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THE DATA CENTER JOURNAL | 13 www.datacenterjournal.com possible aer a power event so that they are ready for another cycle. Most solutions are designed to handle back- to-back events because recharging them can take several hours. Superca- pacitors can be fully charged in sec- onds, whereas lead-acid batteries can take six hours or more. Lithium-ion batteries can be charged in about three hours using standard charge rates. environmental concerns are leading to stricter worldwide environ- mental regulations that force many data-storage companies to use greener alternative components for their equipment. Supercapacitors are the friendliest to the environment, as they contain no hazardous materials. Lead- acid batteries are the least friendly because they contain lead. Safety is also important. Lithium-ion may be the least safe option, but any safety concerns can be mitigated using redundant electronics protection, good thermal and me- chanical designs, good cell selection, and robust manufacturing processes. Supercapacitors and lead-acid bat- teries are safe options that don't need additional circuitry to increase their safety. In unit cost per watt-hour (wh), lead-acid is the most cost ef- fective solution. Supercapacitors are the most expensive, but they are not meant to compete with high-energy batteries. In total cost of ownership (tco) per wh, lead-acid batter- ies tend to be more expensive even though they are initially much cheaper per unit than lithium-ion batteries. Factoring in the replace- ment cost of lead-acid batteries owing to their relatively short calendar life, high maintenance cost, large footprint and higher transportation cost owing to their size, they are not the overall lowest-cost technology. Table 1 summarizes the dif- ferences between the three backup options. Until utility companies can guarantee 100% power availability, the need for backup power solutions to prevent data loss will remain. e best type of backup solution will depend on the servers' load requirements. If short duration (seconds) is all that is required, supercapacitors are a great choice and are becoming popular owing to their high power density, short recharge time, "green" proper- ties, and long calendar and cycle life. When longer backup time is needed, lithium-ion batteries are becoming more popular than their lead-acid counterparts. n about the author: Ilyas Ayub is the Director of Application Engineering, Inventus Power iayub@inventuspower.com SEAlED lEAD AcID (SlA) lIthIum-Ion (lI-Ion) SuPErcAPAcItor Voltage 2V 3.2–3.7V ~2.5V charge time low (6 hours+) medium (1–3 hours) Very fast (10 seconds) Energy Density medium high Very low (1–5Wh/Kg) Power Density low medium Very high (5,000W/Kg+) cycle life low (fewer than 500) medium (500–2,000) Very high (100,000+) operational temperature range –20°c to +60°c –20°c to +60°c –40°c to +65°c maintenance upkeep high low Very low Environmental concerns Very high owing to lead low none Safety concerns low Extreme safety concerns. need robust electronics to avoid safety issues. none regulations low Very high owing to safety concerns low unit cost per Wh low medium Very high tco per Wh medium low Very high table 1. comparison of three backup technologies.