The Better Server Rack Battery Backup For DC Power Distribution
DC power distribution opens the door to rack-level battery backup. Which battery chemistry should you invite in?
The hyperscale providers are leading the adoption of DC distribution at the rack level, rather than a pure AC power distribution architecture. The economic drivers are straightforward: save space and save power.
First, with DC power distribution, data center operators are able to reduce or eliminate the massive central UPS systems and their associated battery strings in favor of in-the-rack AC/DC conversion power supplies and high power density batteries in the rack. This saves space as, once AC power is rectified, DC power distribution has a limited need for additional rectifiers or transformers.
Second, about half of the power supplied to the traditional data center is lost in power conversion and distribution as well as managing the heat from these losses and the IT equipment. DC power distribution improves energy efficiency through a reduction in power conversions, each of which loses energy, mostly as heat. These resulting energy savings reduce costs and the site’s carbon footprint.
“Battery backup units installed within each rack of IT hardware can improve facility reliability because an issue with any one BBU only affects a single rack.”
Now, with DC distribution, backup power can be provided by battery backup units (BBUs) installed within each rack of IT hardware. This approach can improve facility reliability because an issue with any one BBU only affects a single rack, while a centralized UPS approach can impact a variety of downstream equipment. To implement rack-level BBUs, many data center operators have the Open Compute Project (OCP) Open Rack V3 (ORV3) BBU specification to use for guidance.
Rack-level battery backup
The trick is that BBUs are facing stiff competition for space within the server racks. The drive towards digital transformation, and its associated requirement for data analysis applications including artificial intelligence, machine learning and deep learning, have sharply increased demand for computing power and density. Dell, for example, expects that future rack densities will far exceed those of today, while hosting hardware components such as CPUs/GPUs that consume over 300W each.
So, at the rack level, BBUs may be seen as ‘necessary overhead,’ denying space to income-producing IT hardware. As rack power density increases, so does the pressure to reduce BBU footprint.
This is the argument for power density.
The safe high-density option
Up until OCP and DC distribution, the industry has relied on lead-acid backup batteries for data center backup. This familiar technology was adequate when the UPS facility was separate from the server racks, and overall data center backup requirements were less rigorous.
Today’s increasing demands for higher power and smaller footprint are making newer technologies, especially lithium-ion and nickel-zinc (NiZn), more competitive. These alternatives have higher energy and power density than lead-acid batteries, the first key advantages for these battery technologies in rack-level backup.
“Power density, safety and reliability advantages are important in any part of the data center, but especially in the rack.”
The close proximity of these dense batteries to staff and servers highlights a key difference between lithium-ion and nickel-zinc: safety. While some operators have considered lithium-ion as an alternative to lead-acid, many are concerned about reports of indoor thermal runaway events at utility energy storage system (ESS) facilities.
With its demonstrated lack of thermal runaway potential even at the cell level, NiZn is an inherently safer chemistry to deploy in the rack. For a quick explanation of how to evaluate the relative safety of different battery chemistries, see our blog post Ensuring ESS Safety in Data Centers with NFPA 855 – Part 2.
Reliability at the rack level
We mentioned earlier how rack-level BBUs can have a positive impact on data center reliability. This highlights another important distinction between NiZn and other battery chemistries.
Reliability is in large part a function of battery string operation. When a lead-acid or lithium-ion battery cell fails, it creates a high impedance or an open circuit that halts string operation. A weak or depleted NiZn cell, on the other hand, remains conductive, allowing the string to continue operating.
There’s more. NiZn battery strings tolerate string imbalances better and maintain constant power output at significantly lower states of charge and health than the other technologies. And they have the widest operating temperature range of the three BBU options, and so are more tolerant of the higher temperatures inside the server rack.
These power density, safety and reliability advantages are important in any part of the data center, but especially in the rack.
More sustainable too, so learn more
While we are comparing chemistries, it’s also important to consider sustainability, an increasingly important criterion for data center operations. NiZn chemistry provides environmental impact advantages over both lead-acid and lithium-ion batteries. You can see an independent analysis of the options in the blog post Comparing the Climate Impact of Batteries on Data Center Sustainability.
If you are considering DC power distribution in your data center, contact ZincFive to to discuss how nickel-zinc batteries can provide rack-level backup, safely and reliably.
 DC distribution is not just for the giants https://www.datacenterdynamics.com/en/analysis/dc-distribution-is-not-just-for-the-giants/
 Direct current in the data center: are we there yet? https://www.abb-conversations.com/2020/01/dc-in-the-data-center-are-we-there-yet/
 OpenRack/SpecsAndDesigns https://www.opencompute.org/wiki/Open_Rack/SpecsAndDesigns
 Dell EMC’s 2020 Server Trends & Observations, page 13