Technical Classification
Definition
NREL experimental study on the propagation of thermal runaway in battery energy storage systems at module and rack level. Determines critical design parameters for propagation resistance.
Technical Background
Thermal runaway propagation is the spread of a thermal runaway event from one cell to adjacent cells, modules, and racks. Critical parameters: cell spacing (>2mm significantly reduces propagation), thermal management (active vs. passive), cell chemistry (LFP: lower propagation tendency than NMC), state of charge (SOC >80%: higher risk). UL 9540A test at unit level as proof of propagation resistance.
Risks
Total project loss from uncontrolled propagation (container fire). Secondary damage to neighboring infrastructure. Toxic gas release (HF, CO, H2). Insurance loss without UL 9540A certification. Reputational damage to the BESS industry.
Standards & Regulations
UL 9540A (Propagation Test), IEC 62619 (Safety Requirements), NFPA 855 (Installation Standard), VdS 3103 (Lithium Batteries), AGBF Guideline (Fire Brigade Requirements), DIN EN IEC 62933-5-2 (Safety of Grid-Connected EES).
Assessment Procedures
UL 9540A Propagation Test (Cell Level -> Module Level -> Unit Level -> Installation Level). Thermal imaging and temperature mapping. Gas analysis and emission quantification. Post-mortem analysis (CT, metallography). Validation against fire tests.
Typical Deficiencies
UL 9540A test at unit level not available from all manufacturers. High test costs (>EUR 100,000). Aging effects not reflected in tests. Real operating conditions (contamination, maintenance deficiencies) not accounted for. Pass/fail criteria not uniformly defined.
Relevance for Investors, Insurers & Operators
Investors: UL 9540A as minimum requirement for investment decisions. Insurers: insurability directly dependent on propagation test results. Operators: manufacturer selection and safety concept dimensioning.
Assessment by PV-BESS-Assessor
The NREL study investigates the propagation of thermal runaway at module and rack level. The experimental methodology is rigorous and directly relevant for dimensioning safety measures.
Impact on Investors
Determines insurability: systems without demonstrated propagation resistance are difficult to insure. UL 9540A certification at unit level is becoming the investment standard.
Impact on Assessments
Core reference for PV-BESS-Assessor in safety evaluations: thermal runaway propagation is the central risk. Defines testing criteria and acceptance limits.
Technical Risks
Thermal runaway remains the most severe safety risk. Propagation can lead to total loss. BMS quality and thermal management are critical protective barriers.
Regulatory Significance
Drives tightening of safety requirements: UL 9540A increasingly a prerequisite for permits. The AGBF guideline requires propagation certification.
Conclusion by PV-BESS-Assessor
PV-BESS-Assessor considers thermal runaway propagation the most critical risk in BESS. Without UL 9540A certification at unit level, we advise investors against engagement.
Last updated: 16 June 2026