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depend on the chemistry and technology . The chemistry refers to the elements that result in a chemical reaction that charges and discharges the cell .
The chemistry determines the cell voltage and the technology refers to other design characteristics that ultimately determine the amount of energy ( watt-hours ), power ( watts ), energy density ( watt- hours / kg ), power density ( watts / kg ), service life , impact of temperature , stability , and a host of other characteristics .
UPS applications require batteries that can provide a large amount of power capacity for 5-10 minutes . Therefore , UPSs require technologies that can supply a large amount of current in a short amount of time whilst maintaining a safe internal cell temperature . When compared to lead-acid chemistry , Li-ion provides higher energy and power per unit weight , typically referred to as energy density ( Wh / kg ) and power density ( W / kg ). Another key distinction between Li-ion and VRLA is how much of the batteries ’ energy capacity remains after the 5-10 minutes of runtime .
A power cell is designed to provide a relatively large amount of power in a short amount of time while using nearly all of the battery ’ s energy capacity . In a UPS application for example , a power battery solution could provide one to two minutes of runtime at full load while discharging about 80 per cent of the battery energy capacity .
An energy cell however , is designed to provide a relatively small amount of power over a long period of time . In a UPS application , an energy battery solution could provide the same amount of power over the same amount of time detailed above , but will only discharge a 10-30 per cent of the battery ’ s energy capacity .
This means that for this application , an energy battery solution is oversized ( in energy ) and will likely provide much more runtime than required . Depending on the price of the energy cell relative to the price of the power cell , it could be less expensive to use an oversized energy battery solution in a UPS application , rather than a right-sized power battery solution .
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A key conclusion is that Liion batteries can be designed as power cells or energy cells . Conversely , VRLA battery chemistry and technology limit their design solely as energy cells .
How long a battery lasts before you need to replace it is what really matters when it comes to battery lifetime . However , it ’ s important to understand the different metrics suppliers use to measure lifetime . Of particular importance is service life . This is the estimated time a battery will last before it reaches 80 per cent of its energy capacity , the typical definition of end of life for batteries .
Service life assumes the battery is operating under ‘ real world ’ conditions for a stated application and is therefore highly variable . In contrast , calendar life is the estimated time a battery will last if it were to remain trickle charged for its entire life with no power outages at a specified temperature , usually 25 ° C ( 77 ° F ).
VRLA batteries have a service life in the range of three to six years whereas Li-ion batteries can have a service life upwards of 10 years ( estimated using accelerated life testing ). Note that it will be several years before data on actual service life becomes available for newer Li-ion batteries , however , some Li-ion batteries offer warranties in the 10-year range as a hedge against the lack of service data .
Due to the higher energy densities of Li-ion batteries , they are much smaller in terms of footprint or volume compared to VRLA . This space saving is especially attractive to colocation data centres or facilities with higher real estate costs .
Similar to footprint , the higher energy density of Li-ion also contributes to its lighter weight compared to VRLA and inevitably , weight will contribute to increased transportation costs . The most important thing to remember about UPS applications is that providers are required to work closely with reputable Li-ion vendors to ensure safety is at the forefront when it comes to Li-ion batteries . They must find the best combination of chemistry , technology , cell packaging , and
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‘ Due to the higher energy densities of Li-ion batteries , they are much smaller in terms of footprint or volume compared to VRLA .’ |
battery management for specific data centre use .
There are various regulations around shipping any kind of battery including Li-ion or VRLA . These shipping regulations tend to be stricter with Li-ion chemistries due to the higher energy densities and higher volatility of certain chemistries .
Using total cost of ownership ( TCO ) as a metric is gaining traction for certain data centre investments , namely those such as cooling economiser modes and UPS batteries . In the case of Li-ion , certain chemistries and technologies present a favourable TCO over a 10 to 15 year period when compared to VRLA batteries . This happens to be the typical life span range of a UPS before replacement is needed .
For example , the operational battery expenses start at year one and continue till year 10 . Battery maintenance , space lease , and energy costs are incurred every year , while battery refresh costs are incurred at year four and eight .
Our 10-year TCO analysis considers the capital and operational expenses outlined here , and revealed that the Li-ion battery solution had a 39 per cent lower 10-year TCO than the VRLA . This results in a simple payback of 3.4 years to break even from the higher Li-ion capital investment .
In short , large UPS systems utilising Li-ion batteries will address all the disadvantages of traditional batteries , namely footprint , lifetime maintenance , and cooling requirements . And we expect the performance of li-Ion batteries to improve and costs to steadily decrease .
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18 | February 2017 |