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Inside a BESS: The Materials That Keep Battery Storage Safe
A battery energy storage system, or BESS, is a bank of rechargeable batteries packaged with power electronics and controls so it can absorb electricity when it is cheap or abundant and release it when the grid needs it. The industry installed roughly 275 GWh of new battery storage worldwide in 2025, a jump of more than 60% in a single year, with data centers and renewable integration pushing 2026 forecasts higher still.
The parts that get the headlines are the cells and the inverters. Walk through an enclosure with a design engineer, though, and you find another category of component doing constant, quiet work: die-cut and converted materials. Fire barriers, thermal pads, gaskets, insulation, vents. No moving parts, no firmware, no failure alarms. This is a tour of where they live and what each one is doing.
Open the full material reference → Get a part quoted →
What is a battery energy storage system?
At its simplest, a BESS does one job: it moves energy through time. Solar output peaks at midday and demand peaks in the evening. Wind arrives on its own schedule. A data center draws a flat, enormous load and cannot tolerate a sag. Storage smooths all of that, charging when supply is plentiful and discharging when it is not, while also providing services the grid values in their own right, such as frequency regulation and backup during outages.
Physically, most grid-scale systems are containerized. Inside a weather-tight enclosure sit racks of lithium-ion battery modules, a battery management system watching cell voltages and temperatures, a power conversion system converting between the batteries’ direct current and the grid’s alternating current, thermal management to hold cells in their comfortable temperature band, and fire detection and suppression. Lithium-ion dominates the market at four to eight hours of duration; longer-duration chemistries remain a small share of installations today.
Are battery energy storage systems safe?
It is the question people ask first, and it deserves a direct answer rather than reassurance. Lithium-ion cells can, under abuse or manufacturing defect, enter thermal runaway: a self-accelerating release of heat and flammable gas. The risk is real, it is well characterized, and the industry’s response is layered.
Battery management systems watch every cell and shut down abnormal conditions. Gas detection systems sense off-gassing before flame appears. Suppression and cooling systems act on what they detect. Installation codes such as NFPA 855 govern spacing, ventilation and explosion control, and test programs such as UL 9540A prove, with instrumented burns, how a failure propagates and whether it stays contained.
Passive materials are the layer underneath all of those layers. A barrier between two cells needs no sensor to trigger it and no power to hold it in place. It simply has to be there, at the right thickness and density, every hour of a twenty-year service life. That is why current designs treat these materials as engineered, controlled components rather than packaging, and why a fire-test result belongs to the exact material stack that was in the system when it burned.
Every converted part inside a BESS is solving one of eight problems. The interactive version of this map, with the material families attached to each zone, lives on the BESS reference page.
Zones 1–3 are thermal and fire containment. Zones 4–6 are the electrical side. Zones 7–8 keep the environment and the balance of plant honest.
The eight zones inside the box
The map above is the shape of the problem. What follows is what each zone actually asks of a material, because the requirements differ more than people expect.
Walking the zones
1. Between the cells. Cells swell and breathe as they cycle. The pad between them has to keep pushing back for years while also serving as a thermal and flame barrier if a neighbor fails. Cellular-silicone fire barriers do all three jobs in one die-cut part: a compression plateau that tolerates stack-up movement, thermal isolation, and barrier performance when it counts.
2. The tightest gaps. Some designs leave only a few thousandths of an inch for insulation. Polyimide-aerogel films earn their place here, delivering more thermal standoff per mil than nearly anything else available. Where a layer must survive direct flame contact rather than merely slow heat, inorganic mica sheet holds the line, because there is no organic content in it to burn away.
3. Module and rack faces. Between modules and across rack faces, aerogel blankets slow heat transfer in contested space. This is the layer that helps keep a single-module event from becoming a rack event, and it is usually where thickness is fought over hardest.
4. The power conversion system. Inverters concentrate heat in IGBT and SiC modules. Thermal interface materials carry that heat into the heat sink, and most of them insulate electrically at the same time. They are specified by thermal impedance at your actual mounting pressure per ASTM D5470, not by bulk conductivity alone. Graphite spreaders move more heat than any other family here, but they conduct electricity, so they serve only where the module and system isolation scheme is verified elsewhere.
5. Electronics enclosure seams. A container full of fast-switching power electronics is a radio-noise generator, and the grid-facing world regulates that noise. Conductive elastomer gaskets close the seams. Near the battery hazard, designs typically call for a material that pairs MIL-DTL-83528 Type M shielding performance with a UL 94 V-0 flame class, confirmed on the manufacturer’s data sheet for that specific grade and thickness.
6. Busbars and high-voltage runs. As system voltages climb toward 1,500 V, insulation between conductors becomes its own discipline. Aramid paper is the conformable wrap-and-layer default, with high dielectric strength and a 220 °C relative thermal index. Polyimide film packs the highest dielectric strength per mil into the tightest clearances, with corona-resistant variants where partial-discharge exposure exists near fast-switching stages. Rigid laminates such as G-10 provide insulation that also carries structural load, as standoffs and phase barriers machined to a drawing.
7. Doors, panels and the weather. Outdoor enclosures live with UV, ozone, rain and a decade of door cycles. Closed-cell EPDM sponge is the outdoor default; closed-cell silicones step in where temperature, flame class or coastal salt governs. The gasket line also defines the sealed volume that gas detection monitors, so a tired seal is more than a water problem.
8. Breathing and the balance of plant. A sealed enclosure that cycles from cold nights to hot afternoons pumps air in and out. ePTFE protective vents let it breathe without letting moisture march past the gasket line. Around and underneath everything sit the auxiliary parts: vibration-isolation pads under the unit, acoustic barriers on resonant panels, filter media on HVAC intakes.
Why the converting step matters
None of these materials arrives at a battery plant ready to install. Each has to become a part: a specific geometry, thickness and tolerance, often laminated to an adhesive or a facing, sometimes kiss-cut onto liners so an assembler can peel and place hundreds per shift without variation creeping in.
That conversion step is where a data sheet becomes a repeatable component. It is also where a multi-layer stack-up can be converted as a single part instead of assembled by hand at the line, which removes a whole category of build variation. Repeatability is precisely what fire-test results and hazard analyses depend on, so it is worth more attention than it usually gets during sourcing.
How H-O Products helps
H-O Products is a family-owned die-cutter and converter in Winsted, Connecticut, founded in 1971 and an ISO 9001:2015 certified organization. We cut, slit, laminate and kit the material families described above to customer drawings, from prototype quantities through full production runs, working from long-standing relationships with the manufacturers who make these materials.
We are not a battery integrator, a test laboratory or a stocking distributor of finished parts. What we do is turn the specified material into the specified part, consistently, and flag it when a drawing and a material look mismatched before it becomes a production problem.
What to send for a fast answer
Whether you come to us or to another converter, the same handful of details turns a conversation into a part: the drawing or a dimensioned sketch, the material family or simply the problem if the family is still open, the gap or thickness available, the temperature and voltage the part lives with, and, for anything near the battery hazard, the fire-safety context. With those in hand an engineering review can usually say quickly whether the material you have in mind is the right direction or whether an adjacent family fits better.
If you are still narrowing the field, the BESS material reference includes an interactive finder that takes a location and a binding constraint and points to the family that usually leads, along with the data to send with the drawing.
Frequently asked questions
What does BESS stand for?
Battery energy storage system. It refers to the complete installation, meaning the battery modules together with the management system, power conversion equipment, thermal management, enclosure and safety systems, rather than the cells alone.
Are battery energy storage systems safe?
Lithium-ion cells carry a real, well-characterized thermal runaway risk, and the industry addresses it in layers: cell monitoring, gas detection, suppression, code-governed spacing and ventilation, and passive barriers between cells and modules. Installation codes such as NFPA 855 and test methods such as UL 9540A exist specifically to demonstrate that a single cell failure stays contained.
What materials go inside a BESS?
Beyond the cells and electronics, the converted material content typically includes cellular-silicone fire barriers between cells, aerogel films and blankets for thermal isolation, mica sheet where direct flame contact is possible, thermal interface materials in the power conversion system, conductive elastomer EMI gaskets at enclosure seams, aramid paper and polyimide film for busbar insulation, EPDM and silicone sponge door gaskets, ePTFE vents, and auxiliary isolation and filter media.
Does H-O Products make battery energy storage systems?
No. We are a die-cutter and converter. We supply the converted material parts that go into systems built by battery manufacturers and integrators, cut and laminated to their drawings.
Talk through a BESS part with engineering
Tell us the zone, the constraint you are up against, and what the part has to survive. We will confirm the material direction and quote the die-cut part from your drawing.
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
Related articleThermal isolation vs. fire protection: layering a barrier stackRead →
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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). H-O Products is a die-cutter and converter; we do not perform fire testing or issue listings.
References
- NFPA. NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, 2026 edition.
- UL Solutions. UL 9540A, Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems, 6th Edition.
- ASTM International. ASTM D5470, Standard Test Methods for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials.
- U.S. Department of Defense. MIL-DTL-83528, General Specification for Conductive Elastomer Gasket Material, EMI/RFI Shielding.
- UL Solutions. UL 94, Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances.
Product names referenced are trademarks of their respective owners. H-O Products converts these materials and makes no independent certification claims on their behalf. Flame ratings such as UL 94 are grade- and thickness-specific; confirm on the manufacturer's current data sheet.
Code and standard summaries here are informational and reflect published editions as of August 2026. Adoption varies by jurisdiction. Confirm current requirements with your test laboratory and the authority having jurisdiction.