Webasto's Direct Cell Cooling News: Where Battery Thermal Management Meets HVAC

A production announcement with a wider engineering lesson

On August 3, 2026, Webasto announced production of a battery system for the new Mercedes-AMG GT 4-Door Coupé at its Schierling plant. The company describes oil circulating directly around the cells and identifies automated assembly and inspection processes supporting production. The release concerns a specific battery system and manufacturing program. It does not establish that direct cell cooling is appropriate for every vehicle or that a cabin HVAC supplier can deliver the same architecture. [Source]

The announcement is useful because it shows how far the term thermal management can extend beyond an air conditioner mounted on a roof. Buyers may encounter a cooling plate, a liquid circuit, a chiller, an electric compressor and an air-handling unit within the same vehicle. Those items perform related but different functions.

For an OEM or specialist vehicle builder, the first purchasing task is to describe the system boundary. Buying a component before defining the heat path can leave important responsibilities unassigned. This article explains how to prepare a battery-related thermal brief while keeping cabin comfort, battery performance and production evidence separate.

Draw the heat path before choosing the hardware

A useful thermal diagram starts at the source of heat and ends at the environment receiving it. For a battery, the path may include the cell, an interface material, a cooling surface or surrounding fluid, connecting lines and a heat exchanger. Depending on the architecture and operating condition, another circuit may then move heat to outside air.

Each stage has a temperature difference and a resistance to heat transfer. A strong downstream cooling device cannot automatically overcome a poor local interface close to the cell. Likewise, a carefully designed local interface cannot reject heat if the rest of the circuit lacks adequate flow or heat-rejection capacity.

The diagram should identify what each supplier controls. The battery designer may define cell spacing and allowable temperatures. A thermal supplier may provide a module or heat exchanger. The vehicle integrator may control pumps, fans and power allocation. Put those responsibilities beside the interfaces rather than leaving them in separate quotations.

This exercise also clarifies terminology. A refrigerant evaporator serving cabin air is not the same item as a liquid-to-refrigerant chiller, even though both may transfer heat into a refrigerant circuit. Clear names prevent the buyer from comparing proposals that solve different parts of the problem.

Distinguish cooling architectures without declaring a universal winner

An indirect liquid arrangement typically transfers heat through a solid boundary before it reaches the circulating coolant. A direct-contact arrangement places a suitable fluid in contact with surfaces that would otherwise be separated from the cooling circuit. The practical design questions differ because the fluid, materials, sealing and electrical environment differ.

Neither description alone predicts the result in a particular pack. The buyer needs the intended heat load, temperature distribution, operating envelope and service concept. Packaging constraints can make one approach attractive in a specific vehicle while making it unsuitable in another. A production announcement demonstrates one implementation, not a universal selection rule.

Ask the system designer to explain why the proposed architecture matches the duty. Is the main challenge a short peak load, sustained operation, fast charging, cold-weather conditioning or a combination? Which constraints are fixed by the vehicle, and which remain open to design changes? Those answers are more useful than a ranking based on technology names.

The comparison should include manufacturing and service implications as well as thermal performance. A design with fewer external connections may introduce other assembly or inspection requirements. A component that performs well on a bench may become difficult to replace in the installed vehicle. Evaluate the complete lifecycle of the proposed arrangement.

Keep battery targets distinct from cabin targets

Passengers and battery cells do not share a single temperature requirement. Cabin control responds to comfort, air distribution and occupant requests. Battery thermal control responds to limits and targets defined for the cell and pack under different operating states. One system cannot simply inherit the other's setpoint.

If the two functions share a compressor or heat-rejection device, the control strategy must decide how capacity is allocated. A charging event may create a different demand pattern from ordinary driving. The driver may also request cabin conditioning while the vehicle is connected to an external supply. The design brief should include these simultaneous conditions.

A buyer should ask which function has priority when capacity is constrained and how that decision is communicated. The answer should come from the vehicle's system design and applicable requirements, not from an assumption that cabin comfort always wins or always loses. Operating limits and protective behavior need explicit ownership.

For a supplier discussion, prepare a matrix with rows for driving, charging, parked conditioning and storage. In each row, identify the thermal loads, available power and required behavior. This turns a broad request for integrated thermal management into a set of reviewable operating cases.

Use temperature distribution, not only an average

An average temperature can hide a hot location. Two packs may report the same average while having different spreads between their warmest and coolest monitored points. The allowable distribution must be defined by the battery designer, and the measurement plan must be capable of showing it.

Specify where temperatures are measured and why those locations are representative. Include the expected difficult positions, such as areas with less favorable flow or different contact conditions. A test plan should explain how the measurement arrangement relates to the thermal model rather than choosing convenient sensor positions after the hardware is built.

Transient behavior also matters. A system may reach an acceptable steady condition while taking too long to respond to a changing load. Define the operating sequence and the period over which temperatures are assessed. Record the starting condition so that two tests do not appear comparable when one began with a much cooler pack.

For procurement, ask for the relevant evidence at the proposed configuration level. A generic temperature statement for a component does not establish the distribution inside the customer's assembled pack. The component supplier and pack integrator should agree which tests belong to each party and how the results will be combined.

A simple heat-flow calculation and its limits

For a steady single-phase liquid stream, a basic energy balance is heat flow equal to mass flow multiplied by specific heat capacity and the temperature rise across the load. This relationship helps organize a discussion, but it does not by itself design a battery cooling circuit.

Consider a hypothetical load of 2,000 watts and a hypothetical fluid with a specific heat capacity of 3,500 joules per kilogram per kelvin. If the allowed bulk temperature rise is 5 kelvin, the corresponding idealized mass flow is 2,000 divided by 17,500, or approximately 0.114 kilograms per second. These values are invented solely for the arithmetic example and are not specifications for any named product.

The calculation says nothing about local flow distribution, pressure loss, pump efficiency, heat-transfer coefficients or the temperature difference between a cell and the bulk fluid. It also assumes the stated heat load and fluid properties are appropriate. A real design needs those additional inputs and validation under the relevant conditions.

The useful procurement question is therefore not whether a supplier can repeat the formula. It is whether the supplier can identify the assumptions, provide the missing data and show how the proposed design is checked. A transparent calculation is a starting point for engineering review, not evidence that an untested configuration is ready for production.

Define fluid and material compatibility at the system level

The working fluid interacts with seals, metals, coatings, plastics and any surfaces it contacts. In a direct-contact arrangement, that scope can be especially important. A buyer should request a defined fluid specification and a compatibility assessment for the actual materials used in the proposed assembly.

Avoid describing a fluid as suitable merely because it belongs to a familiar category. Formulation, additives, contamination and operating temperature can affect behavior. The responsible designer should specify the approved fluid and any limits on substitution. Service documentation should use the same identification as the production specification.

The test plan should address the conditions relevant to the intended lifetime. Ask how material changes, supplier substitutions and manufacturing residues are controlled. If a seal compound or coating changes, determine whether the previous compatibility evidence still applies. A small purchasing substitution can alter an interface that the original validation assumed was fixed.

These questions do not require a buyer to perform chemical qualification independently. They require the buyer to know which organization owns that qualification and what evidence supports the selected combination. Clear ownership helps prevent a component-level assurance from being mistaken for approval of the complete circuit.

Manufacturing quality belongs in the thermal brief

Thermal performance depends on repeatable manufacturing. Dimensions, surface condition, assembly alignment and cleanliness can influence the heat path or fluid circuit. A prototype assembled with special attention does not automatically represent a production process running at volume.

Ask how critical characteristics are defined and inspected. The answer should connect a manufacturing measurement to a functional requirement. For example, a dimensional check may protect an interface fit, while a leak test may verify an assembled boundary. The test method, acceptance criterion and traceability need to be appropriate to that characteristic.

Production records should link the relevant assembly to its component batches and inspection results. The buyer does not need unrestricted access to proprietary manufacturing details, but it does need an agreed route for investigating a field issue. Traceability is useful only if the recorded information can support a practical containment decision.

During supplier approval, distinguish a process description from evidence that the process is operating as intended. Request representative records and a defined sample approval sequence. Agree how deviations are reported and who can authorize them. This gives both parties a way to manage changes before they affect shipped vehicles.

Plan for isolation and service access

Battery-related thermal equipment may be installed in locations that are difficult to reach. A service plan should identify which assemblies can be inspected or replaced and what vehicle preparation is required. The responsible vehicle manufacturer must define safe procedures for high-voltage systems and other relevant hazards.

Procurement should ask for the service boundary without requesting improvised repair instructions. Is the thermal module replaced as a complete assembly? Are certain connections intended to remain untouched in ordinary service? Which inspections require specialized facilities? These answers affect workshop capability and recovery time.

Fluid handling and contamination control also belong in the service concept. The design team should define how the approved condition is restored after a permitted intervention. A replacement part that fits physically may still require a controlled filling, inspection or commissioning process before the vehicle returns to service.

Document the information a service provider must capture before replacing an assembly. Operating state, fault history and relevant temperatures may help distinguish a thermal-control issue from a local hardware fault. Replacing the most visible component without understanding the system can leave the underlying problem unresolved.

Compare proposals with a responsibility matrix

A thermal proposal often spans several companies. One supplies the battery pack, another the compressor or chiller, another the vehicle controls and another the final installation. Without a responsibility matrix, every quotation can look complete while an important interface remains outside all of them.

Use rows for thermal targets, mechanical packaging, fluid specification, electrical supply, control messages, protection behavior, validation and service documentation. For each row, identify the organization that defines the requirement, the organization that supplies evidence and the organization that accepts the result.

Include the conditions under which responsibility changes. A supplier may validate a module at its connectors while the vehicle builder validates the installed circuit. That division can be reasonable if the boundary conditions are agreed. It becomes problematic when each party assumes the other has tested the complete vehicle.

The matrix should also cover change control. A revised pump, different hose routing or updated control calibration may affect more than one supplier. Specify who reviews the change and which tests must be repeated. This helps the project remain coordinated after the initial design meeting, when individual teams begin making detailed decisions.

An illustrative project review

Imagine a specialist vehicle builder considering a battery cooling module while also adding cabin air conditioning. The buyer initially requests a single cooling capacity figure and a package price. That request leaves unresolved whether the two functions share a refrigerant circuit, whether they operate simultaneously and who controls their priorities.

In a more useful review, the builder first supplies the battery designer's approved temperature targets and the vehicle's operating modes. The HVAC supplier describes the proposed cabin system and its own boundaries. The integration team then maps simultaneous loads and identifies the interfaces that require coordination.

The project can now produce two distinct deliverables: a thermal architecture drawing and a validation responsibility plan. The drawing explains how heat moves. The plan explains which evidence will demonstrate that the proposed arrangement meets the agreed requirements. Neither document needs to claim that the design duplicates the technology in a manufacturer news release.

A sample vehicle is then used to evaluate the agreed conditions. The team records deviations and resolves them before extending the configuration to production. This sequence may reveal that a shared arrangement is appropriate, or that separate systems provide a clearer solution. The decision follows the actual vehicle requirements rather than the appeal of a technology label.

Allocate the temperature budget across the whole path

A simple temperature-budget exercise can help a buyer identify where more information is needed. Suppose a hypothetical design permits a 12-kelvin difference between a defined heat-source point and the final coolant reference point at a particular steady load. The team should not allocate all 12 kelvin to the cooling device while ignoring the interfaces before and after it.

List the physical stages in order: the internal path from the heat source, the mating interface, the local cooling surface and the fluid-side transfer. The responsible engineers can estimate or measure the temperature difference associated with each stage under the same condition. The values must use consistent boundaries so that they can be combined meaningfully.

If one stage consumes most of the available difference, the improvement effort should address that stage. A larger downstream heat exchanger may not solve an unfavorable local contact. Similarly, improving contact may provide limited benefit if the fluid arriving at the assembly is already too warm for the required operating point.

The exercise also exposes responsibilities. The thermal supplier may control the plate or fluid module, while the pack designer controls the cell arrangement and the vehicle integrator controls the incoming fluid condition. A total-temperature requirement cannot be assigned fairly until those contributions and assumptions are understood.

Use the budget as a design aid, not a replacement for validation. Thermal behavior can vary with load, temperature and time, and the simplified allocation may not remain constant throughout the operating envelope. The test plan should examine the cases that determine whether the complete assembly meets its requirements.

For procurement, ask each party to state the evidence behind its contribution. A measured interface result is different from a preliminary estimate. Keeping that distinction visible helps the program decide where an early prototype will provide the most useful information.

Finally, preserve the temperature-reference locations in drawings and reports. If one team measures at a cell surface and another at a different structural point, their results may not answer the same question. Consistent measurement locations make the budget reviewable and reduce disagreement when a later test produces a different number.

Questions that clarify a supplier conversation

Can a parking air conditioner cool a traction battery?

Not by assumption. A parking air conditioner is designed for its stated cabin application. A traction battery requires a dedicated thermal design, approved interfaces and validation defined by the battery and vehicle designers. A supplier should not reclassify a cabin product as a battery cooling solution without that engineering work.

Does direct cell cooling always outperform cooling plates?

The architecture name alone cannot establish a winner. Performance, packaging, fluid compatibility, manufacturing and service requirements must be compared for the specific pack and duty. A successful implementation in one vehicle is evidence of that implementation, not a universal result for all applications.

Can the same compressor serve cabin and battery loads?

Some vehicle architectures coordinate multiple thermal functions, but suitability depends on the complete system design. The buyer needs defined simultaneous loads, control priorities and operating limits. A shared compressor proposal should include the interfaces and evidence needed to support the intended behavior.

How should VETHY be involved in a new thermal project?

Start with the vehicle application and the requested scope. VETHY's HVAC project discussion can help define cabin systems, modules and integration questions for evaluation. High-voltage battery cooling, direct-contact fluids and specialized validation must be confirmed separately; the news discussed here is not evidence of VETHY participation in the named program.

A better definition of thermal expertise

For buyers, thermal expertise is visible in the quality of the system boundary, the relevance of the questions and the evidence behind a proposed design. It is not established by placing several different cooling products under the same heading. A useful partner explains what it can supply, what must be coordinated and what remains to be validated.

The next project brief should therefore begin with operating modes, heat loads, temperature targets and interface ownership. Product selection follows those inputs. That order helps a vehicle manufacturer compare proposals fairly and prevents an attractive component from becoming an incomplete system once installed.