Reusing vehicle batteries can play an important role in conserving resources and lowering CO₂ emissions. Batteries that retain adequate performance and meet safety requirements can be repurposed for stationary second-life applications, ranging from residential energy storage to commercial and grid-scale storage systems.

Researchers at the Development Center X-ray Technology of Fraunhofer IIS and the Hochschule München , developed the AIR system to assess the mechanical integrity of used and accident-damaged high-voltage batteries without opening the battery pack or dismantling components. The system automatically detects cylindrical, prismatic, and pouch cells and identifies geometric irregularities that may indicate cell displacement, deformation, or swelling.

Mechanical Integrity as a Complement to State-of-Health Assessment

The electrical and electrochemical condition of a battery is typically assessed using the so-called State of Health (SoH). However, mechanical integrity is also an important consideration when deciding on battery reuse, transportation, or further diagnostic testing. Aging, vibrations, accidents, or deformation can affect battery cells and modules and may therefore have safety implications. To date, these aspects can only be assessed non-destructively to a limited extent in closed battery packs.

The AIR system addresses this challenge by enabling X-ray inspection of complete battery packs and providing additional information about their internal geometry and potential structural irregularities. The resulting X-ray findings form part of an overall assessment and do not replace electrical testing or case-specific safety evaluations.

Automated Analysis of 2D X-Ray Projections

The AIR system is designed as an X-ray scanning system. Depending on the inspection setup, either a battery pack or an entire vehicle is moved through the X-ray beam. The software developed as part of the project automatically analyzes the resulting 2D X-ray projections. In the test environment used, the evaluation of a battery pack took approximately five minutes. The actual total inspection time depends on several factors, including the size of the battery pack, the inspection setup, and the number of projections acquired.

Case Study: VW ID.3 Following a Frontal Collision

The performance of the AIR system was evaluated in a case study involving a VW ID.3. Prior to the investigation, the vehicle had been subjected to a frontal crash test under laboratory conditions at DEKRA. During the test, the ID.3 collided with the rear of a stationary VW Tiguan at a speed of 79 km/h.

Despite substantial deformation damage to the front section of the vehicle, the X-ray examination of the high-voltage battery installed in the vehicle's underbody revealed no safety-critical structural irregularities visible in the analyzed projections.

The radiographic analysis of the nine battery modules revealed a uniform cell arrangement. Cell boundaries remained parallel in the X-ray projections examined, and no overlaps or bulging were detected. The image analysis software automatically identified the pouch cells and compared the curvature of selected cell edges. No critical deviations indicative of structural damage were observed.

The X-ray examination was complemented by a hazard assessment conducted in accordance with DEKRA guidelines, an evaluation of the Event Data Recorder (EDR) data, analysis of battery management system data, and additional electrical testing.

Based on the overall assessment, the high-voltage battery was classified as EUCAR Hazard Level 0, the lowest hazard category, and deemed suitable for continued use in this particular case.

Cylindrical Cells: Automated Detection of Severe Battery Deformation

The method was also tested on a traction battery containing cylindrical cells following a crash. The AIR system analyzed an X-ray top view of the battery pack and automatically determined the positions of the individual cells.

The analysis revealed a region that had undergone significant deformation due to mechanical loading. Compared with a reference dataset, several cells were visibly displaced. The automated position analysis detected these deviations and delineated the affected area. No indications of cell swelling were observed in the X-ray projections examined.

Despite the absence of indications of swelling, further use of this traction battery is not recommended due to the clearly identifiable mechanical deformation. The findings demonstrate that X-ray analysis can automatically identify damaged areas and thereby support decisions regarding further diagnostic testing, safe storage, transportation, or recycling.

Potential for Battery Reuse, Safety, and Resource Conservation

The results of the AIR project can help improve the assessment of used vehicle batteries for second-life applications. A more comprehensive evaluation of mechanical integrity can also support the handling of accident-damaged vehicles and batteries, for example during transportation, storage, or further diagnostic testing.

The technology may also be relevant for future inspection concepts in the context of periodic vehicle inspections for electric vehicles. Looking ahead, the planned Giant-Eye CT system at the Fraunhofer IIS Development Center X-ray Technology in Fürth is expected to further expand the possibilities for three-dimensional analysis of complete electric vehicles.