Data centers discover the value of LiFePO4 batteries

Data centers discover the value of LiFePO4 batteries

The advent of Li-ion battery packs helps data center managers move to a smaller, simpler, and greener standby-power source

BY RIAD NAKHLEH
Director of Product Solutions
Palladium Energy
www.palladiumenergy.com

International Data Corporation reports that the global annual data generation will reach 35 zettabytes (1021) by 2020. That’s 35 quadrillion gigabytes worth of emails, text messages, documents, images and videos per year, an astronomical number that is likely to only continue growing as data becomes ever more integral to people’s lives.

Computer technology corporations are preparing for this data explosion by building new data centers in strategic locations around the world. The designers of the servers, storage and switches that would go into these facilities are intent on creating a scalable infrastructure that delivers very high service levels at price points that are competitive even in the face of significant commoditization pressures. Consequently, some companies are exploring the feasibility of creating lithium iron phosphate (LiFePO4) powered battery backup units (BBU) to save space, improve emergency runtime by 40% and reduce maintenance costs without compromising data security or service affordability.

While sealed lead-acid (SLA) batteries remain the standard power source for approximately 50% of existing data center BBUs, their qualities, requirements and limitations are widely known. Most data center managers dislike them because SLAs are big, heavy, potentially dangerous, and full of toxic chemicals. They require frequent maintenance and even under ideal conditions have relatively short life spans. The advent of small but powerful lithium-ion battery packs has given data center managers hope that they can, at last, move to a smaller, simpler and greener standby-power source.

What are the benefits of a LiFePO4 BBU?

Longer power durationMore watts of powerImproved operating temperatures and dischargingReduced and simpler maintenance Flexible, scalable deployment architecture

Longer power duration A traditional SLA BBU is typically designed for a three-minute discharge event. However, most data center providers prefer BBUs that deliver five minutes of continuous standby power, providing the margin of safety they need for copying large data sets from cache memory to nonvolatile memory in the event of a power failure.

Creating a LiFePO4 battery pack assembly is actually quite achievable for qualified design teams. LiFePO4 is an energy dense type of battery that offers a high discharge rate, superior safety and long cycle life. LiFePO4s have a charge voltage three times greater than a nickel-cadmium cell and nearly 10 times the energy density of comparable SLA batteries.

More watts Not long ago, standard BBUs discharged 90 to 100 W over their expected runtime. More recent models have offered up to 170 watts.

Using LiFePO4 cells enables battery design engineers to provide the required 300 W in a compact 4S2P (four cells in series and two cells in parallel) form factor. Moreover, this basic design can be scaled up to deliver many more watts of standby power if, necessary.

Improved operating temperatures and discharging All batteries have an optimal temperature range. For lead-acid batteries, it is between 32 and 77F. Temperatures outside this range severely reduce battery life.

Because LiFePO4 uses a very stable cathode technology, the cells perform well when maintained between 50and 108F. It’s not unrealistic to expect a modular LiFePO4 battery pack will be able to deliver up to 500 discharge events over its projected lifetime under optimal temperatures. A comparable lead-acid BBU would probably sustain only 50 to 200 events before becoming unreliable and having to be replaced. This longer life cycle is one of the reasons that LiFePO4 technology is now a cost-competitive alternative to lead-acid batteries.

Reduced and simpler maintenance The lead-acid batteries in traditional BBUs are sealed, but not maintenance free. They must be routinely monitored using voltage checks, load tests and physical inspection. All common battery technologies are subject to sulfation, in which small sulfate crystals form on the battery plates if stored for long periods without recharging. However, LiFePO4 batteries are better able to resist sulfation under partial-charge conditions.

The phosphates component of the LiFePO4 cathode also means the battery can handle high temperatures as well as overcharge and short-circuit conditions quite well. The cells are unlikely to experience thermal runaway and the “memory effect” that affects some other technologies. LiFePO4s also do not require scheduled cycling to prolong service life. As long as the electronic drain is low, they can maintain a minimum state of charge for up to one year with no external power.

Flexible, scalable deployment architecture Data-center BBUs are traditionally located in centralized rooms. This configuration causes the connections to individual servers to be quite long, which can reduce the efficiency and necessitate larger standby-power systems. Some data center operators have started incorporating the BBU into the server rack. This distributed architecture increases efficiency by locating the battery close to the equipment it is supporting.

While a distributed BBU design is attractive from an efficiency standpoint, today’s data centers and battery providers are turning to a hybrid approach that would be more appropriate for a heterogeneous computing environment with different types of servers and data processing requirements. By configuring BBUs as a subset system located near clusters of server racks, providers are able to create a distributed, modular design that improves efficiency while allowing data center customers to use a variety of off-the-shelf servers.

Ensuring performance, safety

LiFePO4-powered BBUs can greatly improve stand-by power capabilities, but creating LiFePO4 battery packs that are safe and reliable requires skill, specialized knowledge and experience. While the cells are extremely safe, assembling multiple cells into a pack with sufficient power and runtime to operate a server requires careful design to deliver optimal performance. All battery packs, but especially LiFePO4 battery packs, need to be part of a properly designed system or they may rupture, ignite or explode when exposed to high temperatures, drops or other abuse.

However, design engineers can develop BBUs that include specific types of protection devices such as integrated circuits (ICs) for controlling battery cell voltage. ICs prevent cells from overcharging or over discharging by controlling a cutoff switch and monitoring voltage across the switch’s metal oxide semiconducting field effect transistor (MOSFET) structures.

Such safeguards are essential for preventing surges or sags in voltage that could damage downstream devices and the battery pack itself. An effective design also includes a thermistor that measures temperature and shuts down the pack if the temperature increases beyond a predetermined maximum.

It’s also important to keep in mind that a BBU is a smart battery solution, which means it should include specially designed recharging management technology for communicating with the battery pack and the recharging source. This technology is essential to determine when a recharge is needed and to protect the battery pack against overcharging. The overall system plays a vital role in promoting cell life and protecting against adverse conditions such as thermal runaway.

LiFePO4 designs offer a superior option for improving the efficiency and reliability of standby power in critical systems such as data center environments. If you want to consider using this exciting technology in your facility, be sure to partner with a company that has extensive experience designing battery-powered devices.

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