Emilie Stone is General Manager for Methode – Active Energy Solutions.
In 1859, Western Union installed one of the first lead-acid battery rooms to provide back-up power for its telegraph services. Since then, advances in power electronics efficiency, battery packaging (sealed versus valve-regulated), and capacity have been made, but the technology is fundamentally the same. The back-up battery and uninterruptible power supply (UPS) market is ripe for innovation. Lithium-ion batteries, with their high energy density, minimal maintenance, and low capacity fade, are poised to upend our existing assumptions about what a UPS is capable of doing.
The term “lithium-ion” is used to describe a class of batteries, typically with rechargeable/secondary cells and a lithium-based cathode. While the precise capabilities of each lithium-ion chemistry vary, they all perform better than lead acid. For example, a common lithium nickel-manganese-cobalt (Li-NMC) cell compared to a common VRLA (valve-regulated lead-acid) cell will have 150 percent of the energy density (W∙h/L) and 275 percent of the specific energy (Wh/kg), meaning it is smaller and lighter for a given capacity. For the data center, the benefit equates to a battery offering more power in a smaller footprint, freeing up valuable data center real estate.
In addition, Li-NMC exhibits 190 percent of the cycle life at a higher depth-of-discharge (80 percent versus 50 percent for VRLA), which translates into more useable capacity for the UPS. NMC cells are typically rated to a full-power operating environment of 40-45°C versus 25°C for VRLA. NMC batteries also maintain 92 percent efficiency at a 1C° discharge versus 60 percent for VRLA. This results in less waste heat in a rack.
There are, however, two major obstacles to widespread lithium-ion use: cost and safety. Just a few years ago, lithium-ion batteries were roughly four times the cost of lead-acid batteries. However, as cell production has increased to support the Electric Vehicle industry and consumer products, the cost will continue to fall. Lithium-ion production is projected to grow by 67 percent over the next five years.
Another cost driver for lithium-ion technologies is the use of a battery management system (BMS). While the primary function of the BMS is to maintain a safe operating environment for the cells, voltage, current and temperature, the BMS also provides invaluable insight into the state-of-charge and health of the battery – all key for making more informed decisions about if and when to replace the battery.
In addition to the BMS, safety technology within a lithium-ion cell, such as current-interrupt devices, positive temperature coefficient fuses, and vents are commonly implemented. Pack-level safeties such as fusing and thermal dissipation measures also guarantee a safe operating environment for the cells, making them a stable solution for data centers.
In a data center environment, these benefits translate into savings in space, weight and replacement that directly contribute to the bottom line. It is now possible to get 6kW of power in a rack in a 2U package, weighing less than 100lbs. Rather than employing a heavily cooled and reinforced battery room, the UPS can be deployed in the rack or end-of-row to offer flexibility and simple power runs. Higher cycle life means a lithium-ion UPS can last up to seven years without service. Combined with fast recharge time, this also means lithium-ion batteries can be used for non-traditional UPS functions like supplementing the grid to load balance to maintain power budget at the rack level. All of these compelling benefits are bringing lithium-ion batteries to the forefront of stationary storage applications and forging the path for their future in the data center.
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