Introduction

As cloud-based services and storage become central to business operations, data centers are under increasing pressure to ensure the resilience of their mission critical facilities. Data centers rely on uninterruptible power supply (UPS) systemsto secure continued operations during power outages and other disturbances, most often powered by lead-acid batteries. However, UPS systems that utilize nickel-zinc (NiZn) battery technology have specific advantages over lead-acid in terms of performance, reliability, safety, lifetime cost and climate impact.

Despite these advantages of alternative battery chemistries, most existing UPS systems were already designed to work with lead-acid batteries. Since each battery chemistry has its own charge/discharge profile, these UPS systems often would require additional hardware and re-engineering to accommodate non-lead-acid batteries. The time and cost needed for such modifications prevent the switch from lead-acid to alternative chemistries.

Thus, the best solution to this dilemma is a “drop-in replacement” battery storage system that not only enables the incorporation of alternative battery chemistries, but also functions in an existing UPS system without the need for additional modifications. This characteristic enables backward and forward compatibility that saves UPS Original Equipment Manufacturers (OEMs) and data centers time and money by integrating new battery types into traditional UPS systems without expensive modifications.

As the world’s first NiZn BESS (Battery Energy Storage Solution) product featuring backward and forward compatibility with megawatt class UPS inverters designed for lead-acid batteries, ZincFive’s BC Series UPS Battery Cabinet offers a drop-in replacement for battery storage systems in both new and existing UPS installations utilizing lead-acid batteries. Through its use, data centers can efficiently upgrade their UPS systems to harness the benefits of ZincFive’s NiZn batteries.

Why Switch from Lead-Acid to Nickel-Zinc?

Historically, most data centers depend on lead-acid batteries to power their UPS systems. The lead-acid battery was the first chemistry used and remains popular today, but alternative battery chemistries such as lithium-ion and nickel-zinc offer compelling value propositions driving many data center end users and OEMs to switch to a new battery chemistry.

Backward and forward compatibility saves UPS Original Equipment Manufacturers (OEMs) and data centers time and money by integrating new battery types into traditional UPS systems without expensive modifications.

Modern data centers bear a larger compute and power burden each year. Digital transformation, 5G, edge computing, the Internet of Things, artificial intelligence and machine learning all require a continually increasing level of computing power and density. For example, Dell expects that future rack densities will far exceed those of today,¹ which have already vastly grown. Since 2012, the amount of computing used for the largest AI training runs has expanded by more than 300,000 times,² and the computer resources consumed by AI double every 100 days.³

The rising power density per rack and the need to provide space for as much compute power as possible in the data center also applies to infrastructure UPS and energy storage densities need to increase to reduce the infrastructure footprint. Additionally, data centers are being designed and built both faster and larger, emphasizing these trends further.

The shortcoming of lead-acid batteries is particularly evident when compared to the capabilities of ZincFive’s NiZn batteries:

Power Density

NiZn batteries have higher power density than lead-acid batteries, so by choosing NiZn battery backup, data center designers can reduce the footprint needed for energy storage. In fact, NiZn batteries have twice the power density, and so can reduce the footprint by as much as 50% compared to lead-acid batteries. Similarly, NiZn batteries can deliver the same amount of power with half the weight of lead-acid batteries. This reduces the structural requirements of the UPS room. Smaller footprint and less weight will reduce capital expenditure (CapEx) costs associated with energy storage.

Safety

Data center operators are responsible for the safety of their facilities and personnel. NiZn technology is inherently safer than lead-acid or lithium batteries. NiZn batteries do not exhibit thermal runaway and are non-flammable, as proven through testing at the cell level using the Underwriters Laboratories UL 9540A test method.⁴ Their superior weight and safety makes the batteries easier to handle.

Reliability

Battery string reliability is key to avoiding unplanned maintenance: when a lead-acid or lithium-ion battery cell fails, it creates 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. This turns an emergency situation with other battery chemistries into a simple battery replacement at the next planned maintenance cycle with little cost and no operational impact.

NiZn String Discharge Reliability

• Unlike lead-acid and lithium-ion chemistries, a weak or depleted NiZn cell remains conductive, allowing the string to continue operating.
• Turns an emergency, unplanned maintenance situation for lithium and lead-acid into a simple string repair at the next planned maintenance cycle.

NiZn reliability delivers planned maintenance with no operational impact! This is a new paradigm in battery string reliability, contributing significantly to improved data center uptime.

Lifespan & Maintenance Costs

For lead-acid batteries, operating expenditures (OpEx) over the life of the UPS system can be more expensive than up-front CapEx. NiZn solutions dramatically lower OpEx through their much longer useful life and simplified maintenance. Additionally, the size and weight advantages of NiZn batteries compared to lead-acid batteries can lower the investment needed to house energy storage within data centers. NiZn batteries also have a wider operating temperature range than other technologies, which reduces demands on data center cooling systems. By lowering OpEx, ZincFive solutions reduce ownership costs over lead-acid based UPS products when looking across the total UPS useful life.

Sustainability

NiZn batteries are mainly composed of common, highly available, non-hazardous materials as opposed to lead-acid batteries containing significant amounts of lead and other hazardous materials. Even after a long operational life, NiZn is one of the most recyclable battery chemistries on the market. Unlike many other materials, both nickel and zinc can be recycled while maintaining their physical and chemical properties. ZincFive’s zinc electrode contains no hazardous lead, cadmium or mercury, and so is environmentally benign. In fact, NiZn batteries have the best climate impact score – 9.4 out of 10 – of backup battery chemistries typically used in data centers, according to a recent 3rd party Climate Impact Report.⁵

As rack-level compute density rises, so do the requirements for backup power in smaller, safer footprints. With their high power density, longer lifespan, sustainability and greater reliability, NiZn batteries are the ideal choice for backing up the increasing data center loads.

Why Alternative Battery Chemistries Need Backward/Forward Compatibility

While NiZn batteries’ characteristics give data centers more than enough reason to make the switch from lead-acid, one challenge often stands in the way: the average data center UPS system was not built with them in mind.

Rather, most data centers were designed specifically to charge lead-acid batteries. Since every battery chemistry has a different charge/discharge profile, alternative batteries can’t charge from the typical UPS system without a costly and time-consuming reengineering process.

Fortunately, one workaround exists to avoid that extra time and cost. A NiZn battery chemistry can, in fact, charge from a UPS made only for lead-acid battery profiles without additional hardware – as long as it has backward/forward compatibility, the ability to seamlessly operate with a UPS designed for a different battery chemistry than its own.

ZincFive’s BC Series UPS Battery Cabinet: the Key to an Easy Upgrade

ZincFive‘s BC Series Battery Cabinets, while housing NiZn batteries with all of their associated benefits, adapt the lead-acid charging system to the needs of the nickel-zinc battery. The battery cabinets’ intelligent charging & monitoring system emulates the profile of lead-acid batteries to the UPS. Since the UPS doesn’t “know” that the NiZn battery string has a different chemistry, it operates the string just as it would for its previous lead-acid inhabitants.

This way, ZincFive’s UPS Battery Cabinet’s specialized profile allows operators to use NiZn battery cabinets as “drop-in” replacements. Able to support 1MW of UPS output power with only four battery cabinets and the industry’s smallest linear footprint, this NiZn Battery Cabinet offers a lower TCO, lower maintenance and higher performance alternative for data center UPS energy storage.

With ZincFive’s BC Series Battery Cabinets, data center operators can easily integrate NiZn’s benefits into existing UPS systems with no need for hardware modifications.

Conclusion

By utilizing ZincFive’s BC Series Battery Cabinets, data center operators can easily integrate NiZn’s benefits into existing UPS systems with no need for hardware modifications. The cabinets’ backward and forward compatibility with megawatt class inverters allows them to function as a straightforward drop-in replacement for existing lead-acid battery cabinets. This allows data centers to take advantage of NiZn’s higher power density, superior reliability, lower costs, longer lifespan, and multiple environmental advantages that modern data centers need: the perfect choice to safely back up their power and maintain critical operations.

Citations

¹ https://www.delltechnologies.com/asset/en-us/products/servers/industry-market/2020-server-trends-and-observations-brief.pdf

² https://www.technologyreview.com/2019/11/11/132004/the-computing-power-needed-to-train-ai-is-now-rising-seven-times-faster-than-ever-before/

³ https://openai.com/blog/ai-and-compute/

⁴ https://www.ul.com/services/ul-9540a-test-method

⁵ https://www.boundlessimpact.net/climate-impact-assessment

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.

Introduction

This white paper provides an informational guide to the United States Codes and Standards regarding Energy Storage Systems (ESS), including battery storage systems for uninterruptible power supplies and other battery backup systems. There are several ESS technologies in use today, and several that are still in various stages of development.

While various technologies, such as flywheels, fuel cells, compressed gas, and others, are either in use or development, the primary focus of most of the jurisdictional Authority Having Jurisdiction (AHJ) is currently being placed on electrochemical storage systems.

These same Codes – and many of the Standards cited – cover all of the currently available ESS technologies, and in some cases, there are additional Codes and Standards cited to cover those specific technologies. For the sake of brevity, electrochemical technologies will be the primary focus of this paper due to being the ESS choice of the vast majority of users. Technologies such as pumped hydro, compressed gas, fuel cells, and various methods of gravity storage are not covered in this paper.

Why do we have Codes and Standards?

A code repository is necessary to increase awareness and improve safety in the energy storage industry. Electrochemical energy storage has a reputation for concerns regarding the ventilation of hazardous gases, poor reliability, short product life, substantial cooling requirements, and high levels of periodic maintenance.

Like the newer lithium battery technologies, the traditional lead-acid technology has developed a stigma. While generally a safe product, damage presents the risk of fire. When damaged, lead-acid, lithium, and some other battery technologies may also leach corrosive chemicals such as mercury, cadmium, and lead into landfills and may contaminate water supplies and ecosystems. These chemicals are dangerous to human health and are expensive to clean up. Users desire a safe, cost-effective alternative that requires less maintenance and is environmentally sustainable.

Definition of Codes and Standards

What differentiates Codes from Standards is the usage. Codes are an overarching statement of best (and safest) practices for an entire industry or technology.

As seen in the provided definitions, a Code is a set (or collection) of mandatory regulations that are established and enforced by a governmental authority. This is generally a collection of safety practices and other associated Standards enforced for the purposes of safety and reliability.

Standards, on the other hand, are technology or product specific, and provide a method to verify that the technology or product meets or exceeds the minimum acceptable level of safety.

What we refer to as “Codes” are actually model Codes, published by one of several nationally recognized organizations, with the purpose of presenting the “best practice” for subjects covered by the intended area of application. Each model code presents the latest consensus information on its related subject. These model Codes are then reviewed and adopted by the various jurisdictions, and when accepted become the legal Code for that jurisdiction. There are several separate model Codes, covering a variety of applications. For the purposes of this paper, only the applicable fire Codes and their related Standards will be considered.

In some cases, these Codes are adopted on a statewide basis. In others, they may be adopted on a county-by-county, or city-by-city basis. In county and local cases, the jurisdiction is allowed to amend the Code to make it more stringent, but in no case may the jurisdiction reduce the Code if it is adopted in full by a higher authority.

There are two primary organizations in the United States that publish these model Codes:

Every jurisdiction in the US has adopted either the NFPA Codes or International Code Council’s I-Codes. Currently (2023), there are eight states that adopt the NFPA 1 Fire Code, and forty-two that adopt the International Fire Code. Interestingly, although there are much more advanced Codes available, there are still several jurisdictions that are using the 2015 revisions of the Codes, and at least one that is still using the 2009 revision.

As one gains understanding of the increasing number of new battery chemistries, and the associated risk factors, it is hard to justify maintaining an outdated Code base unless that Code is regularly amended to maintain the intended safety aspects of the more recent Codes.

Storage Technologies and Electrochemistries

There are several basic groups of ESS technologies in use today. They are usually broken into categories such as Mechanical, Gravity, Electrical and Electrochemical. While some of these may somewhat bridge gaps between technology groups, these are generally recognized as the major categories for Code purposes. It should be noted that technologies falling under the “Gravity” section have very little mention in the Fire Codes.

National Fire Protection Association (NFPA)

The first to be organized was the National Fire Protection Association (NFPA), which was organized in 1896 by several men associated with fire insurance companies. Currently, NFPA sponsors over 300 consensus Codes and Standards covering many safety related areas.

Electrical

Electrochemical Double Layer Capacitors (EDLC) – The EDLC is considered a “supercapacitor” or an “ultracapacitor.” It is an electrostatic-based system with two electrodes producing electrostatic effects with activated carbons. These ultracapacitors can store or deliver energy at tremendous rates.
Supercapacitors – This is a general term for electrochemical double-layer capacitors.

Electrochemistries

Sources: Ricardo Consulting, AABC, National Academy of Sciences, and ZincFive estimates

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Lead-Acid (LA)

The Lead-Acid battery utilizes the chemical reaction between lead and sulfuric acid. During discharge, lead and sulfuric acid react to form lead sulfate and hydrogen ions. During charging, the lead sulfate and hydrogen ions recombine into lead and sulfuric acid. This cycle of chemical reactions between lead and acid allows the battery to store and release electrical energy.

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Nickel-Cadmium (NiCd)

The Nickel-Cadmium battery is based on the redox reaction between nickel hydroxide (NiOOH) and cadmium (Cd). Nickel hydroxide is the anode in the battery, and cadmium is the cathode. When the battery is charging, the reaction is reversed, nickel hydroxide is reduced to metallic nickel, and cadmium is oxidized to cadmium oxide (CdO).

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Nickel-Metal Hydride (NiMH)

The Nickel-Metal Hydride battery is based on a chemical reaction between nickel and hydrogen. When the battery is discharged, the nickel is oxidized, and hydrogen is reduced, releasing electrons to provide electrical power. The reversed reaction occurs when the battery is being charged, which causes the hydrogen to oxidize and the nickel to reduce, allowing electrons to be absorbed from an external source.

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Nickel-Zinc (NiZn)

The Nickel-Zinc battery is the process of chemical reactions between the anode (nickel) and the cathode (zinc) to produce electrical energy. Specifically, the nickel anode reacts with hydroxide ions during discharge, releasing electrons and forming nickel hydroxide. These electrons then travel to the zinc cathode, which reacts with zinc ions, releasing more electrons and forming zinc hydroxide. This process is reversed during charging.

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Zinc-Manganese Dioxide (ZnMnO²)

The Zinc-Manganese Dioxide battery is also known as a zinc-carbon or alkaline battery, a secondary battery that can be recharged and reused multiple times. It is an improved version of the non-rechargeable zinc-carbon battery commonly used in household devices like remote controls, flashlights, and toys. The primary components of a rechargeable zinc-manganese dioxide battery are the anode, cathode, and electrolyte.

• The battery’s anode is composed of zinc metal or a zinc alloy. During the recharging process, an electrical charge is applied to the battery, and the anode’s zinc ions combine with the electrolyte’s hydroxide ions, thus producing zinc hydroxide. The manganese oxide on the cathode is then changed back to manganese dioxide. During the battery operation (and discharge when the battery is in use), the zinc atoms at the anode undergo an oxidation reaction, releasing electrons and forming positively charged zinc ions

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Sodium (High Temperature)

The electrochemistry of sodium batteries is based on the intercalation of sodium ions in graphite anodes and the reduction of sodium ions at a metal-oxide cathode. During charge, sodium ions are inserted into the graphite anode, and during discharge, the sodium ions are released and reduced at the metal-oxide cathode. The main advantage of using sodium instead of lithium is cost, as sodium is much more abundant and less expensive than lithium.

• Sodium Sulfur (NaS) – Sodium sulfur batteries are based on the redox reaction between sodium (Na) and sulfur (S). The reaction occurs in a cell with an anode
  of sodium, a sulfur cathode, and a molten lithium and sodium chloride electrolyte. When the battery is charged, sodium ions move from the anode to the cathode and react with sulfur to produce sodium sulfide. During discharge, the reaction reverses, and the sodium sulfide is converted back into sodium and sulfur. The electrochemistry of sodium sulfur batteries makes them very efficient and can store large amounts of energy. They are also highly durable and have a long life span.
• Sodium Metal Chloride (Na NiCl2) – Sodium metal chloride batteries involve the transfer of electrons between the anode and cathode, resulting in the oxidation of sodium at the anode and chlorine reduction at the cathode. The electrolyte is actually a solid beta-alumina that separates the liquid electrodes. This process is reversed during charging.

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Flow

Flow batteries are electrochemical cells that use two different liquid electrolytes stored in separate tanks and pumped through the cell. The electrolytes are typically held in flow cells and separated by a membrane that allows ions to pass through. During operation, the two electrolytes react in the cell, generating electrical energy. This energy can power various loads, including motors, pumps, and lights. The process can be reversed, allowing a flow battery to act as a generator and store energy for later use.

• Vanadium Redox (VRFB) – Vanadium redox flow batteries are electrochemical energy storage systems that use a vanadium-based electrolyte solution. The vanadium electrolyte is stored in two tanks and is circulated through a cell stack. The cell stack consists of a series of electrochemical cells where oxidation and
  reduction reactions take place. During charging, oxidation occurs in the cell stack, causing the vanadium in the electrolyte to be oxidized. During discharge, the vanadium is reduced. This cycle allows the battery to store and release electrical energy as needed.
• Zinc-Air (ZnO₂) – The zinc-air flow batteries use zinc metal as the anode and air-breathing cathodes. The zinc metal is oxidized at the anode, releasing electrons that travel through the external circuit to the cathode. Oxygen from the air is reduced at the cathode, forming hydroxide ions. The hydroxide ions react with zinc ions in the electrolyte to form zincate ions, which are then released into the electrolyte. The zincate ions are then re-oxidized at the anode, and the cycle is repeated. This process is powered by the flow of electrons from the anode to the cathode, generating electrical energy.
• Zinc-Bromine (ZnBr) – Zinc-Bromine flow batteries are based on the redox reaction between zinc and bromide ions in an aqueous electrolyte stored in two tanks separated by a membrane. During charging, zinc is oxidized at the anode, and bromide is reduced at the cathode, forming a zinc bromide complex. During discharge, the reaction is reversed. The energy is stored in the zinc bromide complex, which can be recharged several times.

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Lithium

Lithium batteries are rechargeable batteries that utilize lithium metal or compounds’ electrochemical properties to store energy. They are the most common type of rechargeable battery used in consumer electronics such as cell phones, tablets, and laptops. Lithium batteries are also used in electric vehicles, medical devices, and military applications. The electrochemistry of lithium batteries is based on the redox reaction between lithium and an electrolyte solution. In a lithium battery, lithium ions are stored in the cathode material and are released into the electrolyte solution during charging. As the lithium ions travel through the electrolyte solution, they react with the anode material to form lithium metal, releasing electrons. During discharge, the lithium ions travel back to the cathode and take electrons from the anode, creating an electric current. (There are over 20 “lithium” chemistries. These are the most widely used in stationary applications.)

Lithium Iron Phosphate (LFP) – Lithium Iron Phosphate (LFP) batteries rely on a reversible redox reaction between the active materials, lithium iron phosphate (LiFePO₄), and carbon. Upon charge, lithium ions (Li+) are inserted into the anode (negative electrode) while electrons are supplied to the cathode (positive electrode). When the battery is discharged, the lithium ions are extracted from the anode and travel through the electrolyte to the cathode. This is where the electrons are recombined to form the LiFePO₄ compound. During this process, the battery releases energy in the form of electricity.

• Lithium Nickel Manganese Cobalt Oxide (NMC) – Lithium Nickel Manganese Cobalt Oxide (LiNiMnCoO₂ or NMC) batteries are rechargeable lithium-ion batteries that have become increasingly popular recently. In these batteries, the positive electrode, or anode, is made of lithium cobalt oxide (LiCoO2). The negative electrode, or cathode, is made of a combination of nickel, manganese, and cobalt oxides (NMC). Lithium ions move from the anode to the cathode during charging, depositing onto the negative electrode. During discharging, the lithium ions move from the cathode to the anode in the opposite direction. This process of shuttling lithium ions back and forth between the anode and cathode provides power for the battery.
• Lithium Nickel Cobalt Aluminum Oxide (NCA) – Lithium Nickel Cobalt Aluminum Oxide (NCA) is a rechargeable lithium-ion battery that utilizes a specific combination of materials in its cathode, including lithium, nickel, cobalt, and aluminum oxides to store and release electrical energy. During discharge, lithium ions move from the anode to the cathode through the electrolyte, creating an electric current. Nickel and cobalt are known for their high energy densities, while aluminum helps stabilize the cathode structure. The NCA cathode chemistry offers several advantages, including increased energy density, a long life cycle, high operating voltage, quick charging, and thermal stability. NCA batteries are commonly used in electric vehicles, power tools, and portable electronics.
• Lithium Titanate (LTO) – Lithium Titanate anodes have been known since the 1980’s. The cathode can be lithium nickel manganese oxide (LMO) or nickel manganese cobalt oxide (NMC) compound, while the anode replaces the graphite with lithium titanate formed in a spinel arrangement. During charging, lithium ions move from the anode to the cathode. During discharge, lithium ions move from the cathode to the anode. This movement of ions creates an electric current, which is used to power a device. Lithium titanate has good fast charge characteristics and performs well at low temperatures but exhibits lower energy density than other technologies and is safer than most lithium batteries. LTO is expensive when compared to other lithium batteries.

How Codes Are Applied

Starting with the NFPA 1-2018 and IFC 2018, the Codes introduced some measure of regulation to ensure the safety of installed Energy Storage Systems (ESS). Although there was limited regulation regarding batteries and their installation, maintenance, and explosion/fire control in earlier revisions of the Codes, it was relatively rudimentary compared to later Codes.

As the adoption of newer battery technologies increased, both NFPA and ICC recognized that these technologies required more stringent regulation. The AHJs were unfamiliar with lithium, sodium, and other new technologies and had limited or no understanding of their potential problems. NFPA had several conflicting Codes and Standards cited in their fire code and recognized that creating a unified Energy Storage System (ESS) Standard was the best way to deal with that issue. This led to NFPA 855, the single ESS Standard NFPA now recognizes. The IFC 2021 revision deals with ESS slightly differently but has a chapter (Chapter 12) dedicated to Energy Systems and a section (Section 1207) dedicated to Electrical Energy Storage Systems. The IFC 2021 essentially copies the Maximum Allowable Quantities table (1206.9) from the IFC 2018, adds Nickel-Metal Hydride and Flow batteries to the table, and reprints almost the same data. There are additional stipulations in the requirements of the IFC 2021, but ICC did not go to the same lengths as NFPA in their 2021 edition. The 2024 edition of the IFC is anticipated to be more closely aligned with NFPA 855-2023.

Both the NFPA 1 National Fire Code and the ICC International Fire Code have adopted significant enhancements to the earlier code language in their 2021 revisions. Many jurisdictions still utilize the 2018 and earlier revisions of the Fire Codes. Still, many jurisdictions with high data center density are adopting the 2021 revisions and/or implementing NFPA 855-2020 as their ESS standard. While the 2021 revisions of the International Fire Code is the most recent available today (Q3, 2023), the new NFPA 1-2024 Fire Code has just been released in September of 2023, and NFPA 855 has already been updated with the NFPA 855-2023 Standard.

In the case of a jurisdiction adopting either of these Fire Codes and their related Standards, there are significant restrictions on some Energy Storage technologies. Any technology not explicitly listed in the relevant tables (Table 9.4.1 in NFPA 855-2023, and Table 1207.5 in IFC 2021), and even some of those listed but not specified as having an unlimited allowable quantity in a given fire area, will have to be approved on a case-by-case basis. Some jurisdictions will not allow a lithium battery to be installed under any circumstances. In contrast, some will allow them with severe restrictions on where they are installed, how much capacity is allowed, and what type of lithium battery is being proposed.

As mentioned earlier, the cited Standards significantly influence the Codes and their implementation. It is necessary to highlight some of these Standards (other than NFPA 855) due to their importance to both Codes. Three of particular importance are UL 1973, UL 9540, and UL 9540A.

List of Major U.S Codes and Standards

This section provides a comprehensive list of major U.S. Codes and Standards. See Appendix A for a concise cheat sheet for Codes and Appendix B for Standards.

Looking Ahead

New Codes and Upcoming Code Updates

Many Codes are due for updated releases next year, which are very important in the ESS world. Among these are the NFPA 1 National Fire Code and the ICC International Fire Code, International Building Code, International Mechanical Code, and several more from ICC, all expected to be released late in 2023. In addition, the NFPA 101 Life Safety Code will be released in late 2023.

Many of the NFPA and UL Standards are either due for a revision release or are in the development cycle for release soon.

As a reminder, checking for the latest amendments or additions to the existing Codes and Standards is extremely important. For example, NFPA will issue Technical Interim Amendments (TIA), which supersede the language in the published document. UL issues Certification Requirement Decisions (CRD) to update the language in their Standards and add or remove requirements for compliance with the Standard in question. In both cases, these changes will be resolved by the Technical Committee responsible for the specific document and incorporated in the document’s next release.

Conclusion

In Summary

It is difficult to overemphasize the importance of maintaining an awareness and understanding of the relevant Codes and Standards affecting modern ESS systems’ selection, use, installation, and maintenance. There are many choices from a broad spectrum of solutions available. Some of these may present challenges in being acceptable to the local AHJ having jurisdiction over Fire Code enforcement, or potentially, even with the AHJ having jurisdiction over the Building Code that applies to their jurisdiction.

Knowing and understanding these Codes and Standards’ impact on implementing a given ESS could have significant financial and scheduling issues for the end user. There is a value in maintaining a knowledge of the most recent revisions and additions to these documents, as this is a rapidly changing landscape, and what is considered acceptable in the Codes and Standards today may change to an unacceptable risk in a very short time.

Works Cited and Useful Links

International Code Council (ICC), https://www.iccsafe.org/
National Fire Protection Association (NFPA), https://www.nfpa.org/
American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), https://www.ashrae.org/
Underwriters Laboratories (U.L.), https://www.ul.com/-Underwriter
Battery University, https://batteryuniversity.com/
Energy Systems and Energy Storage, http://www.eseslab.com/index
DOE/EPRI 2013 Electricity Storage Handbook in Collaboration with NRECA, https://www.sandia.gov/app/uploads/sites/163/2021/09/SAND2013-5131.pdf
Environmental and Energy Study Institute, https://www.eesi.org/papers/view/issue-brief-energy-storage

Appendix A: Codes

Appendix B: Standards