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Compression Pads, Aerogel and Fire Barriers: Choosing BESS Materials by Job
A battery energy storage system asks for four distinct material jobs: a compression pad that holds cell preload as the cells breathe and swell, a thermal-isolation layer that slows heat moving between neighbors, a fire-barrier layer that supports the system's thermal-runaway strategy, and a resilient seal on hot rack and cabinet joints.
The four jobs point at four families, and the language around the third demands particular care: no converted material is "fireproof," and the propagation result belongs to the system evaluated per UL 9540A, not to a sheet.
This guide separates the jobs, maps the families H-O Products converts, and ends with a selector that turns your location into a cautiously framed direction.
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The application problem: four jobs between the cells and the cabinet door
Between a lithium-ion cell and the outside of a BESS cabinet sit several converted layers, each doing a different physical job:
- Cell and module compression: the pad between cells must keep the preload inside the window the cell supplier specifies while the cell breathes a little every cycle and grows thicker over its life from irreversible swelling.
- Thermal isolation: a thin layer that slows heat moving cell-to-cell or module-to-module, both under normal load and in delaying a neighbor heating up.
- Fire-barrier layers: material placed between cells, modules or compartments specifically to support the system's thermal-runaway propagation strategy as evaluated per UL 9540A and installed under NFPA 855.
- Hot enclosure sealing: resilient gaskets on rack and cabinet joints that hold their properties at elevated temperature.
Why the choice is challenging: the jobs sound alike and the claims have hard limits
Compression, isolation and barrier layers all live in the same few millimeters between cells, and the same word, "pad," gets used for all three. But the governing properties differ: the compression pad is governed by its force-deflection curve and compression set per ASTM D3574 and ASTM D395; the isolation layer by thermal conductivity per unit thickness; the barrier layer by its role in the system's fire-safety design.
And the claims have limits that the spec must respect: a material may carry a UL 94 flammability classification, including V-0, which describes its behavior in that specific small-scale test, but that is not a system fire rating, and UL 9540A is a test method that generates system-level data rather than a pass-fail material certificate. Honest selection keeps each claim attached to its owner.
Recommended material families
Each job maps to a family H-O Products converts. Grade-level values such as force-deflection, conductivity, temperature rating and flame classification are confirmed against the manufacturer's current technical data sheet before specifying, and any fire-safety role is confirmed at the system level.
For cell and module compression: microcellular polyurethane
PORON® microcellular polyurethane holds a near-constant push-back force across a growing deflection and resists compression set, which is exactly the duty of a cell-to-cell or module compression pad. Size the thickness so the swollen end-of-life state still sits on the working plateau rather than bottomed out, against the preload window from the cell supplier.
For thermal isolation: aerogel blanket
ArmaGel® aerogel insulation has very low thermal conductivity for its thickness, so a thin die-cut layer slows heat moving cell-to-cell or module-to-module, delaying the onset of a neighbor heating up. Performance is grade- and thickness-specific, and where the layer also plays a part in the fire-safety design, that role is confirmed at the system level.
For fire-barrier layers: dedicated barrier materials
Procell EV fire-barrier material is intended to slow the spread of heat and flame during a thermal-runaway event, as one element of a fire-safety design; the propagation result belongs to the system tested per UL 9540A and designed to NFPA 855. The broader EV battery thermal and fire-barrier family groups the materials used across cell, module and compartment layers; the specific grade follows the system designer's strategy, and H-O converts it to the geometry without asserting fire performance from the material alone.
For hot rack and cabinet sealing: closed-cell silicone
BISCO® HT / BF closed-cell silicone holds a resilient seal across the widest temperature span of the families here, and specific grades reach UL 94 flame classifications, including V-0; the rating is grade- and thickness-specific and confirmed on the data sheet. It serves the hot rack and cabinet joints and high-temperature compartment seals.
Compression, Isolation, Barrier or Seal: Material Direction Selector
A near-constant push-back force across growing deflection with low compression set keeps cell preload inside the supplier's window; size the pad so the swollen end-of-life state stays on the working plateau.
Illustrative only; confirm against current material data and a drawing review. This selector returns a material family direction, not a grade or a guaranteed performance value. Force-deflection, compression set, conductivity, temperature and flame-classification values are confirmed on the manufacturer's current technical data sheet, and any fire-propagation behavior is determined at the system level per UL 9540A within an NFPA 855 installation context.
Design and converting considerations
- Get the preload window from the cell supplier. The compression pad is sized against that window, the expected breathing, and the end-of-life swelling, not against a generic firmness.
- Thickness is the plateau lever. The pad must still sit on the working plateau in the swollen state; too thin bottoms out, too thick wastes stack space and can over-preload at start of life.
- Keep the three between-cell jobs distinct on the drawing. Compression, isolation and barrier layers can be separate parts or laminated builds, but each layer's job and owner of its result should be named.
- Say "supports the fire-safety design," not "fireproof." The barrier grade follows the system strategy; H-O converts it to your geometry and the propagation behavior is confirmed at the system level.
- UN 38.3 belongs to the battery. Compression and cushioning parts support a battery that passes the transport test series; the qualification result belongs to the tested battery and pack.
- Process fit. These parts are die-cut, kiss-cut, profiled and laminated, with adhesive and facing options, supplied flat or kitted per module.
Common failure modes
| Failure mode | Root cause | Consequence |
|---|---|---|
| Preload drifts out of window | Pad sized for day-one gap, not end-of-life swelling | The swollen state lands on the densification knee; size for the plateau across life |
| Pad thins over cycles | High compression set under sustained load | Lost preload; verify set per ASTM D3574 / D395 at temperature on the grade |
| "Fireproof material" on the drawing | A system property assigned to a sheet | An unsupportable claim; the result belongs to the system tested per UL 9540A |
| Isolation layer asked to be the barrier | Aerogel's thermal role conflated with the fire-safety role | The fire-safety design owns the barrier call; many designs use both layers deliberately |
| Seal relaxes on a hot joint | A commodity foam at rack temperatures | Lost compression at heat; closed-cell silicone with the right grade was the call |
| UL 94 V-0 read as a system rating | A small-scale material classification over-extended | The flame class describes the grade in that test, at its thickness; it is not a system fire rating |
How H-O Products helps
H-O Products is a precision die-cut converter serving EV and battery programs. We convert microcellular polyurethane, aerogel blanket, dedicated fire-barrier materials and closed-cell silicone into compression pads, isolation layers, barrier layers and hot-joint seals, cut to your cell, module and cabinet geometry. As an ISO 9001:2015 certified organization, we help you keep the four jobs distinct, match each to a family and grade direction, and convert the stock with the lamination, facing and kitting the build needs.
We do not produce the raw materials; grade values are confirmed against the manufacturer's current technical data sheet, and fire-propagation behavior is determined and owned at the system level by the system designer and testing.
Mapping materials across a BESS build?
Tell us the job at each location, the cell format and preload window, the temperatures, and the fire-safety approach. We will recommend a family and grade direction and convert samples on your geometry.
Samples typically ship in 3 to 5 business days for common die-cut configurations; standard production runs about two weeks; special orders are quoted to the job.
Family-owned in Winsted, Connecticut since 1971 · ISO 9001:2015 certified · OEMs worldwide send us a drawing — and get back their part.
Related reading
ApplicationBESS Cell Compression, Thermal & Fire-Barrier MaterialsRead →
GuideLayering a BESS Fire-Barrier Strategy: Where Compression, Insulation and Barriers Each BelongRead →
GuideHow to Spec a BESS Compression, Thermal or Fire-Barrier Part: What to Send H-O for a QuoteRead →
IndustryEV & Battery materials overviewRead →
Frequently asked questions
How does a cell compression pad hold preload as the cell swells?
By living on the working plateau of its force-deflection curve. A lithium-ion cell breathes a little every cycle and grows thicker over its life from irreversible swelling, so the gap the pad sits in shrinks. A microcellular-polyurethane pad is chosen and sized so that across that growing deflection it stays in the relatively flat middle region of its curve, between the soft initial toe and the stiff densification knee, keeping preload inside the window the cell supplier specifies.
Get the preload window, estimate breathing and end-of-life swelling, and size the thickness so the swollen state still sits on the plateau.
Can a fire-barrier material be called fireproof or certified on its own?
No. A fire-barrier material is intended to slow the spread of heat and flame during a thermal-runaway event as one element of a fire-safety design. The propagation behavior of a battery energy storage system is evaluated by the UL 9540A test method at the cell, module and unit level, and the installation is governed by NFPA 855, so the meaningful result is a property of the tested system. A material may carry a UL 94 classification such as V-0, which describes its behavior in that specific small-scale test, but that is not a system fire rating.
When should I use aerogel versus a dedicated fire-barrier material?
It depends on whether the job is delaying heat transfer or supporting a defined fire-barrier role, and often a design uses both. Aerogel insulation has very low thermal conductivity for its thickness, so a thin layer efficiently slows heat moving cell-to-cell under normal conditions and delays a neighbor heating up. A dedicated fire-barrier material is selected specifically to support the system's thermal-runaway propagation strategy as evaluated per UL 9540A. The system fire-safety design decides the layering and owns the result.
Request a quote for your battery energy storage (BESS) part
Send H-O your drawing or describe the part—engineering reviews every RFQ and converts samples on your geometry.
Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
References
- UL 9540A, Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems. UL Standards.
- NFPA 855, Standard for the Installation of Stationary Energy Storage Systems. National Fire Protection Association.
- UN 38.3, Recommendations on the Transport of Dangerous Goods, Manual of Tests and Criteria, lithium battery test series. United Nations.
- ASTM D3574, Standard Test Methods for Flexible Cellular Materials: Slab, Bonded, and Molded Urethane Foams. ASTM International.
- UL 94, Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances. UL Standards.
This article cites general engineering principles and published standards (UL 9540A for thermal-runaway fire-propagation evaluation, NFPA 855 for stationary energy-storage installation, UN 38.3 for lithium-battery transport qualification, ASTM D3574 and ASTM D395 for force-deflection and compression set, ASTM D1056 for cellular rubber classification, UL 94 for flammability classification). Material names refer to material families H-O Products converts.
Force-deflection, conductivity, temperature and flame-classification values are grade- and thickness-specific; confirm all values against the manufacturer's current technical data sheet and an engineering drawing review. No converted material is fireproof; fire-propagation behavior belongs to the system tested per UL 9540A, and transport qualification belongs to the tested battery. All parts are custom manufactured to order and minimum production run quantities apply.
H-O is a die-cut converter, not a raw-material producer, and does not certify the finished assembly.