The thermal management module (TMM) is a critical safety component within an electric vehicle’s (EV) battery pack. It regulates the temperature of the battery cells through a liquid cooling system, requiring absolute internal and external sealing. Any leakage can lead to coolant loss, electrical short circuits, and ultimately, catastrophic thermal runaway. Consequently, stringent leak testing is a mandatory quality control step. This article details the design, implementation, and validation of a high-precision, automated sealing detection system based on helium mass spectrometry, specifically engineered for the mass production of EV battery pack TMMs.
Traditional methods like pressure decay tests are often inadequate for the stringent requirements of an EV battery pack, especially for detecting very fine leaks. Our system overcomes these limitations by employing helium (He) as a tracer gas, detected by a mass spectrometer. The core innovation lies in a two-stage (rough/fine) detection process with integrated helium recovery, which ensures high reliability, reduces operational costs, and meets the high-throughput demands of modern EV battery pack manufacturing lines.
1. System Principle and Methodology
The fundamental principle is vacuum-mode helium spray testing. The EV battery pack thermal management module is placed inside a vacuum chamber, connected to a helium supply, and the chamber is evacuated. Helium is then introduced into the module’s internal channels. If a leak exists, helium escapes into the chamber and is drawn into a connected helium mass spectrometer leak detector (MSLD), which provides a highly sensitive and quantitative leak rate measurement.
Our system enhances this basic principle with a staged approach to improve robustness and efficiency:
- Rough Leak Check: Before introducing the full test pressure, a brief, low-volume pulse of helium is injected. A subsequent quick MSLD reading identifies gross leaks. This step prevents contamination of the MSLD and vacuum system from a severely leaking EV battery pack component, protecting the instrument and saving time.
- Fine Leak Check: If the rough check passes, the module is pressurized with helium to the specified test pressure. The MSLD then performs a precise measurement to determine if the leak rate is within the acceptable threshold for the EV battery pack’s safety standards.
- Helium Recovery & Nitrogen Purge: Post-test, helium from the module is actively recovered into a storage tank for reuse, significantly lowering gas costs. Finally, the vacuum chamber is flushed with nitrogen to remove any residual helium, preventing cross-contamination for the next test cycle and ensuring measurement integrity.
The workflow is governed by a PLC and can be summarized by the following sequence diagram logic, implemented in software:
1. Load → 2. Evacuate Chamber → 3. Rough Check (Pulse He, MSLD Scan) → IF FAIL → Abort & Purge; IF PASS → 4. Fine Check (Pressurize He, MSLD Measure) → 5. Recover He → 6. Purge with N₂ → 7. Unload.
A self-calibration function using a calibrated helium reference leak is periodically performed to verify the MSLD’s accuracy.
2. System Architecture and Hardware Design
The system is designed for dual-station, alternating operation to maximize the utilization of the MSLD—typically the cycle time bottleneck. While one station is undergoing testing, the other can be purged, unloaded, and reloaded. The key hardware components are:
- Dual Vacuum Chambers: Fabricated from stainless steel with precision-welded seams (surface roughness Ra ≤ 0.3 μm). The chambers are tilted at a 20° angle for ergonomic loading/unloading of the often bulky and heavy EV battery pack module and to facilitate purging. A pneumatic, translational sealing mechanism ensures a leak-tight seal.
- Helium Mass Spectrometer Leak Detector (MSLD): The core sensor. It is positioned lower than the vacuum chambers to prevent accidental contamination by residual helium during the purge cycle.
- Vacuum System: Comprises separate roughing pumps for the chambers and a dedicated fore-vacuum pump for the MSLD.
- Gas Handling Manifold: A network of solenoid valves (V1-V6) controls the flow of helium, nitrogen, and the connection to the vacuum pumps and MSLD. Key components include:
- Helium supply and injection valve (V2).
- MSLD sampling valve (V3).
- Helium recovery valve (V4) leading to a compressor and storage tank.
- Nitrogen purge valve (V5).
- Calibration leak valve (V6).
- Sensors: Pressure transducers monitor test pressure inside the EV battery pack module. A vacuum gauge (Pirani or capacitance manometer) monitors chamber pressure. Position sensors confirm chamber lid status, and product presence sensors initiate the cycle.
- Control Cabinet: Houses the Programmable Logic Controller (PLC), power supplies, and motor drives.
- Human-Machine Interface (HMI): An industrial PC running custom software for parameter setting, cycle control, and data logging.
3. Control System and Software Implementation
The system’s automation is managed by a layered control architecture. The PLC is the real-time workhorse, handling all input/output (I/O) operations:
| PLC Inputs | PLC Outputs |
|---|---|
| Product Presence Sensors | Solenoid Valves (V1-V6) |
| Lid Open/Close Sensors | Vacuum Pumps (Start/Stop) |
| Safety Light Curtains | Audible/Visual Alarms |
| Operator Buttons (Start, Stop, E-Stop) | – |
| Analog: Pressure, Vacuum Gauge Signals | – |
The HMI software, developed in LabVIEW, provides the supervisory layer. It sends commands to the PLC, reads back process data (like leak rate from the MSLD via serial communication), displays real-time trends, stores all test results (Pass/Fail with leak rate value) for each EV battery pack module, and allows for recipe management for different product types. The software logic follows a strict state machine pattern to ensure safe and repeatable operation.
4. Performance Validation and Experimental Results
The system’s performance was validated against the stringent leak rate requirement for EV battery pack thermal management modules, typically on the order of $$1 \times 10^{-5}$$ mbar·L/s or lower. The test environment was controlled at 25°C and 50% RH. The total cycle time was optimized to 60 seconds. A certified helium reference leak with a known leak rate of $$1.5 \times 10^{-6}$$ mbar·L/s was used to simulate a defective part.
Experiment 1: Repeatability on a Known Good EV Battery Pack Module.
A leak-tight TMM was tested 15 consecutive times. The results demonstrate the system’s high precision and stability.
| Test # | Measured Leak Rate (mbar·L/s) | Result |
|---|---|---|
| 1 | 7.886 × 10-8 | Pass |
| 2 | 7.914 × 10-8 | Pass |
| 3 | 7.763 × 10-8 | Pass |
| 4 | 7.461 × 10-8 | Pass |
| 5 | 6.927 × 10-8 | Pass |
| 6 | 7.531 × 10-8 | Pass |
| 7 | 7.094 × 10-8 | Pass |
| 8 | 7.628 × 10-8 | Pass |
| 9 | 7.109 × 10-8 | Pass |
| 10 | 8.026 × 10-8 | Pass |
| 11 | 7.288 × 10-8 | Pass |
| 12 | 6.780 × 10-8 | Pass |
| 13 | 7.521 × 10-8 | Pass |
| 14 | 5.871 × 10-8 | Pass |
| 15 | 5.607 × 10-8 | Pass |
The repeatability (standard deviation of the measurements) is calculated using the Bessel formula:
$$s_x = \sqrt{\frac{1}{n-1} \sum_{i=1}^{n} (x_i – \bar{x})^2}$$
Where \(x_i\) is the individual measurement, \(\bar{x}\) is the mean of all measurements, and \(n\) is the number of tests. For the data above (\( \bar{x} \approx 7.08 \times 10^{-8} \) mbar·L/s), the repeatability is:
$$s_x \approx 7.08 \times 10^{-9} \text{ mbar·L/s}$$
This exceptionally low value confirms the system’s high measurement precision.
Experiment 2: Detection of a Simulated Leak.
The same good TMM was tested with the attached $$1.5 \times 10^{-6}$$ mbar·L/s reference leak. The system must reliably identify this as a failure.
| Test # | Measured Leak Rate (mbar·L/s) | Result |
|---|---|---|
| 1 | 1.403 × 10-6 | Fail |
| 2 | 1.407 × 10-6 | Fail |
| 3 | 1.424 × 10-6 | Fail |
| 4 | 1.433 × 10-6 | Fail |
| 5 | 1.411 × 10-6 | Fail |
| 6 | 1.697 × 10-6 | Fail |
| 7 | 1.447 × 10-6 | Fail |
| 8 | 1.414 × 10-6 | Fail |
| 9 | 1.467 × 10-6 | Fail |
| 10 | 1.399 × 10-6 | Fail |
| 11 | 1.368 × 10-6 | Fail |
| 12 | 1.369 × 10-6 | Fail |
| 13 | 1.391 × 10-6 | Fail |
| 14 | 1.394 × 10-6 | Fail |
| 15 | 1.386 × 10-6 | Fail |
The mean measured leak rate is $$1.427 \times 10^{-6}$$ mbar·L/s, closely matching the reference leak, and all tests correctly resulted in a “Fail.” The repeatability for this leak level is $$s_x \approx 7.9 \times 10^{-8}$$ mbar·L/s, demonstrating consistent performance across different leak magnitudes.
The system’s Minimum Detectable Leak (MDL) is effectively below $$1.5 \times 10^{-6}$$ mbar·L/s, which is more than sufficient for the quality gate of $$1 \times 10^{-5}$$ mbar·L/s for an EV battery pack thermal management module.
5. Conclusion and Outlook
The developed helium mass spectrometry-based sealing detection system provides a robust, reliable, and cost-effective solution for the high-volume production of EV battery pack thermal management modules. The implementation of a two-stage (rough/fine) testing protocol, combined with helium recovery and nitrogen purging, addresses key challenges of operational cost, cross-contamination, and instrument protection. The dual-station design maximizes throughput, making it suitable for integration into fast-paced manufacturing lines.
Experimental validation confirms that the system meets and exceeds the sensitivity requirements for EV battery pack components, with excellent measurement repeatability. The current system provides a quantitative total leak rate. Future development will focus on integrating a helium “sniffer” probe mode to not only detect but also precisely locate the source of a leak on a failing EV battery pack module, further aiding in root cause analysis and process improvement. This evolution will enhance the system’s utility from a simple pass/fail tool to a comprehensive diagnostic instrument for quality assurance in EV battery pack assembly.