Runtime Optimization: As Data Centers Reduce UPS Runtimes, The Right Batteries Become More Critical

Executive Summary

When utility power goes down, data center Uninterruptible Power Supply (UPS) systems immediately switch over to backup batteries, which provide power to the facility until the generators come online. This transitional period is known as “battery runtime” or “ride-through time.” In the past decade, data center designs have continued to reduce battery runtime.

The reduction in battery runtime provides an opportunity for data center owners to re-examine the batteries they use for backup power in their facilities. Due to limits in their chemistries, traditional lead acid and lithium-ion batteries cannot be optimized for runtimes of less than 5 minutes. In either case, data centers must oversize their battery bank, buying more batteries than they actually need in order to provide adequate short-term power.

As data center owners adopt data center designs with faster automatic failover from UPS to generator, they should invest in a battery that:

1. Can be optimally sized for runtimes of 5 minutes or less;
2. Provides a small footprint with reliable and safe short-term power burst;
3. Offers total cost of ownership (TCO) savings, by requiring a smaller number of batteries to support UPS designs with shorter runtimes.

Nickel-zinc (NiZn) battery technology offers all these advantages and is a smart choice for the future of data center backup power as UPS technologies continue to improve.

The Industry Moves Towards Shorter Runtimes

An Uninterruptible Power Supply (UPS) system serves as part of the fail-safe power mechanism for a data center. Should the utility power experience an outage, the backup power system seamlessly transitions from UPS battery backup to longer duration backup generators. This ensures business continuity for servers housed at that facility, and prevents costly downtime, loss of revenue, and potential damage to the company’s reputation.

In the past decade, automatic failover in data centers has gotten faster and smoother, and battery runtimes have been significantly reduced. Ten years ago, a 30-minute runtime was considered normal. Five years ago, a 15-minute runtime was expected. Today, many data centers are operating with a 5 minute runtime and new designs are focusing on switching their facilities over to generator power in 3 minutes or less.

The Trend in Reduction of Battery Runtime is Due to Several Factors:

Improvements in generator technology – Today’s generators are able to start up faster. When a power outage hits, the generators can usually start up and come online in under 50 seconds. When the UPS sees that the main input has been transferred to the generators, they slowly transition the facilities load over from the batteries to the generator typically over a 10 second ramping period known as the walk-in period. In some facilities, this total startup and transition to generator happens within 30 to 45 seconds.

Improvements in power delivery architecture – Generators may not be the site’s only backup power source and may not even be the first option during automatic failover. For example, if a facility has dual power feeds and one utility feed goes down, the UPS will automatically switch the facility over to the second utility feed. The backup batteries may still provide power to the facility during this switchover which only typically requires around 5 seconds to make the transfer.

Some corporations have achieved multiple-site redundancy by setting up data centers in several cities, with servers running synchronized applications. For example, if a Denver data center which is hosting eCommerce applications goes down, the company’s IT systems will automatically switch customers over to those same applications hosted on servers in their Atlanta and Phoenix data centers.

Multiple-site redundancy doesn’t directly affect UPS failover times. The switchover of applications usually happens in less than 1 minute – but in this case, the switchover is software-driven. Still, having data centers in several cities means that a business will not be interrupted if one facility goes down. This gives a company peace of mind since it lowers the risk of losing business due to a data center outage.

Even with faster automatic failover and reduced battery runtime, UPS systems are as reliable as ever in providing continuous uptime to data center facilities with reliability reaching 99.99999%. In fact, some data center owners are now adopting new and more robust automatic failover plans to accommodate faster turnover. They now expect their generators to come online faster, and count on the UPS to reliably transfer the data center’s load to generator power in less than 1 minute.

With data center design and construction teams adopting these new and more robust failover plans and reducing their battery runtimes, it creates a highly competitive landscape and is causing other providers to reevaluate their runtime requirements to better optimize their UPS and battery systems. They have an opportunity to choose a battery solution that supports this inevitable move to shorter runtimes.

A few key qualities they should consider include:

1. Reliability and safety during short-term, high-power discharge.
2. High-density power delivery that enables right-sizing of battery loads for data center UPS systems with runtimes moving below 5 minutes.
3. Total Cost of Ownership (TCO) savings.

Choosing a Reliable Battery for Shorter UPS Runtimes

There are batteries available today that offer reliable power delivery for UPS systems for shorter runtimes. Due to output current limitations, a majority of these battery solutions have to be oversized to meet the UPS power requirements. Nickel-zinc batteries are uniquely capable of safely supporting extremely high-power discharges in very small footprints, consequently reducing the UPS room’s overall space requirements and the quantity of cabinets necessary for installation.

The right choice of a high-power battery solution for short runtimes can simplify the site installation and reduce the site risk, provided that it is a safe and reliable solution.

As seen in Figure 1 in a lead acid or lithium-ion battery string, failed cells are typically accompanied by a steep rise in internal resistance which will block the current flow from surrounding cells when the battery string is expected to provide runtime for an outage. This impedance block will create an open circuit, and subsequent failure to discharge. A single bad battery cell can bring down an entire string. Data center owners usually try to compensate for this by adding additional redundant battery strings – which increases the TCO of the battery system.

Contrary to the other chemistries, a weak or depleted nickel-zinc cell will see a reduction in internal impedance and, instead of creating an open circuit, will remain conductive and allow the battery string to discharge. This means that even with a weak or depleted cell the nickel-zinc battery system will continue to deliver runtime, reducing the need for redundant battery strings required by other chemistries. That is inherent reliability at the electrochemical level.

Nickel-Zinc String Discharge Reliability

Figure 1: Lead-Acid or Lithium-Ion vs. Nickel-Zinc String Discharge Reliability

Choosing a Safe Battery for Short-Duration Discharge

It is possible to choose a battery that not only delivers a reliable electric charge for a short-duration runtime, but also delivers that charge safely.

Nickel-zinc batteries are alkaline batteries. Unlike lead acid and lithium-ion batteries, the nickel-zinc battery has zero risk of thermal runaway. This gives the battery several advantages when it comes to safety:

1. A nickel-zinc battery can provide a high-power, short-term discharge without the risk of thermal runaway. This makes the battery a safe option for UPS systems with shorter runtimes.
2. In the case of a bad battery cell, the nickel-zinc battery can still provide a safe, sustained discharge, thanks to the battery’s electrochemistry.
3. The nickel-zinc battery can provide high levels of current without posing a safety risk.

Lithium chemistries specifically rely on Battery Management Systems (BMS) to ensure the cells are functioning and to prevent any safety risk that could occur. The risk this poses to the reliability of the lithium battery system is that if a single battery cell is determined to be out of tolerance, the BMS must protect the battery string and disconnect itself from the system as not to cause any adverse conditions or potential safety concerns. If this happens and the battery system is disconnected by the BMS, that means no runtime for the data center when it is needed most.

Unlike with other types of batteries, the BMS for nickel-zinc batteries does not automatically shut down a battery string or prevent the battery from discharging for any reason.

Nickel-zinc batteries do not require these controls as they are safe at the cell level. These systems do not have discharge parameters that are required to be managed by a BMS as there is no safety risk or thermal runaway concerns with this chemistry. This means the battery system can provide runtime for the data center without any concern of the BMS preventing a discharge.

Changing workloads and corporate priorities have caused data center builders and operators to take a closer look at UPS runtimes. With the explosive market growth in AI comes different and often reduced backup power requirements. Meanwhile, larger facilities mean the difference between 3 minutes and 7 minutes of battery runtime likely doesn’t impact the end result if a generator fails to start. Focusing specifically on achieving enough runtime to ride through generator starting and load transfers allows our data center clients to reduce their capex, ease their supply chain burden, and reduce equipment footprint, allowing them to maximize the sellable space in their facilities.”

Gary Russinko, PE & Managing Principal,
kW Mission Critical Engineering

Right-Sizing Battery Banks for Faster UPS Runtimes

Right sizing your battery bank begins with selecting a battery that can be optimized for shorter battery runtimes. Data center designers and operators are reducing their runtime requirement to less than 5 minutes, with some targeting 3 minutes at 10 years as an optimized solution.

Let’s look at a common industry example of a 1MW UPS design for a side-by-side comparison. In this example, the data center can switch over to generator power in 1 minute or less, and because of this, it is determined that they only require a 3-minute (at 10 years) battery runtime design.

Figure 2 shows how this setup might work using different types of batteries.

Most data centers today use either traditional lead acid or lithium-ion batteries as their backup power source but these batteries may not be the best option for UPS systems with faster automatic failover. Due to limits in their battery chemistries, lead acid and lithium-ion batteries may not be optimized for UPS runtimes under 5 minutes whereas nickel-zinc solutions can.

Lead acid batteries have a high discharge rate but a low energy density. This means you need a larger number of batteries to handle your data center load.

Lithium-ion battery cabinets, on the other hand, have a higher energy density but have a lower discharge rate. This is due to limitations in maximum current and the safety risk an overcurrent scenario poses to thermal runaway risks. If the discharge current goes above this limit, the BMS that controls the lithium-ion batteries will shut the cabinet down. These overcurrent protection limits increase the risk of cutting off the system in the middle of battery runtime, preventing the system from providing the high-power discharge required to support the critical load.

In either case, with lead acid or lithium-ion batteries, you have to oversize your battery bank to reach your target load of 1 MW and provide a sustained, short-duration uninterrupted, high-power discharge. This means the data center must buy a larger number of battery cabinets to serve a UPS system with a runtime of less than 3 minutes.

To achieve 1 MW power requirements for 3 minutes at 10 years you would need:

Figure 2: Nickel-Zinc vs. Lithium-Ion vs. Lead Acid Battery Cabinet Footprint

On the other hand, nickel-zinc batteries offer three times the power density of lead acid, two times the current carrying capability of industry leading lithium-ion batteries, and a BMS that is passive during a discharge and will not interrupt the operation as there is no safety risk to do so.

Using nickel-zinc batteries, data centers can safely optimize their battery load for runtimes of 5 minutes or less. In the example above they would only need 3 cabinets of nickel-zinc batteries to reach 1 MW target load, and provide a 3-minute short-term, high-power discharge.

Many data centers are selecting runtimes less than 5 minutes to optimize cost and minimize footprint. These customers have realized long ago that 5 minutes of UPS battery runtime does not provide added protection. If a generator fails to start, the problem cannot be corrected in 5 minutes or even 15 minutes.”

Harry Handlin, U.S. Data Center Segment
Leader at ABB

Achieving Total Cost of Ownership Savings with Shorter Runtimes

Today’s UPS systems can provide TCO savings for data center owners who take advantage of shorter battery runtimes. If one knows how to optimize a battery bank to provide a sustained discharge for less than 5 minutes, one can reduce the quantity of batteries required to meet the facility’s load.

Since NiZn batteries can be optimally sized for runtimes of 5 minutes or less, one only needs to buy the minimum number of battery cabinets to support the load and runtime requirements.

This reduction in batteries that nickel-zinc battery technology can offer for shorter runtimes has large implications to the overall total cost of ownership decrease. The largest contributing factors are the footprint reduction, installation costs and lower overall maintenance costs over the life of the system. These can be compounded – and the savings even larger – if the UPS and battery system are containerized in a modular power system where footprint reduction can translate to larger savings.

Conclusion

Shorter runtimes may not suit every data center’s needs. Many facilities maintain longer battery runtimes during automatic failover for various reasons. For such applications, nickel-zinc batteries continue to offer a robust and dependable backup power source.

Nickel-zinc batteries are a smart choice for the future. As UPS systems and generators improve on transfer time, this failover will become even faster and more reliable as data centers are able to switch over to generator power (or from one utility feed to another) more seamlessly than ever before. With these improvements, there will be a high demand for a battery that can be right-sized and optimized for shorter runtimes, while still delivering a high-power discharge.

The nickel-zinc battery offers the power density, reliability, safety, and reduced runtime capabilities that the data centers require.