In battery pack design, foam may appear to be a relatively simple cushioning layer.

In practice, however, selecting the right foam can be a much more complex engineering decision.

How thick should the foam be?

Should you choose 1 mm, 2 mm, or 3 mm?

Does a higher density always provide better support?

Is softer foam better at accommodating cell expansion?

And perhaps most importantly:

How much compressive force will the foam provide over its service life?

These questions cannot be answered by thickness or density alone.

For battery pack compression applications, the more important factors are the foam's compression behavior, compression force, recovery, stress relaxation, and long-term compression performance.

1. Why Is Foam Used in Battery Packs?

Lithium-ion cells are not dimensionally static components.

During charging and discharging, cells can undergo changes in thickness. Depending on the cell chemistry, construction, and aging condition, irreversible swelling may also develop over time.

This is particularly important for pouch and prismatic cells, where dimensional changes need to be managed within the battery module or pack structure.

A compression foam layer can help:

• Accommodate cell thickness variations

• Maintain controlled contact pressure

• Compensate for assembly tolerances

• Reduce relative movement between components

• Provide cushioning against vibration and mechanical impact

• Maintain mechanical support as the cell dimensions change

Therefore, foam inside a battery pack should not simply be considered a gap-filling material.

It is a functional mechanical component that interacts with the cell throughout the battery's operating life.

This is why battery compression materials are often evaluated using parameters such as Compression Force Deflection (CFD) rather than thickness and density alone.

2. A Common Misconception: Does Thicker Foam Provide Better Cushioning?

Not necessarily.

Suppose a battery pack design has a 2 mm available gap.

It may seem reasonable to select a 3 mm foam and compress it into the available space. A thicker foam may appear to provide more room for cushioning.

However, thickness only describes the geometry of the material.

What the cell actually experiences is the compressive force generated by the foam at a specific compression level.

For example, two foams may both have an initial thickness of 3 mm:

• Foam A may generate relatively high compressive force during compression.

• Foam B may generate lower and more gradual compressive force.

Although their thickness is identical, their effect on the cell can be very different.

The same principle applies when comparing different thicknesses of the same material.

A thicker foam may provide more displacement capacity, but if it generates excessive compressive force at the operating compression level, it may not be suitable for the application.

The key question is therefore not:

"How thick should the foam be?"

It is:

"What compressive force will the foam generate across the actual operating compression range?"

3. Which Foam Parameters Matter in Battery Pack Applications?

When evaluating compression foam for a battery pack, several parameters should be considered together.

3.1 Compression Force Deflection (CFD)

Compression Force Deflection, or CFD, describes the force generated by a foam when it is compressed to a specific deflection.

For example:

• 10% compression

• 25% compression

• 50% compression

The resulting force at each compression level provides a much clearer picture of the material's mechanical behavior than density alone.

For battery applications, the objective is generally not to select the hardest material available.

Instead, the foam should provide an appropriate and controlled compressive response within the cell's allowable pressure range.

This makes the compression curve an important part of material selection.

3.2 Compression Curve

A single CFD value does not always provide enough information.

For example, a supplier may provide:

Compression Force @ 25% = XX kPa

This value is useful, but what if the actual application operates between 10% and 40% compression?

The foam's behavior across the entire compression range may be more important than its performance at one specific test point.

Consider two hypothetical materials:

The two materials may show similar performance at 25% compression.

However, their behavior can become significantly different at higher compression levels.

If cell expansion increases the compression of the foam during service, this difference can become important to the overall pack design.

For this reason, engineers should evaluate the force-deflection behavior across the expected compression range, rather than relying on a single data point.

4. Long-Term Compression: Will the Foam Maintain Its Force?

Initial compression performance is only part of the equation.

A battery pack is expected to operate through repeated charging and discharging cycles.

The cell may expand and contract repeatedly, while long-term aging can introduce additional dimensional changes.

The foam therefore needs to maintain its mechanical function over time.

This is where stress relaxation and compression set become important.

Stress Relaxation

Stress relaxation occurs when a material is held at a relatively constant deformation while the force it generates gradually decreases over time.

For example:

A foam may initially generate 100 kPa at a given compression level.

After remaining compressed for an extended period, the force may decrease.

If the original design only considers the initial compression force, the actual long-term performance may differ from the initial test result.

Compression Set

Compression set describes the material's tendency to retain permanent deformation after being compressed.

A high compression set may reduce the material's ability to recover its original thickness and maintain consistent mechanical support.

For battery compression applications, both properties should therefore be considered when evaluating long-term performance.

5. Does Softer Foam Always Provide Better Cell Expansion Management?

Again, not necessarily.

It is tempting to assume that softer foam will automatically be better because it can accommodate more expansion.

However, an excessively soft material may not provide sufficient mechanical support.

On the other hand, an excessively stiff material may generate excessive compressive force as the cell expands.

The objective is to find the appropriate balance between:

Cell expansion
→ Foam thickness
→ Compression ratio
→ Compression force
→ Recovery
→ Long-term force retention

This is why battery pack foam selection is fundamentally a mechanical matching problem, rather than simply a material hardness decision.

6. Why Is Microcellular PU Foam Considered for Battery Applications?

Microcellular polyurethane (PU) foam can be engineered to provide different compression and recovery characteristics.

Depending on the formulation and structure, microcellular PU foam can offer a combination of:

• Controlled compression

• Cushioning

• Elastic recovery

• Vibration management

• Stress distribution

• Long-term compression performance

These characteristics make microcellular PU foam a potential material solution for applications such as:

• EV battery packs

• Battery modules

• Pouch cell compression

• Cell-to-cell cushioning

• Cell-to-module cushioning

• Battery expansion management

However, "PU foam" by itself is not a sufficient material specification.

Different PU foams can have significantly different:

• Density

• Hardness

• Cell structure

• Compression force

• Recovery behavior

• Compression set

• Temperature performance

Therefore, the correct question is not:

"Can PU foam be used in a battery pack?"

The more useful question is:

"Which PU foam provides the compression behavior required by the specific battery design?"

7. A Practical Approach to Battery Pack Foam Selection

If you are currently evaluating compression foam for a battery application, start with the actual operating conditions rather than the foam specification.

Step 1: Define the Cell Type

Identify whether the application uses:

• Pouch cells

• Prismatic cells

• Cylindrical cells

The mechanical requirements can vary significantly between different cell designs.

Step 2: Define the Available Gap

For example:

Available gap: 2.5 mm

The available space is one of the key factors determining the required foam thickness.

Step 3: Define the Target Compression

Determine how much the foam will be compressed during assembly and operation.

For example:

Initial compression: 20–30%

The actual value should be determined by the specific battery design rather than applying a universal target.

Step 4: Define the Required Compression Force

This is one of the most important parameters.

Instead of simply telling a foam supplier:

"We need a soft foam."

It is much more useful to specify the required compression force or pressure range.

Step 5: Consider Cell Expansion

Estimate how the cell thickness may change during operation and aging.

The foam should then be evaluated across the corresponding compression range.

Step 6: Evaluate Long-Term Performance

Consider:

• Compression set

• Stress relaxation

• Recovery

• Temperature resistance

• Compression cycling performance

Step 7: Select the Foam Thickness and Density

Only after the above parameters are understood should thickness and density be finalized.

In other words:

Do not start with thickness and then search for a foam.

Start with the mechanical requirements, then determine which foam construction and thickness can meet them.

8. What Information Should You Provide When Requesting Battery Foam Samples?

A general inquiry such as:

"Please recommend a foam for EV battery applications."

usually does not provide enough information for a meaningful material recommendation.

A more useful technical inquiry would include:

Application: Cell-to-cell / cell-to-module compression pad

Cell type: Pouch / Prismatic / Cylindrical

Available gap: XX mm

Target foam thickness: XX mm

Initial compression: XX%

Required compression force: XX–XX kPa

Operating temperature: XX–XX °C

Compression cycling: XX cycles

Required compression set: XX%

Additional requirements: Flame retardancy, electrical insulation, thermal resistance, adhesive backing, die-cutting, or other requirements

The more clearly the application conditions are defined, the more accurately a foam supplier can evaluate potential materials.

9. How to Evaluate a Battery Compression Foam from a Supplier

When comparing different materials, it is easy to focus on the first few numbers on a technical data sheet:

Density.
Thickness.
Hardness.

These parameters are useful, but they do not tell the complete story.

For a compression application, it is often more useful to ask for:

• Compression Force Deflection data

• Force-deflection curves

• Compression set data

• Stress relaxation data

• Recovery characteristics

• Temperature performance

• Long-term compression performance

• Sample testing under the actual application conditions

This approach allows engineers to evaluate the material based on its functional performance, rather than simply comparing catalog specifications.

10. A Better Way to Think About Battery Pack Foam Selection

For battery applications, the selection process can be simplified into one engineering chain:

Cell behavior
↓
Available gap
↓
Foam thickness
↓
Compression ratio
↓
Compression force
↓
Stress relaxation / compression set
↓
Long-term mechanical performance

This is a more complete way to evaluate compression foam than simply asking whether a material is "soft" or "hard."

For XY Foams, microcellular PU foam solutions such as the INF series can be evaluated based on the specific requirements of the application.

The material selection should be based on the required compression range, compression force, recovery behavior, temperature conditions, and long-term performance rather than on product grade alone.

Conclusion: Battery Foam Selection Is Not Just About Thickness

If there is one thing to remember when selecting foam for a battery pack, it is this:

Thicker foam does not automatically mean better cushioning, and softer foam does not automatically mean better expansion management.

The right material needs to match the actual mechanical behavior of the battery system.

The key questions are:

• How much will the cell expand?

• What is the available gap?

• What compression range will the foam experience?

• What compressive force is acceptable?

• How will the foam recover?

• How much force will it retain after long-term compression?

• How will temperature and cycling affect its performance?

A suitable battery compression foam should therefore be selected based on compression behavior and long-term performance, not thickness or density alone.

For battery pack developers evaluating compression foam, providing the cell type, available gap, target compression, required compression force, operating temperature, and long-term performance requirements is the best starting point.

Because in a battery pack, the most important question is not how thick the foam is.

It is how the foam behaves when the cell expands—and how that behavior changes over the life of the battery.