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Lithium batteries have been used for years to power cell phones, laptops, and electric vehicles, and are now coming into use in UPS applications. There are several types of lithium battery chemistries. "We use a lithium iron phosphate (LFP) chemistry in our batteries," says Tom Lynn, technical director for LiiON, LLC, a lithium battery manufacturer. "We found that LFP is the most effective lithium chemistry to use in UPS batteries."
(Note: LFP is the standard industry term for lithium iron phosphate battery chemistries. The acronym is based on its chemical formula: LiFePO4.)
• Cycle Life and Lifespan: Lithium iron phosphate batteries have a cycle life of 1,000 to 3,000 discharges, far more than lead acid batteries. Manufacturers estimate their lifespan at 10 to 15 years.
• Discharge Rate and Recharge Time: A lithium iron phosphate battery is capable of a 2-minute (30C) discharge rate, which makes it highly effective in delivering power to UPS systems. Also, an LFP battery can be recharged in about 2 hours.
"The high-voltage capability of lithium iron phosphate batteries is better than lead acid batteries," says Lynn. "What's more, lithium batteries can sustain that high voltage discharge a lot longer over the course of their lifetime. Lead acid batteries tend to lose their ability to provide a high discharge rate the more times you use them."
• Size and Weight: Lithium is the lightest metal, and lithium batteries weigh 60 to 80 percent less than lead acid batteries. Additionally, lithium ion batteries are smaller in size, and provide a 50 percent savings in floor space.
"Since lithium batteries are lighter and smaller, they are easier to install and move around in the data center," says Lynn. "Also, you don't have to reinforce the floor to handle extra weight, as you sometimes do with lead acid batteries."
• Cooling Requirements: Lithium-ion batteries can operate at temperatures of 86 F (30 C), nearly 10 degrees higher than most lead acid batteries. This can provide a savings in cooling costs, since data center owners can raise temperatures in their facilities.
• Maintenance: Lithium batteries require maintenance once a year, but do not require quarterly testing like lead acid batteries. The built-in battery management system makes it easier to monitor the health and performance of lithium batteries.
• Initial Cost vs. Total Cost of Ownership: Lithium battery systems currently cost 1.2 to 1.5 times more than lead acid batteries, but first cost isn’t the whole story.
"Lithium batteries may last for the lifetime of your UPS system," says Lynn. He contends that savings in replacement costs, labor and maintenance and energy savings from higher cooling temperatures provide a total-cost-of ownership savings that justifies the initial cost.
Since lithium batteries are new to UPS applications, their exact lifespan and total cost of ownership have yet to be determined.
• Safety: In the past, there have been safety concerns over lithium batteries. However, the stability of the battery depends on the chemistry being used.
"Lithium iron phosphate has a thermal runaway temperature of 518 F (270 C), which is much higher than other lithium or lead acid chemistries," says Lynn. "It can produce a high discharge while generating very little heat. If the battery casing is pierced, the LFP chemistry bond will not break down and release oxygen, producing an explosion or fire."
UPS Energy Storage Option 2: Lithium-Ion Batteries