Battery Storage Fire Safety That Actually Works

Most people love the idea of clean energy and home batteries—right up until someone mentions “thermal runaway” or “explosion risk.” That’s the tension sitting underneath almost every solar-plus-storage conversation in 2025.

Here’s the thing about green technology: if it isn’t safe, it isn’t scalable. Utilities won’t approve it, insurers won’t underwrite it, and communities won’t tolerate it. That’s why the latest wave of large-scale fire testing (LSFT) for battery storage from Sungrow, Wärtsilä and Chint Power Systems matters way more than a dry standards update might suggest.

This post breaks down what those tests really prove, how they connect to UL9540A/UL9540B and NFPA 855, and what it all means if you’re a developer, EPC, facility owner, or sustainability leader trying to build reliable, bankable energy storage projects.

Why Battery Fire Safety Is Now a Hard Requirement

Battery storage fire safety is no longer a “nice to have” line in an RFP. It’s become a gating factor for permitting, insurance, and financing.

Authorities having jurisdiction (AHJs) and fire marshals have watched a small number of high-profile battery fires dominate headlines and slow down projects. Their response has been predictable and, frankly, reasonable: prove it’s safe at full scale, under worst-case conditions, or it doesn’t get built.

What’s changed over the past two years is the type of proof required:

• Early-stage safety: cell-level abuse and small-scale tests
• Mid-stage safety: module and unit-level UL9540A data
• Current standard: Large-Scale Fire Testing (LSFT) that shows what happens when whole cabinets or containers actually burn, with suppression disabled

For green technology to earn trust, the industry has had to move from paper-based compliance to real-world evidence. The recent test campaigns by Sungrow, Wärtsilä and Chint are exactly that.

UL9540A vs UL9540B vs LSFT: What’s Really Being Tested?

If you’re trying to make sense of all the acronyms around battery safety, here’s the short version.

UL9540A is a thermal runaway propagation test. It’s designed to answer a specific question:

If one battery cell fails catastrophically, does the fire spread to other cells, modules, or enclosures?

Traditionally, UL9540A has covered:

• Cell level
• Module level
• Unit or enclosure level

The data helps AHJs, engineers and insurers understand how a system behaves, but it’s not a pass/fail certificate by itself. It’s more like a detailed crash test report.

UL9540B is newer and targets residential energy storage systems up to 20 kWh. It focuses on large-scale fire behavior in home-sized systems:

• Forced thermal runaway at cell level
• Fire propagation assessment at system level
• Impact on surrounding equipment and structures

Meanwhile, Large-Scale Fire Testing (LSFT) under procedures such as CSA/ANSI C800:25 goes even further for commercial, industrial and utility-scale projects. It answers questions like:

• What happens when a cabinet or container burns for hours with suppression disabled?
• Do flames, heat, or explosions spread to adjacent cabinets or aisles?
• Are there dangerous gas build-ups or projectile hazards?

NFPA 855’s 2026 edition will make LSFT effectively mandatory for many larger installations. That’s why so many system integrators are rushing to complete and publish their LSFT results.

Sungrow: Raising the Bar for Residential Battery Safety

Sungrow’s recent test on its SBH Series residential ESS isn’t just marketing. It quietly resets expectations for what “safe enough” looks like in the home storage segment.

What Sungrow Actually Proved

UL Solutions ran UL9540B Large-Scale Fire Testing on a modular, stackable SBH unit (10 kWh increments). The test forced a battery cell into thermal runaway, turned off fire suppression, and then watched what happened under worst-case conditions.

Key outcomes:

• Fire spread was contained inside the initiating cabinet
• Adjacent units saw internal surface temperatures of 21°C, well below the 154°C venting threshold
• Open flames self-extinguished after around one hour without active suppression
• No explosions, combustion of surrounding equipment, or projectiles occurred
• The cabinet involved in the fire remained structurally intact

For a residential ESS under 20 kWh, that’s exactly the kind of behavior you want. One unit can fail catastrophically without turning a garage or utility room into a multi-cabinet event.

Why Homeowners and Installers Should Care

If you’re selling or installing home batteries as part of a green technology offering, this matters because:

• AHJs, especially in states aligned with the 2022 California Fire Code, increasingly expect LSFT data, not just legacy UL9540A reports
• Insurers are scrutinizing residential ESS installs more carefully as adoption grows
• Homeowners are more safety-conscious as systems move closer to living spaces (garages, basements, utility closets)

A residential battery that self-contains fire, limits temperatures, and extinguishes flames without suppression is much easier to defend to a fire marshal or a cautious customer.

Wärtsilä: Worst-Case Testing for Utility-Scale Storage

On the utility side of the green technology stack, Wärtsilä has taken a more aggressive approach: stress systems until they fail, then prove the failure stays where you want it.

Quantum 3 Under UL9540A and LSFT

Wärtsilä’s Quantum 3—a 5 MWh grid-scale BESS—recently went through:

• UL9540A unit-level thermal runaway propagation testing
• Large-Scale Fire Testing (LSFT) following CSA/ANSI C800:25
• Validation of its Active Ignition Mitigation System (AIMS)

Results Wärtsilä has highlighted:

• Under UL9540A, when a cell was forced into thermal runaway, fire did not propagate to adjacent modules or other BESS enclosures
• Under LSFT, with suppression disabled, a Quantum 3 unit burned for 22 hours, and the fire remained contained in the initiating unit
• AIMS successfully ignited flammable gases early, preventing dangerous build-up and potential explosion scenarios

It’s worth underlining: UL9540A doesn’t formally say “pass” or “fail.” It produces data. But Quantum 3’s behavior matched or exceeded current performance expectations for non-propagation.

Why “Igniting Gas on Purpose” Is Smart Engineering

AIMS sounds counterintuitive at first: a system that deliberately ignites flammable gases. But from a safety engineering standpoint, it’s a strong move.

In a thermal event, gases released from lithium-ion cells can create an explosive atmosphere if they accumulate in an enclosure. By igniting them early and in a controlled way, AIMS:

• Reduces peak overpressure
• Lowers the probability of catastrophic explosions
• Makes the fire more manageable instead of allowing an unpredictable blast

Wärtsilä previously ran similar tests on its Quantum 2 product line with flammable gases intentionally released inside the enclosure. That pattern—design, test under extreme conditions, publish data—builds exactly the kind of confidence grid operators and investors need.

For large-scale green technology deployments, no propagation plus controlled gas management is about as good as you can realistically get with today’s chemistries.

Chint Power Systems: Proving Safety in C&I Environments

Commercial and industrial (C&I) sites are a different story. Batteries are close to people, property, and often mission-critical operations. Chint Power Systems (CPS) addressed that reality head-on in its recent LSFT.

How the CPS Test Was Structured

CPS put a C&I ESS cabinet through LSFT in Suzhou, China, following CSA 800 procedures and a draft UL9540A that includes LSFT. The layout was designed to mimic a real installation:

• Four cabinets total
• One initiating cabinet forced into thermal runaway
• Two target cabinets on either side
• One cabinet across an aisle, simulating a row gap or service corridor

The initiating cabinet’s cells and modules were pushed into thermal runaway, then actively ignited using a propane burner.

Timeline highlights:

• 1:09 pm – Active ignition of battery equipment
• 1:25–2:20 pm – Fire at peak intensity
• 4:02 pm – Fire burned out and extinguished

CPS reported:

• No thermal runaway or fire in target cabinets
• Internal temperatures in target cabinets stayed below venting threshold
• Only slight exterior damage on adjacent cabinets—described by the fire engineer on site as “normal” under such conditions

Why This Matters for Facilities and C&I Owners

If your C&I site hosts a battery system near employees, equipment, or inventory, your risk tolerance is understandably low. CPS’ test offers three practical assurances:

1. A single cabinet fire doesn’t automatically become a row fire when systems are properly designed
2. Temperatures can stay within controlled bounds even under aggressive burn conditions
3. Physical separation (aisles, cab spacing) still matters and can be validated with real data

For C&I customers using energy storage for demand charge management, backup power, or onsite solar optimization, this level of test evidence helps satisfy internal risk teams, insurers, and local regulators.

What This Means If You’re Planning Storage in 2025–2026

Most companies get battery safety wrong by treating it as a late-stage checkbox. That approach is going to hurt more and more as LSFT and stricter codes kick in.

Here’s a better way to approach it.

1. Bake Fire Safety Into Technology Selection

When you evaluate ESS vendors, ask for:

• UL9540A reports at module and unit levels
• UL9540B reports for residential or small systems (≤20 kWh)
• LSFT reports following CSA/ANSI C800 or equivalent, showing real burn data
• Documentation on gas management strategies (venting, active ignition, deflagration panels, etc.)

If a vendor can’t produce credible third-party data, you’re inheriting their risk.

2. Align Early With AHJs and Insurers

Don’t assume your AHJ or fire marshal will accept older standards alone. Many are now:

• Asking explicitly for LSFT data, not just UL9540A
• Referencing the latest NFPA 855 updates
• Looking to California and other early-adopter regions as benchmarks

Bring them into the conversation early with concrete test results from vendors like Sungrow, Wärtsilä, CPS and others. It speeds permitting and reduces redesign headaches.

3. Frame Safety as a Business Enabler, Not a Cost Sink

There’s a direct link between demonstrable fire safety and project economics:

• Lower perceived risk can mean better insurance terms
• Bankability improves when lenders see industry-standard tests passed at scale
• Community acceptance rises when you can show evidence of non-propagation under worst-case conditions

For any serious green technology strategy—whether residential portfolios, C&I campuses, or grid-scale assets—proven fire performance is now part of the ROI story, not separate from it.

Where Battery Fire Safety Goes Next

Energy storage is becoming the quiet backbone of the clean energy transition, and 2025 is the year where safety standards finally catch up with the speed of deployment.

Sungrow’s UL9540B test for home storage, Wärtsilä’s LSFT and AIMS validation for 5 MWh grid systems, and CPS’ multi-cabinet burn for C&I all point in the same direction: real-world, large-scale fire data is the new baseline.

If your organization is serious about green technology—whether that’s decarbonising operations, enabling more solar and wind, or adding resilience—you’ll need partners and products that can prove they behave predictably under stress, not just in a spec sheet.

The projects that move fastest over the next two years will be the ones that treat battery fire safety as part of strategy, design and procurement from day one.

If you’re planning storage deployments and want help evaluating safety data, standards alignment, or vendor options, now’s the time to get that work started—before LSFT goes from “emerging best practice” to “non-negotiable rule.”