UL 9540A Test Method: Battery Energy Storage Systems (BESS) are growing rapidly as renewable energy, electric vehicles, and grid-scale projects expand worldwide. While storage technology brings efficiency and sustainability, it also raises safety challenges—especially related to thermal runaway and fire propagation. This is where the UL 9540A Test Method plays a vital role. It is […]
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Liquid vs Air Cooling System in BESS – Complete Guide: Battery Energy Storage Systems (BESS) are transforming how we store and manage renewable energy. But one often overlooked factor that determines their safety, performance, and lifespan is the cooling system. Effective thermal management ensures batteries operate within safe temperature ranges, preventing overheating, fire risks, and […]
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UL 9540A Test Method: Battery Energy Storage Systems (BESS) are growing rapidly as renewable energy, electric vehicles, and grid-scale projects expand worldwide. While storage technology brings efficiency and sustainability, it also raises safety challenges—especially related to thermal runaway and fire propagation. This is where the UL 9540A Test Method plays a vital role. It is […]
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Liquid vs Air Cooling System in BESS – Complete Guide: Battery Energy Storage Systems (BESS) are transforming how we store and manage renewable energy. But one often overlooked factor that determines their safety, performance, and lifespan is the cooling system. Effective thermal management ensures batteries operate within safe temperature ranges, preventing overheating, fire risks, and […]
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UL 9540A Test Method: Battery Energy Storage Systems (BESS) are growing rapidly as renewable energy, electric vehicles, and grid-scale projects expand worldwide. While storage technology brings efficiency and sustainability, it also raises safety challenges—especially related to thermal runaway and fire propagation.
This is where the UL 9540A Test Method plays a vital role. It is the most widely accepted safety test for energy storage systems, referenced by codes such as NFPA 855 and the International Fire Code.
In this guide, we’ll explain:
What UL 9540A is
The four levels of testing
How it relates to UL 9540 and UL 9540B
The major 2025 updates
Why manufacturers and project developers should care
What is the UL 9540A Test Method?
The UL 9540A Test Method for Evaluating Thermal Runaway Fire Propagation in BESS is the only national standard designed to measure how battery systems react under thermal runaway conditions.
It answers critical safety questions such as:
Will a single cell failure spread to neighboring cells?
How will a battery module or system behave in case of fire?
Can suppression systems prevent explosions, flames, or reignition?
Unlike general safety certifications, UL 9540A focuses only on fire propagation risks, making it the go-to test for engineers, installers, and regulators.
Battery systems are often installed in homes, businesses, and utility-scale projects. If a fire starts and spreads, it can have devastating consequences. UL 9540A ensures that:
Manufacturers can prove their systems are safe.
Installers get clear installation requirements, such as spacing and suppression.
Authorities have a reliable, consensus-based safety test.
It is directly referenced in NFPA 855, which governs the installation of stationary energy storage systems, making it essential for compliance.
BESS integrators – ensuring compliance with NFPA 855 and local fire codes.
Developers and EPCs – gaining project approvals faster.
Home and commercial installers – understanding spacing and suppression requirements.
Without UL 9540A testing, it becomes nearly impossible to get approval for large BESS installations in the U.S.
Benefits of UL 9540A Testing
Compliance: Meets NFPA 855 and IFC requirements.
Design Optimization: Provides data for safe spacing and fire suppression.
Market Access: Required for many U.S. and Canadian projects.
Risk Reduction: Lowers fire and explosion risks in installations.
Conclusion
The UL 9540A Test Method is the backbone of energy storage safety in 2025. With its updated fifth edition, it provides a clear, science-based framework to evaluate thermal runaway risks at cell, module, unit, and installation levels.
For manufacturers, passing UL9540A testing means faster certifications, safer designs, and wider market acceptance. For regulators and installers, it provides confidence that battery energy storage systems meet the highest safety standards.
As BESS technology evolves, UL 9540A will continue to be the key standard shaping how we store renewable energy safely and effectively.
The UL 9540A Test Method is a fire safety test designed to evaluate the thermal runaway characteristics of battery energy storage systems (BESS). It helps manufacturers and system designers understand how a battery reacts under stress, including fire, gas release, and propagation risks.
Q3. How does UL 9540A differ from UL 9540 certification?
Q2. Why is UL 9540A testing important for energy storage systems?
UL 9540A testing provides critical data for fire safety, helping system integrators, regulators, and building authorities approve energy storage projects. It ensures that batteries are safe for use in residential, commercial, and grid-scale installations.
Q4. What are the levels of UL 9540A testing?
UL 9540A includes four test levels:
Cell level – Tests how a single cell behaves in thermal runaway.
Module level – Examines propagation within a battery module.
Unit level – Assesses fire and gas risks in the entire system.
Installation level – Evaluates safety in a real-world installed environment.
Q7. Does passing UL 9540A mean a battery is certified?
No, UL 9540A is not a certification. It is a test method that provides data used to achieve UL 9540 certification or to comply with fire codes. Certification comes from successfully meeting UL 9540 requirements.
Q8. How long does UL 9540A testing take?
The duration depends on system size and complexity. A full series of tests (cell, module, unit, installation) can take several weeks to months, depending on scheduling and regulatory requirements.
Q9. Can UL 9540A testing be skipped if a battery is already safe?
No. Even if a system has proven safety records, AHJs and certification bodies often require UL 9540A data as part of the approval process, especially for large-scale installations.
Q10. How does UL 9540A testing impact project approval timelines?
Providing UL 9540A data often accelerates permitting and approval by demonstrating compliance with NFPA 855, IFC, and local fire codes. Without it, projects may face delays or additional scrutiny.
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Among the various methods available, liquid cooling and air cooling stand out as the two most common approaches. Each has unique advantages, costs, and applications. In this post, we’ll compare liquid vs air cooling in BESS, and help you understand which method fits best depending on scale, safety, and compliance needs.
Why Cooling Matters in BESS
Battery cells generate heat during charging and discharging. If not managed properly, this heat can cause:
Air cooling is the most widely used thermal management method in small to medium BESS setups. It works by blowing cool air across the battery racks with fans or forced ventilation.
Advantages of Air Cooling
Lower upfront cost
Simpler system design
Easier maintenance
Limitations of Air Cooling
Less effective for high-density, utility-scale systems
Struggles in hot or humid climates
Uneven cooling across battery modules
Best Use Case: Residential or small commercial BESS paired with solar PV or EV charging.
Liquid Cooling Systems in BESS
Liquid cooling uses water-glycol mixtures or dielectric fluids circulated through cold plates or coolant channels around the battery cells. This method transfers heat more efficiently than air cooling.
Advantages of Liquid Cooling
High thermal efficiency
Better temperature uniformity
Ideal for grid-scale energy storage PCS and high-density BESS
Scalable and safer in demanding climates
Limitations of Liquid Cooling
Higher initial investment
More complex installation and monitoring
Requires leak-proof design and maintenance
Best Use Case: Utility-scale BESS, energy storage PCS integration, and applications requiring long-duration reliability.
👉 Learn more about Energy Storage PCS and how cooling supports PCS performance.
Liquid vs Air Cooling: Side-by-Side Comparison
Factor
Air Cooling
Liquid Cooling
Cost
Low
Higher
Efficiency
Moderate
High
Scalability
Limited
Excellent
Maintenance
Simple
Technical
Best for
Residential & small commercial
Utility-scale & grid applications
In large-scale deployments, liquid cooling dominates due to higher efficiency and better safety margins. For smaller systems, air cooling remains cost-effective.
Cooling and Compliance
Thermal management directly influences regulatory compliance. Global frameworks such as:
UL 9540 & UL 9540A for safety testing
UL 9540A Test Method for thermal runaway evaluation
All emphasize the role of cooling in preventing fire hazards.
This makes cooling systems a critical design choice, not just an engineering afterthought.
Choosing the Right Cooling System
When selecting between liquid vs air cooling, consider:
System Size: Larger BESS requires liquid cooling.
Environment: Hot climates favor liquid systems.
Cost vs Performance: Air cooling suits budget-sensitive projects.
Compliance Needs: Regulatory approvals may depend on cooling efficiency.
For projects exploring advanced storage technologies such as green hydrogen storage, cooling strategies also play a role in integrated system safety.
Conclusion
The debate of liquid vs air cooling in BESS isn’t about which is better overall—it’s about which is better for your application.
Air cooling is cost-effective and simple for residential or small commercial setups.
Liquid cooling is the gold standard for utility-scale, high-capacity BESS where safety, scalability, and compliance are critical.
As energy storage adoption grows, smart cooling design will define the future of battery system safety and efficiency.
FAQs – Liquid vs Air Cooling in BESS
1. What is the difference between liquid and air cooling in BESS?
Air cooling uses fans to move air across battery modules, while liquid cooling uses fluids circulated through channels or plates to absorb heat more effectively.
2. Which cooling system is better for large-scale BESS?
Liquid cooling is preferred for utility-scale and high-density BESS because it provides superior thermal management, reduces hot spots, and improves safety.
3. Is air cooling still used in modern BESS?
Yes, air cooling is still used in residential and small commercial BESS where costs are lower and power density is moderate.
4. How does cooling affect battery safety?
Proper cooling reduces the risk of overheating and thermal runaway. Standards like UL 9540A Test Method specifically evaluate how BESS cooling impacts fire safety.
5. Does cooling impact regulatory compliance for BESS?
Air cooling is more affordable upfront. However, liquid cooling may deliver better long-term value by extending battery lifespan and ensuring compliance in large-scale systems.
✅ Next Step: Learn more about Energy Storage PCS and how Sunlith Energy helps integrate cooling with PCS design for optimal BESS performance.
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