Custom Cell-to-Cell & Cell-to-Module Compression Pads for EV Batteries

A cell-to-cell compression pad is an engineered foam placed between two adjacent cells in a battery stack. Its job is to hold a near-constant push-back force as the cell changes thickness. Every lithium-ion cell breathes reversibly each cycle as lithium moves into and out of the anode, and swells irreversibly over life – cells commonly run about 10–20% thicker at end of life.

The pad is specified for a flat compression-force-deflection plateau and low compression set so its force stays inside a useful window across that growing deflection, keeping electrode layers in contact without crushing the cell. A cell-to-module pad does the same job between a cell group and the rigid end plate.

There is no single universal number; the target is a window that depends on the cell chemistry, format, and the cell maker's guidance. Research indicates a light-to-moderate preload commonly helps capacity retention and life, while over-compression may accelerate aging. Pouch cells have no rigid housing and rely more heavily on external compression for stability; prismatic cells are stiffer but still benefit from controlled stack pressure.

Because published beneficial and harmful pressure levels vary widely across studies and cell types, the practical approach is to set the preload window from your cell supplier's recommendation and your stack stiffness, then pick a pad whose CFD plateau keeps the force inside that window from the beginning-of-life gap through end-of-life swelling.

Both hold compression on the cell stack, but they sit in different places and see different duty. A cell-to-cell pad is the thin foam layer between two adjacent cells; it manages the gap between that pair and is usually a softer, thinner grade. A cell-to-module pad is the thicker cushion between the whole cell group and the rigid module wall or end plate; it absorbs the accumulated swelling of the entire stack and holds preload against the frame, so it is usually a firmer, thicker grade.

In cell-to-pack designs the module frame is deleted and the pad also takes up tolerance and damps vibration. All of them follow the same rules: flat CFD plateau, low compression set, low stress relaxation across the deflection the joint actually sees.

It comes down to the housing. A pouch cell is wrapped in a flexible laminate with no rigid case, so it relies on external compression to keep its electrode layers in contact and to manage breathing and swelling – without a pad, the layers can delaminate and lose contact. Prismatic cells have a stiffer can but still benefit from controlled stack pressure as they swell.

Cylindrical cells are wound inside a rigid metal can that largely constrains the jelly roll on its own, so they are generally self-constraining and do not need an external compression pad between cells the way pouch and prismatic stacks do. That is why cell-to-cell and cell-to-module compression pads are primarily a pouch-and-prismatic concern.

Compression force deflection (CFD) is the stress a foam pushes back with at a given percent compression, measured per ASTM D3574 Test C (the related ISO method is ISO 3386). It matters because the shape of the CFD curve is what makes a pad work. Microcellular foam shows a roughly linear-elastic region at low compression, a wide plateau in the working zone where the force barely changes, and a steep densification region at high compression.

A compression pad is designed to live on that plateau: as the cell swells and the pad compresses further, the push-back force stays nearly constant rather than climbing out of the useful window. When you specify a pad, ask for the full CFD curve and confirm your working deflection sits on the flat plateau, not in the densification knee.

They are built for opposite jobs. PORON microcellular polyurethane is a compression material: it is engineered for a flat CFD plateau, low compression set, and low stress relaxation, so it holds a controlled push-back force as a cell swells. A thermal gap filler is engineered for thermal conductance and tolerance take-up – its stress-strain shape is wrong for holding a controlled spring force, and used as a compression pad it may relax and let the preload decay or stiffen and spike the force out of the window.

For cell-to-cell or cell-to-module compression, specify a true compression material (PORON microcellular PU, PORON EVExtend, or BISCO silicone foam). Reserve gap-fill foam for the thermal job it was made for; if one part must do both, call it out so engineering can confirm a stack-up that serves both duties.

For most EV battery modules, lead with PORON microcellular polyurethane: it delivers the flattest CFD plateau and the lowest compression set across normal module temperatures, which is exactly what a compression pad needs. Choose BISCO silicone foam when temperature or flame rating governs – it offers higher continuous-temperature capability and stronger flame performance, with UL 94 ratings available up to V-0 on specific grades.

The trade-off is that silicone foam has a different force-deflection behavior than PU, so confirm the CFD curve and compression-set value for the grade. In short: PORON PU for the best mechanical behavior in normal temperatures; BISCO silicone foam for higher heat or a stringent UL 94 requirement.

Start from three numbers: the beginning-of-life gap the pad has to fill, the end-of-life swelling allowance the cell will add, and the target push-back force inside your preload window. The pad and grade are then chosen so the compression at the beginning-of-life gap and the compression at end-of-life swelling both land on the flat part of the CFD curve – not so light that it falls below the window early, and not so thick or firm that end-of-life swelling drives it into densification.

A slightly thicker pad on a flatter grade often holds the window better than a thin pad that runs out of plateau as the cell grows. Send those numbers with your stack drawing and engineering will confirm a grade and thickness against the TDS.

A cell changes thickness in two ways. Reversible "breathing" is the small, repeating thickness change each charge-discharge cycle as lithium moves into and out of the graphite anode – the cell gets slightly thicker on charge and recovers on discharge. Irreversible swelling is the gradual, permanent growth over life from gas generation and solid-electrolyte-interphase growth; it does not recover, and it commonly leaves cells about 10–20% thicker at end of life.

A compression pad has to handle both at once: it flexes a little each cycle with the breathing, and it absorbs the slow one-way swelling over years – all while keeping its push-back force inside the useful window. That dual demand is why a flat CFD plateau and low compression set matter so much.

Compression set is the permanent thickness a foam loses after being compressed, held, and released – the part of the deflection the material never recovers, expressed as a percentage. Lower is better. It is measured per ASTM D395 (Method B, constant deflection, is the most specified) or ASTM D3574 for flexible PU foam; note that D395 and D3574 report against the original height while D1056 reports against the compressed height, so values from different methods are not directly comparable.

Low compression set is critical because a pad that takes a permanent set thins out under sustained load, pushes back with less force, and can eventually go slack – letting the cell move and losing contact. Purpose-built microcellular PU battery grades are specified precisely because they hold a low compression set across a long service life.

A compression pad's primary job is mechanical – holding preload and accommodating swelling – not fire suppression. That said, flame-rated grades are available and matter where the pack design calls for them. Foam flame performance is classified under UL 94: low-density foams carry HBF and HF-1 classes, and V-0 is the most stringent vertical-burn rating, available on specific silicone-foam grades. Ratings are grade- and thickness-specific, so verify the exact rating on the TDS for your gauge.

At the pack level, the relevant references are UN 38.3 for lithium-cell transport safety and UL 9540A for thermal-runaway propagation testing of energy-storage systems; a compliant pad-level material can support a pack's compliance with those standards, but the pad is not itself "certified to" UN 38.3 or UL 9540A – those apply at the pack and system level.

Yes. H-O is a die-cutter and converter, and every compression pad is made-to-order to your stack drawing. We die-cut, kiss-cut, laser-cut, and waterjet PORON microcellular PU, PORON EVExtend battery grades, and BISCO silicone foam to your profile, laminate adhesive where the pad needs to stay located, and stack layers to a target thickness, with liners and pull-tabs for repeatable line installation.

Samples typically ship in 3–5 business days for common configurations on materials we keep on hand, and standard production lead time after drawing approval is 2 weeks; special orders run custom lead times. MOQ varies by material and part; prototype quantities through full production runs are equally accepted. Send your stack drawing, the gap and swell numbers, and the target force through the form below for a specific quote.