BYD Automotive Company, Ltd.
Battery Energy Storage System
MC Cube 623S4 ESS
NFPA 69 Ventilation Analysis Rev0
Date: May 8, 2025
05/08/2025
Prepared by Fire & Risk Alliance, LLC
7640 Standish Place
Rockville, MD 20855
THIS DOCUMENT AND THE INFORMATION CONTAINED HEREIN ARE CONFIDENTIAL. ALL SUCH INFORMATION IS COPYRIGHTED BY FIRE &
RISK ALLIANCE AND BYD AUTOMOTIVE CO., LTD. ANY UNAUTHORIZED REPRODUCTION, COPY, OR DISTRIBUTION OF THE INFORMATION
CONTAINED HEREIN WITHOUT OUR WRITTEN CONSENT IS STRICTLY PROHIBITED.
MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
Executive Summary
Fire & Risk Alliance, LLC (FRA) was requested by BYD to perform an NFPA 69 ventilation analysis for their MC
Cube 623S4 Energy Storage System (ESS) product (MC Cube ESS). The MC Cube ESS is a lithium-ion battery
energy storage system (BESS) with an energy capacity of 5,365 kilowatt hours (kWh). It is equipped with ten
individual Cubes (or Units) installed on a skid for ease in transportation and installation. Each Cube is equipped
with a combustible gas concentration reduction system (emergency ventilation system) designed in accordance with
NFPA 69, Standard on Explosion Prevention Systems. During a thermal event, the explosion control system is
designed to activate and ventilate the Cube to maintain the combustible gas concentration below its explosive limit.
The primary objective of this analysis was to analyze the gas concentrations that develop within a single Cube when
the battery cells vent during thermal runaway and subsequent reduction in gas concentration achieved after
activation of the emergency ventilation system. To evaluate compliance to NFPA 69 requirements, FRA performed
Fire Dynamics Simulator (FDS) computational fluid dynamics (CFD) modeling of the venting of the battery cells.
The emergency ventilation system consists of two fans to supply fresh air into the Cube and two outlet louvers to
allow hazardous gases to be exhausted from the Cube. The gas release rate calculations for this analysis were based
on UL 9540A test data and gas generation data provided by BYD. Overall, the goal of this analysis was to ascertain
whether the system could maintain an average gas concentration within the Cube below the 25% lower flammability
limit (LFL), as required by NFPA 69. This assessment was conducted with the off-gas detection system
(Honeywell’s Li-ion Tamer) triggering the ventilation system and incorporated time delays associated with the
initiation of the ventilation system.
Summary of Findings
A ventilation analysis was conducted to evaluate the capability of the emergency ventilation system in limiting the
accumulation of flammable gases inside the MC Cube 623S4. The analysis evaluated the model RS20053B24UH
axial fan manufactured by Dongguan RUNDA Cooling Fan Co. Ltd. rated at 793 cubic feet per minute (CFM)
maximum fan flow rate at a pressure of 0 pascals (Pa). Two release cases were evaluated for this analysis:
1. Worst-case constant steady-state release rate based on information from the UL 9540A Unit Level Test Report
(per FRA recommendation).
2. Release rate considering single cell continuous release (per BYD request).
The important findings can be summarized as follows:
1. The combustible concentration shall be maintained at or below 25 percent of the LFL for all foreseeable
variations in operating conditions of material loadings [NFPA 69, 2024 Edition §8.3.1]: With the available
test reports and within the limits of the geometric details and assumptions presented in this analysis, the
emergency ventilation system is able to maintain the mean battery gas concentration within the Cube below
25% of the battery vent gas LFL (at steady state) for the release scenarios considered (Case 1 and Case 2).
2. The temperature at the inlet ventilation fans was analyzed and remains below 25°C during the simulation.
Additionally, an NFPA 68, Standard on Explosion Protection by Deflagration Venting, partial volume deflagration
hand-calculation was performed to evaluate the deflagration overpressure on the Cube walls due to ignition of the
partial gas volume in the Cube exceeding 100% LFL.

For the Case 1 release scenario, the analysis demonstrates that an overpressure of nominally 0.307 bar-g
(4.46 psig) would occur at the Cube walls resulting from the ignition of a residual partial volume gas cloud
concentration above 100% LFL at the time of the initial peak. Additionally, an overpressure of nominally
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
0.046 bar-g (0.67 psig) would occur during a deflagration at steady state after the ventilation system
activates.

For Case 2 release scenario, the analysis demonstrates that an overpressure of nominally 0.106 bar-g (1.54
psig) would occur at the Cube walls resulting from the ignition of a residual partial volume gas cloud
concentration above 100% LFL at the steady state condition after the ventilation system activates.
Information related to the strength of the MC Cube has not been provided for this analysis. As such, Factory Mutual
datasheet FMDS 07-76 §2.3.3.3.A states that when strength data is not available for a piece of equipment, a 2.9 psig
maximum allowable pressure (Pred) for normally constructed weak rectangular vessels can be assumed, with a note
that some vessel deformation may occur. Based on the calculated overpressures noted above, potential for damage
to the Cube enclosure is low for the steady state conditions for Case 1 and Case 2. However, there is a likelihood
of damage due to the initial peak deflagration scenario for Case 1. Despite this, the initial venting phase (duration
of approximately 17s) where the 4.46 psig overpressure may occur, is not a significant concern as the chances of
personnel being present during the period of initial gas buildup are low. FRA advises that an appropriate emergency
response guide (ERG)/emergency response plan (ERP) and training procedures be provided to address all potential
failure scenarios for the MC Cube ESS.
Disclaimer
This report and its contents are provided for informational purposes only and are based on the specific conditions,
data, and product specifications available at the time of its preparation. The recommendations and conclusions
presented herein are applicable solely to the specific product, site, or application described in this report. Note, FRA
was not responsible for the design of the emergency ventilation system and did not provide oversight to BYD during
their internal design process. This report is an analysis of the scenarios summarized herein.
The results presented in this report do not constitute a guarantee or warranty of performance in the field. All designs,
calculations, and recommendations should be verified through appropriate field testing and site-specific evaluations.
The accuracy and applicability of this report’s findings may be subject to changes in conditions, technology, and
standards that are beyond the scope of this analysis.
It is the responsibility of the owner, contractor, or designated party to conduct comprehensive testing and obtain all
necessary approvals to confirm the validity of the design in the field. The authors, engineers, and firms involved in
the creation of this report assume no liability for performance, errors, omissions, or failures that may arise during
construction or operation, and no warranty of fitness for a particular purpose is implied.
Note, this executive summary is an abbreviated list of findings. Refer to the main report for details of the analysis.
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
Table of Contents
Executive Summary................................................................................................................................................... ii
1
Introduction ........................................................................................................................................................1
2
UL 9540A Test Summary ..................................................................................................................................3
3
CFD FDS Analysis Methodology.......................................................................................................................5
4
CFD Model Inputs ..............................................................................................................................................7
5
Summary of Modeling Results and Analysis ...................................................................................................11
6
Summary of Findings .......................................................................................................................................18
7
Revisions ..........................................................................................................................................................20
8
Appendix ..........................................................................................................................................................21
List of Figures
Figure 1. UL 9540A Unit Level Test Initiating Module Configuration .....................................................................4
Figure 2. Key Features of an MC Cube (Left) and an MC Cube ESS (Right) ...........................................................7
Figure 3. Interior of MC Cube 623S4 ........................................................................................................................7
Figure 4. Location of Emergency Ventilation Fans and Louvers ...............................................................................8
Figure 5. FDS Model Geometry Depicting Exhaust Fan Vents (Yellow), Outlet Vents (Black), Gas Detector
Location (Red), and Gas Release Vents (Blue) ..........................................................................................................9
Figure 6. Volume Averaged Battery Gas Concentration (Case 1) ...........................................................................11
Figure 7. Gas Composition from 0 % to 100% LFL Color Bar Range (Case 1) ......................................................12
Figure 8. Gas Temperature Output (Case 1).............................................................................................................12
Figure 9. Free Air Volume Above 100% LFL (Case 1) ...........................................................................................13
Figure 10. Volume Averaged Battery Gas Concentration (Case 2) .........................................................................14
Figure 11. Gas Composition from 0 % to 100% LFL Color Bar Range (Case 2) ....................................................15
Figure 12. Gas Temperature Output (Case 2)...........................................................................................................15
Figure 13. Free Air Volume Above 100% LFL (Case 2) .........................................................................................16
Figure 14. FDS Slice Output for Centerline Velocity at the Inlet and Exhaust Vents .............................................17
List of Tables
Table 1. Constituent Concentrations from UL 9540A Cell Level Test Report ..........................................................3
Table 2. Cell Flammability Properties from UL 9540A Cell Level Test Report .......................................................3
Table 3. FDS Model Inputs ........................................................................................................................................9
Table 4. UL 9540A Unit Level Test Report Observations .......................................................................................10
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
1 Introduction
Fire & Risk Alliance, LLC (FRA) was requested by BYD to perform an NFPA 69 ventilation analysis for their MC
Cube 623S4 Energy Storage System (ESS) product (MC Cube ESS or Cube). The MC Cube ESS is a lithium-ion
battery energy storage system (BESS) with an energy capacity of 5,365 kilowatt hours (kWh). It is equipped with
ten individual Cubes (or Units) installed on a skid for ease in transportation and installation. Each Cube is equipped
with a combustible gas concentration reduction system (ventilation system) designed in accordance with NFPA 69.
During a thermal event, the explosion control system is designed to activate and ventilate the Cube to maintain the
combustible gas concentration below its explosive limit.
The primary objective of this analysis was to analyze the gas concentrations that develop within a single Cube when
the battery cells vent during thermal runaway and subsequent reduction in gas concentration achieved after
activation of the emergency ventilation system. To evaluate compliance to NFPA 69 requirements, FRA performed
Fire Dynamics Simulator (FDS) computational fluid dynamics (CFD) modeling of the venting of the battery cells.
1.1 Scope
The scope of this analysis was limited to conducting a ventilation analysis and a resulting partial volume
deflagration analysis for a fully populated MC Cube 623S4 (single unit with 16 modules in a rack) with lithium
iron phosphate (LFP) model CBFAF cells (26 cells per module). The scope aimed to assess battery gas
concentrations within the Cube. This assessment relied on battery vent gas release rates derived from the UL 9540A
tests and gas data provided by BYD and considered the presence of a predetermined forced ventilation flow rate
from the inlet fans. The emergency ventilation system is activated based on the signals from the off-gas detection
system (Honeywell Li-ion Tamer) within the Cube. The analysis of the detection system itself is not part of this
scope of work. The methodology and analysis presented in this report are based on a sealed, damage-free Cube.
The analysis does not consider site-specific conditions and is focused on the MC Cube 623S4 general configuration
and operation. This scope of work does not include a detailed compliance review to NFPA 69 and is only intended
to evaluate the performance objectives noted below.
1.2 Performance Objective
The objective of this analysis is to follow the requirements of NFPA 69, 2024 Edition §8.3.1 to assess the ability of
the emergency ventilation system to maintain the average combustible gas concentration (for the noted failure and
gas venting scenarios within the Cube) at or below 25% of the battery vent gas lower explosive/flammability limit
(LEL/LFL)1 when the gas detector is triggered at 100 parts per million (ppm) (proposed BYD detection threshold).
A partial volume deflagration evaluation was also conducted following the requirements of NFPA 68, 2023 Edition
§6.3.1.1 to assess the overpressure resulting from the ignition of flammable gases within the Cube before and after
activation of the NFPA 69 emergency ventilation system. NFPA 68 provides guidelines on explosion prevention
by deflagration venting.
1.3 Acronyms and Abbreviations
Battery Energy Storage System
Computational Fluid Dynamics
Energy Storage System
Emergency Response Guide
BESS
CFD
ESS
ERG
Large Eddy Simulation
Lower Flammability Limit
Lithium Iron Phosphate
National Fire Protection Association
LES
LFL
LFP
NFPA
1
Note, LEL and LFL can be used interchangeably to refer to the lowest concentration for which a mixture is able to sustain a
flame. LFL is used throughout the remainder of this report for consistency with NFPA 69 and UL 9540A result reporting.
1
MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
Emergency Response Plan
ERP
Fire Dynamics Simulator
Fire & Risk Alliance
Lower Explosive Limit
FDS
FRA
LEL
National Institute of Standards and
Technology
SmokeView
Lower Flammability Limit
NIST
SMV
LFL
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
2 UL 9540A Test Summary
UL 9540A testing (4th edition) was performed at the cell2, module3, and unit level4 for the MC Cube 623S4. The
cell vent gas composition determined from the cell level test is provided in Table 1 and the flammability
characteristics are provided in Table 2. The cell level test report indicated that a battery cell produces approximately
224 L of battery gas when the cell undergoes thermal runaway.
Table 1. Constituent Concentrations from UL 9540A Cell Level Test Report
Table 2. Cell Flammability Properties from UL 9540A Cell Level Test Report
Each battery module (Model E206) contains 26 cells with a nominal voltage rating of 83.2 V and nominal capacity
rating of 403 Ah. In the module level test, a Mica film heater was used to initiate thermal runaway in a single cell
and overall, three cells exhibited thermal runaway within the module (cell to cell propagation occurred within the
module). For the unit level test, each unit (Model MC-B536-U-R4M04) consisted of 16 modules with a nominal
voltage rating of 1,331.2 V and nominal capacity rating of 536 kWh (1P16S electrical configuration). Similar to the
module level test, a Mica film heater was used to initiate thermal runaway in a single cell. Refer to Figure 1 for the
test configuration. Initiating module 7 contains 26 cells with cell 12 serving as the initiating cell.
2
4th Edition UL 9540A Cell Test Report, Project #4791184956, Dated 07/19/2024
4th Edition UL 9540A Module Level Test Report, Project #80232592, Dated 11/01/2024
4 th
4 Edition UL 9540A Unit Level Test Report, Project #80232593, Dated 01/01/2025
3
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
Figure 1. UL 9540A Unit Level Test Initiating Module Configuration
Overall, six cells exhibited thermal runaway within one module during the test (cell to cell propagation within the
module occurred). No flaming was observed in either test. Limited information regarding the gas generation rates
and composition/volumes of vented gases was provided in the unit level report. The unit level test report provides
timelines of when cells vented and went into thermal runaway. This information was used in determining the gas
release rate for the Case 1 scenario. Additional gas generation data provided by BYD yielded inputs for the Case 2
scenario (details provided in later sections).
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
3 CFD FDS Analysis Methodology
FRA utilized the CFD program FDS to model battery gas venting resulting from the thermal runaway of battery
cells in the MC Cube 623S4. This analysis included an evaluation of battery gas concentrations (expressed as a
percentage of the battery gas LFL) within the Cube under conditions of forced ventilation from the emergency
ventilation system (i.e. exhaust fans). The forced ventilation was implemented within the initiating Cube as a safety
measure to prevent explosions and catastrophic enclosure failures by maintaining combustible gas concentrations
below the LFL.
The following steps were used to complete this analysis:

Define Performance Objectives: Performance objectives for the emergency ventilation system were
determined in accordance with NFPA 69.

Gas Source Term Development: Develop conservative battery gas release source term based on UL
9540A cell, module, and unit level test results (presented in Section 2 of this report).

Model Development: Construct the MC Cube 623S4 geometry (including the ventilation fans, inlet/outlet
louvers, battery modules/racks, gas detector, and other internal components impacting the free air volume
within the enclosure) using Pyrosim.

Model Analysis: Evaluate the conditions inside the MC Cube 623S4 during battery venting assuming a
five-second delay in ventilation system activation (to mimic fan ramp up) following gas detector activation
at 100 ppm.

Model Results: Calculate the average battery gas concentration inside of the MC Cube 623S4 based on the
interior free air volume over time. Compare the average battery gas concentration in the Cube to the NFPA
69 performance objectives to determine if the performance of the ventilation system meets the intent of
NFPA 69.
The programs used in performing this analysis are described in the following sections.
3.1 Pre-Processor
The 3D models for the analysis were developed in Pyrosim which is a graphical user interface and pre-processor
for FDS. Pyrosim provides a visual depiction of the developed model geometry and provides error checking for
meshing in the model to reduce the potential for numerical instabilities that affect the FDS input file.
3.2 FDS Solver
FDS is a fire modeling software that uses CFD to simulate fire behavior in complex 3D environments such as
buildings, equipment, and surrounding structures. Developed by the National Institute of Standards and Technology
(NIST), FDS uses the Large Eddy Simulation (LES) technique to analyze fluid flows in fires. The LES technique
separates fluid flow into two parts: the large-scale turbulence, which is computed directly from fluid motion
equations, and the small-scale turbulence, which is estimated using a sub-grid model. This technique is ideal for
modeling fire growth, smoke movement, and heat radiation, which are driven by large-scale structures in fluid flow.
With over 500 peer-reviewed publications and over 20 years of continued development, FDS has been extensively
validated and proven to be an effective tool for fire simulations5,6.
CFD models, such as FDS, divide the model space into thousands or millions of tiny volumes or cells. Inside each
small volume, variables such as gas temperature and velocity are considered uniform in the cell volume, changing
5
McGrattan, K., et al.(c) National Institute of Standards and Technology. Fire Dynamics Simulator Technical Reference
Guide Volume 2: Verification. Sixth Edition, June 28, 2022.
6
McGrattan, K., et al.(d) National Institute of Standards and Technology. Fire Dynamics Simulator Technical Reference
Guide Volume 3: Validation. Sixth Edition, June 28, 2022.
5
MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
only with time. The model then solves simplified “low Mach number” equations of fluid and energy flow for each
cubic volume within the computational domain, thereby describing the large eddy fire phenomenon over all of the
cubic volumes in the computational domain as a function of time.
The portions of the FDS code used to model various aspects of fire physics (i.e. fire growth and spread, smoke
development and spread) have been rigorously tested against experimental data. These studies have dealt with areas
involving fire growth and spread, suppression, and dispersion of smoke and hot gases throughout a large-scale
environmental domain.
3.3 Post Processing and Visual Representations
Smokeview (SMV) is a post processor which is developed by NIST and is used to provide visual outputs that are
obtained from the FDS solver. The FDS solver develops *.smv files which are then accessed using SMV software
to analyze and present various results.
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
4 CFD Model Inputs
4.1 MC Cube 623S4 ESS
Figure 2 presents the overall layout of a single MC Cube 623S4 and an MC Cube ESS skid. Each Cube has a single
rack of 16 battery modules, with each module having 26 LFP cells. For this analysis, a single Cube has been
evaluated.
There are two emergency ventilation fans mounted at the bottom of the Cube door. The emergency ventilation fans
supply air into the enclosure through louvers. The enclosure is also equipped with two exhaust louvers, located near
the top of the Cube door, which open upon activation of the emergency ventilation fans. The emergency ventilation
system is activated using a Li-on Tamer gas detection system manufactured by Honeywell.
Figure 3 and Figure 4 present the interior geometry and emergency ventilation system components of the MC Cube.
Figure 2. Key Features of an MC Cube (Left) and an MC Cube ESS (Right)
Figure 3. Interior of MC Cube 623S4
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
Figure 4. Location of Emergency Ventilation Fans and Louvers
4.2 FDS Model Geometry
The Cube was modeled with nominal internal dimensions of 2.3 m (H) x 1.1 m (W) x 1.0 m (D). The unit has a
single rack composed of 16 modules. Based on the UL 9540A test results, it was assumed that a failure was limited
to a single battery module. The modules are modeled with external dimensions of 0.95 (D) x 0.8 m (W) x 0.13 m
(H) to align the modules to the computational grid.
Figure 5 presents the general FDS model layout for the two scenarios analyzed in this report and shows the location
of the emergency ventilation exhaust fans and outlet louvers. The Cube FDS model included all the module
obstructions as well as the structural steel and auxiliary equipment that occupy solid volume within the enclosure.
The Cube is equipped with Li-ion Tamer gas detection which activates the emergency ventilation fans and louvers
(100 ppm detection threshold). The louver openings were modeled as vents with an effective free area of 8,000
mm2, based on the mechanical data sheet provided by BYD and to align with the computational grid.
The emergency ventilation fans and exhaust louvers were assumed to activate with a five second delay to account
for the delay associated with the fan ramp up. The fans were assumed to immediately ramp up to speed based on
the pressure differential across the fan inlet and outlet.
The battery gas was released inside the Cube from two small venting surfaces placed on a module located near the
floor of the Cube to present the worst-case location of venting. The vents were modeled to release battery gas at the
rates detailed in Section 4.4. The volume averaged battery vent gas concentration within the Cube as a function of
time was measured using devices in the FDS model.
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
Figure 5. FDS Model Geometry Depicting Exhaust Fan Vents (Yellow), Outlet Vents (Black), Gas Detector
Location (Red), and Gas Release Vents (Blue)
4.3 Grid Size and Environmental Conditions
Table 3 summarizes the grid size and environmental conditions utilized in this analysis.
Table 3. FDS Model Inputs
FDS Parameter
Value
Grid Size7 (m)
0.01
Ambient Temperature (°C)
20
Ambient Pressure (bar)
1.03
4.4 Design Gas Release Scenarios
The design scenarios for this analysis were developed based on the requirements of NFPA 69, information shared
by BYD, and information from the UL 9540A cell, module, and unit level test reports. The gas mixtures observed
in the UL 9540A cell level testing included total hydrocarbons, carbon dioxide (CO 2), carbon monoxide (CO), and
hydrogen (H2) with an American Society for Testing and Materials (ASTM) determined LFL of 7.15% and a 25%
LFL of 1.79%. There were two cases modeled for this analysis based on the specific inputs from the above
information.
4.4.1 Case 1 – Worst-Case Constant Steady-State Release Rate Based on UL 9540A Unit
Level Test Report
For Case 1, the source term was developed based on the information in the UL 9540A unit level test report. Based
on the timeline provided in the test results, the first two cells vent and go into thermal runaway over a span of three
minutes (see results excerpt in Table 4). During this period, there is an overlap in time where both cells are releasing
battery gas simultaneously. Meaning that the overall release duration can conservatively be assumed to be between
2-3 minutes (which is a typical conservative estimate for battery cell gas releases). Given that a single cell releases
224 L of gas (per the cell level test report), and a total of about 448 L of gas release for 2 cells, the average gas
7
The grid resolution has been determined for such BESS enclosure ventilation analyses to be appropriate to capture the
necessary fluid dynamics and details of the flow conditions within an enclosure, with sufficient resolution across the
modeling surfaces.
9
MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
release rate over a 2.5-minute duration would be approximately 180 L/min. With a calculated density of 0.721 kg/m 3
for the cell vent gas composition mentioned in Table 2, the battery gas mass release rate was determined to be
0.0022 kg/s or 2.2 g/s. This mass release rate was then applied as a constant indefinite release source term for the
Case 1 model.
Table 4. UL 9540A Unit Level Test Report Observations
4.4.2 Case 2 – Release Rate Considering Single Cell Continuous Release (BYD Data)
For Case 2, a second source term was developed based on direction from BYD which assumed a single cell
continuous release of the vent gases with a dynamic release rate. The data shared by BYD for this particular case is
expected to be released as a report for stakeholder consumption and the report will outline the data and the methods
used for determination/calculation of that data.
For this case, the gas generation rate was modeled based on detailed gas generation data provided by BYD from
one of the cell level tests conducted. This involved a ramp from the beginning of the simulation to approximately
35 seconds at which a peak gas generation rate of 213 L/min was reached. With a calculated density of 0.721 kg/m 3
for the cell vent gas composition mentioned in Table 2, the battery gas mass release rate was determined to be
0.0026 kg/s or 2.6 g/s. This mass release rate was then applied as a constant indefinite release source term for the
Case 2 model.
4.5 Emergency Ventilation Fan Flow Rate
The analysis was performed to examine the volume averaged battery gas concentration observed within the Cube
with two ventilation fans installed as inlet fans in the Cube, supplying outside air into the Cube while hazardous
gases are exhausted out of the louvers located at the top of the Cube door. The fan model that is proposed and was
utilized for modeling is a Dongguan RUNDA axial exhaust fan (model no. RS20053B24UH) rated at 793 CFM
maximum fan flow rate at 0 Pa. The fan datasheet is shared in Section 8.1 of this report.
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
5 Summary of Modeling Results and Analysis
5.1 Case 1 – Worst-Case Constant Steady State Release Rate Based on UL
9540A Unit Level Test Report (FRA)
Figure 6 displays the results for Case 1, where the worst-case gas release rate was assumed to be based on the UL
9540A unit level test with a 2.2 g/s constant indefinite battery gas release rate. The plot shows the volume averaged
battery gas concentration in percent of battery gas LFL.
Figure 6. Volume Averaged Battery Gas Concentration (Case 1)
The simulation was set up for a duration of 120 seconds (two minutes) and steady state conditions were achieved
within the first 60 seconds. The average gas concentration in the battery compartment exceeded the 25% LFL
threshold initially. Gas detection occurred at approximately 7.5 seconds, followed by the emergency ventilation
system activating 5 seconds later at 12.5 seconds.
The initial peak exceeding 25% LFL can be attributed to the small size of the Cube and the small free air volume.
However, after the activation of the emergency ventilation system, the battery gas steady-state concentration within
the Cube was maintained at approximately 7.7% LFL. This demonstrates that under the expected worst-case gas
release rate (determined by FRA) from the UL 9540A unit level test, the emergency ventilation fans and exhaust
louvers maintain the average battery gas concentration at steady state within the enclosure below 25% of the battery
gas LFL8 after system activation for the simulated release scenario. Figure 7 below shows the gas composition
within the Cube at steady state.
8
This assumes that the fans are able to continue operating throughout the cell venting process and are not impacted by any
elevated temperatures that may be experienced.
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
Figure 7. Gas Composition from 0 % to 100% LFL Color Bar Range (Case 1)
5.1.1 Steady State Gas Temperature Analysis
Figure 8 depicts the gas temperatures within the enclosure at steady state conditions. As illustrated, gas temperatures
at the inlet vent are close to the assumed ambient temperature of 20°C and do not exceed 25°C.
Figure 8. Gas Temperature Output (Case 1)
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
5.1.2 Partial Volume Deflagration Overpressure Analysis
Based on the analysis in this report, there is a residual quantity of gas that exceeds 100% LFL even with the
emergency ventilation system running. This results in the potential for an overpressure event, posing a deflagration
hazard.
The total volume inside the enclosure that was above 100% LFL battery gas concentration was calculated by the
model and is illustrated in Figure 9. The free air volume above 100% LFL was observed to be 0.066 m 3 for the
initial peak and 0.01 m3 for the peak present during the steady state condition.
Figure 9. Free Air Volume Above 100% LFL (Case 1)
Using the FDS model predictions for gas concentrations, hand calculations were performed using the guidelines of
NFPA 68. The total volume inside the enclosure above 100% of battery vent gas LFL was determined to have
equivalent average gas concentration of 15%. Based on the gas properties from the UL 9540A cell level test report
and the calculated stoichiometric volume concentration, the unvented enclosure pressure resulting from a partial
volume deflagration was calculated to be 0.307 bar-g (4.46 psig) for the initial peak and 0.046 bar-g (0.67 psig) for
the steady state condition. The spreadsheet calculations are attached in Section 8.2 of this report.
5.2 Case 2 – Release Rate Considering Single Cell Continuous Release (BYD
Data)
Figure 10 displays the results for Case 2, where the gas release rate was assumed to be based on a single cell venting,
based on data provided by BYD, with a 2.6 g/s constant indefinite battery gas release rate applied after an initial
(lower) release rate during the initial 35 second ramp period. The plot shows the volume averaged battery gas
concentration in percent of battery gas LFL within the Cube.
13
MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
Figure 10. Volume Averaged Battery Gas Concentration (Case 2)
The simulation was setup for a duration of 300 seconds (five minutes) over which a steady state condition was
observed (within the first minute) as demonstrated from the plot. The rate of rise of the average gas concentration
within the Cube was lower than what was observed for Case 1, which was a result of the initial (reduced) release
rate prior to the constant indefinite rate. Despite the lower release rate, the gas detection occurred at a similar time
(8 seconds) with the emergency ventilation system activating at 13 seconds. As the peak release rate takes 35
seconds to be reached, the overall gas concentration remains lower within the Cube in this scenario compared to
Case 1. Steady-state conditions are achieved within the enclosure within 40 seconds, and the average concentration
is maintained at approximately 9% LFL. This demonstrates that with a ramp-up to the peak gas generation rate
(from the data provided by BYD), the emergency ventilation system maintains the gas concentration within the
enclosure below 25% of the battery gas LFL9. The average gas concentration within the enclosure was below 25%
LFL at all times during the venting event. Figure 11 presents the outputs of the simulation and shows the gas
contours within the Cube.
9
This assumes that the fan is able to continue operating throughout the cell venting process and is not impacted by any
elevated temperatures that may be experienced.
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
Figure 11. Gas Composition from 0 % to 100% LFL Color Bar Range (Case 2)
5.2.1 Steady State Gas Temperature Analysis
Figure 12 depicts the gas temperatures within the enclosure at steady state conditions. As observed from the results,
the gas temperatures at the inlet vent are close to the assumed ambient temperature of 20°C and do not exceed 25°C.
Figure 12. Gas Temperature Output (Case 2)
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
5.2.2 Partial Volume Deflagration Overpressure Analysis
Based on the analysis in this report, there is a residual quantity of gas that exceeds 100% LFL even with the
emergency ventilation system running. This results in the potential for an overpressure event, posing a deflagration
hazard.
The total volume inside the Cube that was above 100% LFL battery gas concentration was calculated by the model
and is shown in Figure 13. The free air volume above 100% LFL was observed to be 0.023 m 3 for the peak present
during the steady state condition.
Figure 13. Free Air Volume Above 100% LFL (Case 2)
Using the FDS model predictions for gas concentrations, hand calculations were performed using the guidelines of
NFPA 68. The total volume inside the Cube above 100% of battery gas LFL was determined to have equivalent
average volumetric battery gas concentration of 20%. Based on the gas properties from the UL 9540A cell level
test report and the calculated stoichiometric volume concentration, the unvented enclosure pressure resulting from
a partial volume deflagration in the Cube was calculated to be 0.106 bar-g (1.54 psig) for the steady state condition.
The spreadsheet calculations are attached in Section 8.3 of this report.
5.3 Expected Volumetric Fan Flow Rate at the Inlet and Outlet Vents
This section provides information on the expected volumetric fan flow rate at the inlet and outlet vents of the
ventilation system.
The FDS simulation for the analysis included air velocity outputs at the vents of the MC Cube 623S4 FDS model.
The FDS model consisted of two vents each for the inlet and outlet. Each of the four vents was modeled with
dimensions of 0.1 m x 0.08 m. The centerline velocity from the simulation output for the FDS model was observed
to be around 19.5 m/s at each of the two top exhaust vents and 17 m/s at each of the two bottom inlet vents. The
FDS slice output for centerline velocity at the inlet and exhaust vents is shown in Figure 14.
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
Using the volumetric flow rate equation Q=AV (where ‘Q’ is the volumetric flow rate in m 3/s; A is the area of the
surface in m2, and V is the centerline velocity of the gas through the surface in m/s) the expected volumetric flow
rate at the top exhaust and bottom inlet vents was calculated. The centerline velocity was assumed to exist over the
entire vent surface area. Substituting the values in the equation described above, with vent surface area of 0.008 m 2
and the centerline velocity observed from the FDS output, the calculated volumetric flow rate was 0.156 m 3/s (330
CFM) through each of the two top exhaust vents and 0.136 m 3/s (290 CFM) through each of the two bottom inlet
vents.
Therefore, the three main volumetric flow rates quantified from this analysis are as follows:

Rated Max Volumetric Fan Flow Rate per fan from the fan spec sheet: 793 CFM.

Expected Volumetric Fan Flow Rate at the inlet vents: 290 CFM.

Expected Volumetric Flow Rate at the exhaust vents: 330 CFM.
Note: Differences may be observed due to numerous factors such as variations in construction of the ESS, the
specifics of the site, weather conditions, and other variables that may impact the airflow and performance of the
explosion control system. Field testing should be performed to validate that the design and installation meets the
intent of the system performance.
Figure 14. FDS Slice Output for Centerline Velocity at the Inlet and Exhaust Vents
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
6 Summary of Findings
A ventilation analysis was conducted to evaluate the capability of the emergency ventilation system in limiting the
accumulation of flammable gases inside the MC Cube 623S4. Two release cases were evaluated for this analysis:
1. Worst-case constant steady-state release rate based on information from the UL 9540A Unit Level Test Report
(per FRA recommendation).
2. Release rate considering single cell continuous release (per BYD request).
The important findings can be summarized as follows:
1. The combustible concentration shall be maintained at or below 25 percent of the LFL for all foreseeable
variations in operating conditions of material loadings [NFPA 69, 2024 Edition §8.3.1]: With the available
test reports and within the limits of the geometric details and assumptions presented in this analysis, the
emergency ventilation system is able to maintain the mean battery gas concentration within the Cube below
25% of the battery vent gas LFL (at steady state) for the release scenarios considered (Case 1 and Case 2).
2. The temperature at the inlet ventilation fan was analyzed and remains below 25°C during the simulation.
Additionally, an NFPA 68 partial volume deflagration hand-calculation was performed to evaluate the deflagration
overpressure on the Cube walls due to ignition of the partial gas volume in the Cube exceeding 100% LFL.


For the Case 1 release scenario, the analysis demonstrates that an overpressure of nominally 0.307 bar-g
(4.46 psig) would occur at the Cube walls resulting from the ignition of a residual partial volume gas cloud
concentration above 100% LFL at the time of the initial peak. Conversely, an overpressure of nominally
0.046 bar-g (0.67 psig) would occur during a deflagration at steady state after the ventilation system
activates.
For Case 2 release scenario, the analysis demonstrates that an overpressure of nominally 0.106 bar-g (1.54
psig) would occur at the Cube walls resulting from the ignition of a residual partial volume gas cloud
concentration above 100% LFL at the steady state condition after the ventilation system activates.
Information related to the strength of the MC Cube has not been provided for this analysis. As such, Factory Mutual
datasheet FMDS 07-76 §2.3.3.3.A states that when strength data is not available for a piece of equipment, a 2.9 psig
maximum allowable pressure (Pred) for normally constructed weak rectangular vessels can be assumed, with a note
that some vessel deformation may occur. Based on the calculated overpressures noted above, potential for damage
to the Cube enclosure (and subsequent shrapnel or projectile risk for personnel) is low for the steady state conditions
for Case 1 and Case 2. However, there is a likelihood of potential shrapnel or projectile hazards for the initial peak
deflagration scenario for Case 1. Despite this, the initial venting phase (duration of approximately 17s) where the
4.46 psig overpressure may occur, is not a significant concern as the chances of personnel being present during the
period of initial gas buildup are low. FRA advises that an appropriate ERG/ERP and training procedures be provided
to address all potential failure scenarios for the MC Cube ESS.
Disclaimer
This report and its contents are provided for informational purposes only and are based on the specific conditions,
data, and product specifications available at the time of its preparation. The recommendations and conclusions
presented herein are applicable solely to the specific product, site, or application described in this report. Note, FRA
was not responsible for the design of the emergency ventilation system and did not provide oversight to BYD during
their internal design process. This report is an analysis of the scenarios summarized herein.
The results presented in this report do not constitute a guarantee or warranty of performance in the field. All designs,
calculations, and recommendations should be verified through appropriate field testing and site-specific evaluations.
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
The accuracy and applicability of this report’s findings may be subject to changes in conditions, technology, and
standards that are beyond the scope of this analysis.
It is the responsibility of the owner, contractor, or designated party to conduct comprehensive testing and obtain all
necessary approvals to confirm the validity of the design in the field. The authors, engineers, and firms involved in
the creation of this report assume no liability for performance, errors, omissions, or failures that may arise during
construction or operation, and no warranty of fitness for a particular purpose is implied.
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
7 Revisions
Date
Revision
Reason for Issue
Developed By
Reviewed By
Approved By
May 8, 2025
0
Initial
SM/DKB
GM
NR
Revision
Section
Change Noted
20
MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
8 Appendix
8.1 Appendix 1 – Emergency Ventilation Fan Specification Datasheet
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MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
8.2 Appendix 2 – NFPA 68 Partial Volume Deflagration Hand Calculations
(Case 1)
30
MC Cube 623S4 ESS NFPA 69 Ventilation Analysis
8.3 Appendix 3 – NFPA 68 Partial Volume Deflagration Hand Calculations
(Case 2)
33