Energy storage: Failure analysis from cells to containers

Energy storage using Battery Energy Storage Systems (BESS) has become a key infrastructure for electrical grids, renewable energies, and industrial backup. However, the increase in energy density in lithium-ion batteries has also heightened the risks associated with thermal runaway propagation (TRP) events.

In a BESS system, a failure can originate in a single cell and scale up to complete modules, racks, and containers, compromising safety and operational reliability. For this reason, standards such as UL 9540A and NFPA 855 establish criteria to evaluate thermal propagation, flammable gas release, and mitigation strategies in energy storage systems.

What is a battery energy storage system?

A battery energy storage system (BESS) stores electricity in rechargeable battery modules and supplies it when a facility, a microgrid, or an electric utility company needs power. A complete BESS is more than just a battery. It includes battery modules, a battery management system (BMS), a power conversion system (PCS), an energy management system (EMS), thermal management equipment, controls, enclosures, and safety systems.

As additional information, the following video, courtesy of The Power Hub, explains how BESS technology captures and releases energy, supporting the electrical grid, providing backup power, and revolutionizing our dependence on fossil fuels. Discover the different types of BESS and how they work to stabilize the grid. Explore the advantages and disadvantages, and find out how BESS can benefit businesses and contribute to a sustainable future. Ready to harness the potential of BESS for your renewable energy source?

Types of energy storage: the context of bess

To understand the current landscape, it is necessary to classify storage technologies according to their operational nature:

• Mechanical: Pumped-storage hydroelectric plants and compressed air energy storage (CAES).
• Thermal: Latent or sensible heat storage (e.g., molten salts).
• Chemical: Green hydrogen and synthetic fuels.
• Electrochemical: Lithium-ion batteries stand out here, dominating the market thanks to their high efficiency, lifespan, and energy density, making them the core of modern BESS systems.

Structural configuration of bess energy storage systems

An industrial BESS system is composed of a modular and hierarchical architecture integrated by cells, modules, packs, racks, and containers. Understanding this configuration is fundamental to analyzing how a failure can start at the electrochemical level and progressively propagate through the system. The components are detailed below:

• Cell: The basic electrochemical unit that stores energy.
• Module: A set of cells connected in series or parallel to achieve a specific voltage and capacity.
• Pack: A grouping of modules with physical protections and primary monitoring.
• Rack: A vertical structure that holds multiple packs connected in series to raise the system voltage.
• Container: The plant-level solution that houses the racks, ventilation systems, fire suppression, and environmental control.

Thermal runaway: from cell to container (TRP)

The greatest latent risk in lithium-ion batteries is Thermal Runaway. This phenomenon occurs when an internal failure triggers an uncontrolled exothermic reaction.

Mechanism of failure and propagation (TRP)

The Thermal Runaway Propagation (TRP) process follows a destructive domino effect:

• Cell failure: A manufacturing defect, mechanical damage, overcharging, or an internal short circuit raises the local temperature. Upon exceeding the critical threshold, the cell separator melts, releasing flammable gases (CO, H2, CH4) and extreme heat.
• Escalation to the module and pack: Radiant heat and thermal conduction transfer energy to neighboring cells, causing them to undergo chain thermal runaway.
• Catastrophe in the container: If the propagation is not stopped in time, the volume of flammable gases accumulated inside the container can reach the lower flammability limit (LFL), resulting in deflagrating fires or destructive explosions.

Control ecosystems: BMS, EMS and PCS

Failure mitigation depends on a coordinated technological triad that supervises the BESS:

• BMS (Battery Management System): It is the first line of defense. It monitors critical variables (voltage, current, temperature) at the cell and module level. If it detects an anomaly (e.g., a cell heating up unusually or a voltage imbalance), it isolates the pack electrically before thermal runaway begins.
• PCS (Power Conversion System): The bidirectional inverter that transforms the current (DC to AC and vice versa). It responds to the orders of the BMS by interrupting the power flow in the event of serious failures in the grid or in the battery bank.
• EMS (Energy Management System): The software brain at the plant level. It dispatches energy according to the market or the grid, optimizing load profiles to prevent premature thermal stress of the system.

How to analyze failures from cell to container bess?

Failure analysis in BESS systems requires a tiered technical evaluation to identify how an event initiated in a cell can propagate to entire modules, racks, and containers. This process combines electrochemical analysis, thermal evaluation, operational review, and post-incident structural verification.

The first stage consists of identifying the point of origin of the failure. To do this, historical logs from the BMS system are reviewed with the objective of determining the initiating cell, the possible thermal source, the associated electrical event, and the operational conditions prior to the incident, including state of charge, abnormal currents, overtemperatures, or voltage deviations.

Subsequently, a thermal reconstruction of the event is performed to understand how the propagation occurred. In this phase, the heat transfer route, the thermal propagation speed between components, the zones of highest energy concentration, and the ignition sequence within the system are analyzed. These evaluations are usually supported by CFD simulations and thermodynamic models to reproduce the behavior of the system during the event.

The analysis must also include the characterization of gases released during the thermal runaway. Depending on the battery chemistry, hydrogen, carbon monoxide, hydrogen fluoride (HF), and mixtures of flammable hydrocarbons can be generated. The accumulation of these gases represents one of the main risks in thermal runaway propagation (TRP) events, especially in containers with insufficient ventilation.

Another critical aspect corresponds to the review of protection and control systems. The behavior of the BMS, the response of the EMS, the operation of the PCS, and the activation of suppression or isolation systems are evaluated. This stage allows determining whether the protection barriers responded within the design parameters or if there were detection failures, response delays, or loss of communication between subsystems.

Finally, the post-incident analysis verifies the level of structural and electrical damage to the system. The integrity of the container, the condition of racks and modules, damage to power and control systems, as well as the possible presence of residual chemical contamination derived from toxic gases, vaporized electrolytes, or combustion residues are inspected. This evaluation is essential to determine the viability of recovering the system and to establish corrective measures to prevent future events.

The following video presents a summary of how proper battery management system (BMS) management can prevent overheating and internal short circuits through safe operating and maintenance practices. Furthermore, it shows how to extend battery life and optimize the performance of energy storage systems by properly controlling critical operating limits. Source: WattsApp with Power.

Regulatory Framework and Safety: UL 9540A and NFPA 855

The industry responds to these risks through strict battery safety standards to validate that containers are safe for installation.

What does UL 9540A require to mitigate propagation?

The UL 9540A standard is the standard test method for evaluating thermal runaway fire propagation in energy storage systems. It is not a “pass/fail” certification, but rather a rigorous four-level test:

• Cell level: Thermal runaway is forced to analyze the onset temperature and the composition of the gases emitted.
• Module level: It evaluates whether the failure propagates to adjacent cells within the same module.
• Rack level: It verifies whether the design of the rack prevents vertical or horizontal propagation to other packs.
• Container level: It analyzes the behavior of the external fire and the remaining risk of explosion.
• Key requirement: UL 9540A requires quantitative demonstration that, in the event of a fire or thermal runaway in a rack, the gas ventilation and fire suppression systems effectively prevent catastrophic propagation to the rest of the container and adjacent structures.

NFPA 855: the installation standard

While UL 9540A evaluates the equipment, NFPA 855 regulates the safe installation of these systems. It requires minimum spacing between containers (typically 3 meters), deflagration control systems, water supply for firefighters, and emergency response plans based precisely on data obtained from UL 9540A testing.

Conclusions

An effective failure analysis requires integrating operational data, thermal models, and post-incident physical evaluation. After an event, BMS logs, temperature, voltage, and current trends are correlated with CFD simulations and analysis of the affected components. Identifying the root cause, from internal cell defects to limitations in ventilation or thermal containment, is fundamental to improving the safety, resilience, and reliability of BESS systems.

Failure analysis from cell to container currently represents one of the greatest technical challenges in the safety of BESS systems based on lithium-ion batteries. Thermal propagation, or thermal runaway propagation, can transform a localized failure into a large-scale event capable of compromising complete modules, racks, and containers if adequate thermal, electrical, and structural barriers are not in place.

Standards such as UL 9540A and NFPA 855 provide the technical framework to evaluate propagation, design mitigation strategies, and improve the operational safety of energy storage installations.

As BESS systems increase in capacity and energy density, effective integration between BMS, EMS, PCS, and advanced thermal containment strategies will be decisive in ensuring long-term reliability, resilience, and industrial safety.

References

1. International Electrotechnical Commission (IEC). (2024). Electrical energy storage (EES) systems – Safety requirements for battery management systems. IEC Standards Publication.
2. Doughty, D., & Roth, E. P. (2012). A general discussion of Li ion battery safety. The Electrochemical Society Interface, 21(2), 37–44.
3. Feng, X., Ouyang, M., Liu, X., Lu, L., Xia, Y., & He, X. (2018). Thermal runaway mechanism of lithium ion battery for electric vehicles: A review. Energy Storage Materials, 10, 246–267. https://doi.org/10.1016/j.ensm.2017.05.013
4. UL Standards. (2023). UL 9540A: Test method for evaluating thermal runaway fire propagation in battery energy storage systems. Underwriters Laboratories.