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Smart composite casing with embedded sensors for aircraft battery applications

Using the latest developments in thermoplastic composite processing and flexible printed electronics, IPC and its partners developed a lightweight, smart battery casing for aircraft applications. With optimised materials selection, part design and sensor embedding, the composite casing meets the sector’s functional and safety requirements, including resistance to cell thermal runaway. This article has been published in the JEC Composites Magazine N°149.

Smart composite casing with embedded sensors for aircraft battery applications
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5 minutes, 50 secondes

In order to provide electrical power in airplanes, it is necessary to integrate higher energy storage capacities in the next generation of aircraft, and this trend is true for all airplane segments. The battery technology expected to overcome this challenge currently uses lithium-ion batteries for their higher power density. However, the batteries’ weight is significantly increased by the need to add a metal enclosure to contain the well-known, feared event associated to lithium-ion technology: thermal runaway. To overcome this drawback, a lightweight composite packaging capable of resisting a cell thermal runaway was developed, with sensors integrated inside the casing structure to detect a potential thermal runaway and provide information on this possible critical in-flight situation. The development presented in this paper is conducted in the framework of the OASIS H2020 project (Open Access Single entry point for scale-up of Innovative Smart lightweight composite materials and components). The goal of this project is to develop the market potential of nano-enabled multifunctional lightweight composites, particularly polymer-matrix and aluminium composites. As part of this project, Thales is leading a specific showcase to develop an innovative composite battery casing equipped with sensors. This work was conducted in the continuity of previous studies devoted to the composite casing, which should be improved using the nano- technology product lines proposed in the OASIS project.

A high-performance composite casing
The new composite casing is expected to show the following properties:
– 10-20% higher gravimetric energy density of the battery system (ratio between the stored energy quantity and the battery’s weight) compared to a reference aluminium casing.
– Resistance to thermal runaway of one cell in the battery system and reduced fire propagation risk.
– Reduced thermal runaway risk through early detection.

Figure 1: The assembled smart composite casing

Thales’ showcase is subjected to several constraints that directly influence the material selection for the composite casing. Due to the aircraft environment, the casing should be as little sensitive as possible to fire and chemicals (DO-160 standard). It is also required to be insulated at U > 1300 V and impervious to gas, liquid, vapour and flames. The material should resist a long-time exposure to temperatures ranging from -20°C to 85°C for 10 years.
Lithium batteries for aircraft applications have to comply with the DO-311 standard and one of the strength requirements is to prove capable of containing the thermal runaway effect. When a battery cell runs into thermal runaway, the pressure in the casing strongly increases due to the limited gas evacuation through the venting hole. In addition, flames, fumes and cell electrolyte vapours are produced rapidly. Finally, the air temperature inside the casing grows up to 1300°C for a very short time (below one second for this cell format). Therefore, the casing has to resist high temperatures and limit flame propagation.

Smart thermoplastic composite casing concept
To meet these demanding specifications, a full-thermoplastic casing with embedded sensors was developed, providing both lightweight and monitoring functions. The casing is composed of two parts: a cylindrical body produced by injection moulding of a short fibre-reinforced technical thermoplastic polymer, and a hybrid cover produced from a thermoplastic composite prepreg and an overmoulded polymer using the stamping/overmoulding process.
This process is usually divided into three steps: first, a thermoplastic composite prepreg is heated above its melting temperature; then, the prepreg is stamped in a mould with the targeted shape; and finally, the stamped prepreg is overmoulded in specific areas to bring additional functions to the part.

The process developed for the smart cover makes it possible to integrate a printed sensor during the overmoulding step. This way, the temperature sensor is fully integrated in the composite part, thus simplifying the installation, reducing the wiring requirements, allowing more reliable temperature measurements and improving the battery’s safety.

Material selection and validation
Process-wise, some polymer matrices require high processing temperatures (370 to 400°C), which makes it difficult to integrate sensors. All the previously defined specifications must be taken into account to ensure that the casing will resist a thermal runaway and thus reduce the risk of fire propagation. In order to meet these requirements, various thermoplastic matrices were compared to select the most appropriate materials for this showcase. Table 1 summarizes the properties of these thermoplastic matrices regarding the material specifications for designing the casing.

Table 1 summarizes the properties of these thermoplastic matrices regarding the material specifications for designing the casing.

In addition to the properties reported in Table 1, a thermal simulation conducted by IPC indicates that the casing material stays below 115°C in case of a thermal runaway. The simulation also reveals that a pressure peak occurs when the material reaches a temperature of 40 °C. Based on these results, a PPS (polyphenylene sulphide) matrix was selected for both the composite prepreg and the overmoulding matrix. Using the same polymer ensures an optimum compatibility between the composite and the overmoulding material. To optimise the casing’s mechanical performance while limiting its weight, a PPS/carbon prepreg was selected. To meet the required dielectric properties, a PPS/glass ply was added on both sides of the PPS/C prepreg.

The mechanical properties were measured on material specimens by tensile, compression, in-plane shear stress and interlaminar shear strength (ILSS) tests. The tests confirmed that the selected materials meet the initial specifications and the experimental results were used later for simulations. The physico-chemical properties of the materials were analysed by DSC (differential scanning calorimetry) and TGA (thermo-gravimetric analysis) in order to validate the process window. Fire tests were performed on representative coupons to confirm the limited fire propagation. The dielectric properties were also evaluated to meet the electrical insulation specifications of the NF EN 60243 standard. Finally, a scanning electron microscope (SEM) analysis was conducted to inspect the materials’ microstructure (voids, delaminations, etc.). The fibre orientation is clearly visible on the SEM picture in Figure 2. No porosity was identified on the prepreg. The carbon fibre (in the middle of the pictures) and glass fibre (top and bottom) can be clearly identified. All these analyses made it possible to validate the selected materials in accordance with the battery casing’s specifications. Therefore, the composite prepreg used for the smart cover is composed of six plies of PPS/carbon for mechanical properties, with one ply of PPS/ glass on each side of the PPS/carbon ply stack for dielectric resistance. The overmoulding matrix is also a PPS resin reinforced with 65% short glass fibre.

Figure 2: SEM analyses of PPS/C + PPS/G

Mechanical simulation and design optimization

Based on an initial design, the dimensions and shape were adapted to composite materials and to IPC’s HCIM (Horizontal Composite Injection Moulding) process. A prepreg composite was chosen to provide the required mechanical properties. Overmoulding makes it possible to integrate additional functions such as sensors, localized reinforcements and assembly features.

Figure 3: Topological simulation on Optistruct

Mechanical and topological simulations (Figure 3) in static mode using the Computer Aided Engineering (CAE ; Altair® Optistruct and Abaqus® software) resulted in a design with several ribs on the covers and curves on the prepreg composite to improve the mechanical properties. The simulated maximum deformation of the casing was 1.62 mm in the middle of the cover, which is in line with the requirements. With this optimised casing design, the simulation revealed no crack or break in the composite structure, and showed resistance to the internal pressure target.

Sensor development
Thin temperature sensors were developed by the ARES (Activités de recherche sur l’électronique structurelle – Research Activities on Structural Electronic, formerly PICTIC) platform at CEA-Grenoble, using a specific process flow to withstand the…

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