The engineering decision
Release the enclosure only after loads, cell movement, tolerance stack, electrical clearances, thermal interfaces, assembly sequence and service access are resolved together.
Have these inputs ready
- Cell dimensions, tolerances and expansion behavior
- Equipment loads, vibration and mounting points
- Ingress, electrical clearance and service requirement
- Assembly process and inspection access
Mechanical design interfaces
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Trace loads from each cell to the equipment mount
Define how acceleration, vibration, handling and installation loads pass through cells, spacers, frames and enclosure mounts. Avoid relying on wires, tabs or thermal pads as structural restraints unless they are designed and validated for that function.
Cell dimensional change over state of charge, temperature and life can alter preload and contact. Obtain the cell supplier’s mechanical limits and use a tolerance stack for the complete assembly.
| Interface | Question | Evidence |
|---|---|---|
| Cell to restraint | What limits movement and compression? | Tolerance and preload analysis |
| Conductor to cell | Can movement load the terminal or weld? | Displacement and fatigue review |
| Pack to equipment | Where do service loads enter? | Mount load case and fastener definition |
| Cover / seal | Can it be assembled repeatedly? | Compression, flatness and leak test |
Reserve space for electrical and thermal functions
Busbars, fuses, switching, sense harnesses and insulation need controlled routing and clearances. The enclosure should protect these parts from abrasion and service damage while preserving inspection access.
Thermal pads and cold plates require defined contact pressure, flatness and material thickness. Their tolerance stack belongs in the mechanical drawing, not only in a thermal simulation.
| Output | Must define |
|---|---|
| Envelope drawing | Datums, keep-outs, connectors and service clearance |
| Cell restraint | Material, preload, tolerance and assembly method |
| Insulation system | Materials, overlap, routing protection and inspection |
| Thermal interface | Contact area, thickness, pressure and allowable gap |
Resolve cell failure behavior and the enclosure gas path
Use the failure behavior of the exact cell construction. Many cylindrical and hard-case prismatic cells have designed pressure-relief features. Pouch cells, often sold as Li-polymer, can swell and release gas through seal opening or pouch rupture. The Li-polymer label does not establish a valve, opening pressure or predictable release direction.
At pack level, assess gas and ejecta release, enclosure pressure, neighboring cells and exposure of people or equipment. Define pressure relief or containment from the hazard analysis and representative tests. Check that restraints, insulation and thermal interfaces leave any specified cell vent functional. Ingress sealing and normal pressure equalization must be reviewed alongside the fault gas path.
| Boundary | Review input | Release evidence |
|---|---|---|
| Cell construction | Specified vent, pouch seams and credible rupture behavior | Cell mechanical data and representative failure evidence |
| Pack enclosure | Gas route, pressure, debris and installed orientation | Validated relief or containment design and protected outlet location |
| Adjacent cells / equipment | Heat, gases and electrical energy from a failed cell | Propagation and integration assessment for the defined configuration |
Prove the assembly sequence before design freeze
Use a pilot build to confirm tool access, insertion direction, cable routing, torque sequence, inspection points and rework limits. Capture steps that can damage insulation or alter thermal contact.
A mechanically valid CAD assembly can still be impractical to build. Release drawings with the build and inspection method that preserves the validated configuration.
Measurement & validation procedure
| Test / review | Record | Acceptance condition |
|---|---|---|
| Resolve tolerances and load paths | Cell dimensions, permitted restraint, mounting loads and stack analysis | Movement, preload and clearances remain within documented limits |
| Perform a representative pilot assembly | Tool access, fastener sequence, routing, insulation inspection and thermal contact | The assembly can be built and inspected without damaging functional interfaces |
| Review cell failure and enclosure pressure | Cell-specific failure evidence, gas-path drawing and specialist test report for the defined configuration | Relief or containment and propagation behavior meet the application hazard criteria |
| Release the mechanical definition | Drawings, material revisions, torque requirements and service sequence | Production and service reproduce the qualified assembly |
Continue this engineering task
Related engineering guidance
Prepare the evidence for review
- Cell mechanical data and tolerance stack
- Equipment load cases and mounting drawing
- Enclosure, insulation and fault gas-path drawings
- Assembly, inspection and cell-failure review
Technical sources & claim support
References support the stated engineering principles. Worked examples use their explicitly stated assumptions.
- Texas Instruments: Designing high cell count automotive applications with BQ79616-Q1 ↗
Slides 12, 14–19 and 35–37: high-current and temperature-gradient challenges, wiring and mechanical design, sensing, balancing and system requirements
System design reference; component limits and architecture must be verified for the selected implementation.
- NREL: Battery Thermal Characterization (2016) ↗
NREL/PO-5400-66260: measured heat capacity, heat generation, thermal imaging and model validation
Laboratory methodology overview; obtain properties and duty-specific measurements for the actual assembly.
- IEC 62619:2022 ↗
Official scope: industrial secondary lithium cells and batteries
Industrial-cell and battery safety scope; develop the conformity test plan from the full standard and applicable edition.
- NASA: Alternative Orion Small Cell Battery Design Support ↗
Thermal-runaway controls, enclosure venting path and propagation tests
Spacecraft design example supporting separate gas-path and propagation review; its dimensions and performance do not transfer to other packs.
- Comprehensive Hazard Analysis of Failing Automotive Lithium-Ion Batteries in Overtemperature Experiments (2020) ↗
Section 5.1.2: pouch rupture and gas emission
Experimental results for the tested pouch construction and conditions; obtain failure data for the actual cell.
Key claims and evidence
| Claim | Basis | Reference |
|---|---|---|
| Mechanical design and harness routing are coupled BMS system concerns | documentation | Texas Instruments: Designing high cell count automotive applications with BQ79616-Q1 |
| Overtemperature experiments observed gas release through pouch rupture in the tested construction | documentation | Comprehensive Hazard Analysis of Failing Automotive Lithium-Ion Batteries in Overtemperature Experiments (2020) |
| The cell release mechanism and enclosure gas path require separate review | inference | NASA: Alternative Orion Small Cell Battery Design Support; Comprehensive Hazard Analysis of Failing Automotive Lithium-Ion Batteries in Overtemperature Experiments (2020) |
| The interface tolerance and pressure review is a proposed mechanical design check | workflow | engineering workflow |
| Pilot assembly is required to verify access and repeatability | workflow | engineering workflow |