operate for 10 years at 25 °C (77°F) approximately would last only 5 years at 33 °C (92°F), and would last only 2.5 years at 42 °C (107°F).
Nickel–cadmium (NiCd) batteries fail in a different manner. NiCd batteries are susceptible to failures caused by short circuiting due to dissolution/crystallization reactions occurring at the negative electrode, which can result in dendrite growth of Cd creating a short to the positive plate.
NiMH batteries do not fail in the same way as NiCd or lead-acid batteries. NiMH batteries fail in two predominant modes that are somewhat interrelated. The metal hydride material used for the negative electrode undergoes gradual corrosion in a strong alkaline environment. This corrosion results in less negative active material for hydrogen storage and also consumes water from the electrolyte.
This results in a gradual loss of power as water is consumed, increasing the cell resistance and a gradual loss in capacity as active material is converted to corrosion products. By optimization of the alloy composition, this corrosion process can be controlled at very low rates. The rate of corrosion will be impacted by various factors including temperature, State of Charge (SoC), and control of overcharge and oxygen recombination. Studies under controlled overcharge conditions predict that the rate of battery degradation will double for approximately each 20 °C (36°F) rise in temperature. Thus, a battery designed to operate for 20 years at 25 °C (77°F) would last 10 years at 45 °C (113°F). Extrapolation beyond 45 °C is not linear since other failure modes, caused by decreasing charge acceptance resulting in positive electrode swelling and thermal instability, could control the battery life.
Read more about this topic: Low Self-discharge Ni MH Battery, Applications