With so much emphasis today on electric cars – the development of which will be encouraged in the US by the Obama administration seeking to rid the country of its dependence on oil – the
spotlight has fallen on battery technology.
This is already having spin-off benefits for the IT industry and can be seen in the rapid development and increasing sophistication of batteries that power uninterruptible power supply (UPS) systems.
One of the fastest developing technologies surrounds the rechargeable nickel metal hydride (NiMH) battery. NiMH has been proven for some time in applications in laptops, cellphones and even space shuttles. Now the technology is proving to be popular in HEV (hybrid electric vehicle) applications as well as UPS systems.
Sharing some similarities with nickel cadmium (NiCd) batteries, NiMH has notable differences and features that are attractive to both automotive and UPS system designers alike, the most significant of which is the NiMH battery’s virtually unlimited cycling capability. This is in stark contrast to lead acid battery products which have inadequate cycling capacities – particularly in light of the demands being made of them in modern-day applications.
From a technical perspective, the components of NiMH batteries include a cathode of nickel-hydroxide, an anode of hydrogen absorbing alloys and a potassium-hydroxide electrolyte. An NiMH cell can have two to three times the capacity of an equivalent sized NiCd and is smaller and lighter than the VRLA (valve-regulated lead acid) battery equivalent.
NiMH batteries are also viewed as safer without any toxic substances to be concerned about – such as lead, acid, cadmium, mercury or hydrogen evolution common to other battery types. Therefore, no special or separate battery room is needed. NiMH also has a higher temperature tolerance than lead acid; this allows the product to be paired with a non-airconditioned (but properly ventilated) UPS module and the pair can be rated at up to 40°C if required.
In common with VLRA batteries, NiMH batteries are sealed, however they do not exhibit the type of unpredictability or early open circuit failure rates common to VRLA cells. This is directly attributable to the nickel – alkaline chemistry involved. The combination of rare earth-based hydride alloy and nickel/metal compounds greatly limits electrode growth during charge-recharge functions and allows the use of a steel casing for strength. The result is a long, stable life even at the highest cycle rates and elevated temperatures.
Reliability is one of the NiMH battery’s strong suits, as is the predictability of its cells. Ten-year ‘full replacement’ warranties are becoming commonplace. No lead acid product – not even the highly regarded flooded lead acid cells – can match this level of longevity.
Significantly, for installation in often crowed data centres where space is at a premium, a NiMH battery pack can occupy up to 80% less space then a corresponding VLRA battery and weigh between 10% and 25% less. This is despite the fact that the NiMH’s individual cell voltage is only 1,2 V and many cells are required to make up high voltage batteries.
The design of NiMH batteries has progressed dramatically in the last three years. Up until 2005, NiMH cells historically had a somewhat higher self-discharge rate (equivalent to internal leakage) than NiCd cells. This used to stabilise at around 1% per day depending on room temperature. In 2005 a new type of cell was introduced that reduced self-discharge, giving NiMH batteries the ability to retain up to 85% of their capacity after one year – again dependent on temperature – without being subject to a charge.
For more information contact Philip Hampton, Powermode, +27 (0)11 235 7750, philiph@powermode.co.za, www.powermode.co.za
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