Coatings for battery energy storage systems and electric mobility

The transition toward renewable energy supply and electric mobility is creating entirely new requirements for surface coating. While corrosion protection or appearance used to be the primary focus, today additional electrotechnical properties are required, properties that must be provided by the coating itself.
One key topic is the insulation of metallic components to provide reliable protection against electrical flashover.
Conventional paints cannot meet these requirements. New coating systems, combined with the right application method and coating process, can. As a surface specialist, BENSELER has been addressing these new challenges in the automotive and energy industries for several years and offers field-proven solutions.

At BENSELER, the focus is on industrial and automotive applications that require a high level of insulation safety for metallic components made of aluminum or steel in order to protect electrical energy storage systems. Due to their favorable structural properties, these metal components are frequently used, for example as enclosures, covers, cooling plates or side plates in battery systems for electric vehicles.
At the same time, the non-automotive segment is also growing. Large-scale battery storage systems help buffer energy generated by renewable power plants using wind and solar energy. The battery systems required for this purpose, known as stationary energy storage systems, must be installed safely in robust racks.
While plastics offer the advantage of being electrically non-conductive, they often do not meet the required levels of rigidity and stability. In addition, they are less heat-resistant. For this reason, metal components remain indispensable in vehicle body construction as well as in the installation of large-scale battery energy storage systems.
What is required are cost-effective coating processes that ensure permanently reliable electrical insulation.

Electrical insulation is not just another coating
The requirements for an electrical insulation coating differ significantly from those of conventional coating systems. To ensure complete insulation, a standard coating is usually not sufficient, even when combined with a prior cathodic electrocoating / e-coat (KTL) primer. Even the smallest pores, defects or surface irregularities in the coating can enable electrical flashover and must therefore be eliminated through suitable process measures. This requires targeted adjustments to both the coating process parameters and the temperature profile. Both have a direct impact on the reaction behavior of the coating material as well as on the pre-treatment and post-treatment of the components.

End-to-end quality assurance
Due to the system-critical safety function of coated components, quality assurance requirements are significantly higher than for conventional corrosion protection coatings. A simple visual inspection is not sufficient for an electrical insulation coating. Instead, a technical 100% dielectric strength test across the entire surface of every component in a production series is generally required, including complete documentation. In addition to specific technical expertise, this also requires dedicated testing equipment and trained personnel. Without such high-voltage breakdown testing, it is not possible to make a reliable statement about the quality of the electrical insulation. Because of the safety-critical nature of the coating, any irregularity leads to the immediate rejection of the coated components. This can result in high scrap rates and increased component costs.

New concepts for cost-effective rework and masking
For BENSELER, the focus in large-scale coating production is always on the economic viability of the solution for the customer. The selected processes must meet both the technical requirements and the economic framework conditions. This includes developing suitable rework concepts before series production begins in order to keep scrap rates as low as possible.
In the context of new electrical insulation coatings, one key question is how components that do not fully meet the coating requirements can be safely reworked so that they pass the subsequent insulation test and achieve the required durability values for the insulating layer. Any reworked area must meet the same technical standards as the original coating system. Various processes may be used for this purpose, but they must always be individually aligned with the functional requirements of the component, the coating material used, the layer structure and the durability requirements.
There is therefore no universal off-the-shelf solution. BENSELER develops the right solution in close coordination with coating material manufacturers and customers. For example, partial or complete stripping followed by recoating of insulation-coated components is generally not economically feasible due to the high layer thicknesses involved. This makes forward-looking planning of masking concepts during the development phase all the more important. In many cases, not all component surfaces may be coated with an insulation coating — for example electrical contact areas or later assembly or bonding surfaces. For these areas, it is necessary to develop a cost-effective masking concept in advance that is also suitable for large-scale production.
Adhesive masking as well as silicone covers may be used for this purpose. However, both must meet the increased requirements of the coating system and, in some cases, modified oven profiles. Due to the sometimes very high coating thickness of insulation coatings compared with conventional corrosion protection coatings, edge build-up or burr formation at masked areas is also significantly more pronounced and may require additional rework.

Successful large-scale coating production
BENSELER demonstrates in daily practice that these challenges can be successfully mastered in large-scale series production of electrical insulation coatings, both in the automotive sector and in the energy industry.
Among other applications, BENSELER coats housing covers for battery control units in electric vehicles, with average annual volumes in the high six-digit range. Coating thicknesses of up to 250 µm ensure high dielectric strength at voltages of 1,500 volts and significantly above.
In the field of components for stationary energy storage systems, BENSELER achieves coating thicknesses of 300–500 µm using a two-layer powder coating process, providing protection against electrical flashover at voltages above 7,500 volts.
The insulation-coated aluminum add-on components used in battery racks prevent electrical flashover between neighboring battery packs.
Complete testing and traceable documentation of all coated components are mandatory.

Increasing requirements: combining electrical insulation and corrosion protection
Demand for suitable solutions is high. Electrical insulation coatings are becoming increasingly important, while the required performance levels and coating thicknesses are rising just as quickly as the voltage levels within the systems. Experienced handling of higher coating thicknesses is therefore essential.
In ongoing projects, BENSELER demonstrates that cost-effective and reliable insulation of metallic components in large-scale series production is possible with adapted coating solutions. The prerequisites are forward-looking planning and close coordination with coating material manufacturers and customers, as the coating methods and process parameters must be individually tailored to the specific requirements of each component.
Based on the insulation values required by the customer and the definition of which areas of a component are to be coated, which areas serve as transition zones and which areas must remain uncoated, BENSELER’s surface specialists develop well-balanced coating solutions with regard to technical performance and cost efficiency — across the entire process chain, from pre-treatment and primer coating through to the final electrical insulation coating.
At the same time, the high requirements for corrosion protection remain unchanged. Vehicles, as well as stationary battery energy storage systems, are exposed to weather and environmental influences for decades. BENSELER therefore combines the electrical insulation coating, among other options, with a prior cathodic electrocoating / e-coat (KTL) or WBK pre-treatment, washing, pickling and preservation.
In this way, the coating system fulfills not only the requirements for electrical insulation, but also the desired level of corrosion protection — up to heavy-duty corrosion protection, for example in combination with a high-build KTL coating.

Next step: fire protection — safeguarding the worst-case scenario
Development is continuing. In addition to protection against electrical flashover and corrosion, BENSELER’s specialists are already focusing on how system safety can be further increased in a worst-case scenario. Combinations of powder coating and wet paint coating / liquid coating in particular offer promising approaches for meeting increasing fire protection requirements and integrating additional safety-related functions. The approach is deliberately technology-open: processes such as zinc flake coating can also be integrated into corresponding safety concepts. In this field, BENSELER has a particularly strong market position and is one of the leading providers in Germany.

Author:
Björn Dehne, Head of Sales, BENSELER Beschichtungen Sachsen GmbH & Co. KG