EV Engineering & Infrastructure

How are EV battery packs tested for leaks?

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Electric vehicle (EV) battery packs must resist water ingress and retain coolant while maintaining electrical isolation. This article explores how automotive engineers verify sealing integrity across three areas: the pack enclosure, coolant circuit, and electrical feedthroughs. It also explains how test requirements guide the selection of helium or forming gas and discusses the role of venting in equalizing pressure without compromising water resistance.

Enclosure leak testing

As shown in Figure 1, engineers test individual enclosure components and assembled housings on dedicated stations. Each station seals a defined volume and pressurizes it with air or tracer gas.

Figure 1. Two EV battery-pack enclosure leak-test stations. The station on the left performs pressure-decay or differential-pressure-decay testing and backfills failed parts with tracer gas for sniffer-based localization. The large-format station on the right compensates for atmospheric-pressure changes during pressure-decay testing. (Image: Cincinnati Test Systems)

For high-throughput screening, production lines use air-based pressure-decay testing, which avoids tracer-gas consumption. Differential pressure decay compares the test part with a reference volume to resolve smaller pressure changes. Large enclosures take longer to fill and stabilize thermally, while wall deflection can produce apparent pressure loss and limit accuracy.

When the rejection limit falls below the practical sensitivity of pressure-decay testing, integral tracer-gas testing provides a more sensitive measure of total leakage.

Hard-vacuum systems detect tracer gas as it enters an evacuated chamber. In contrast, accumulation systems track rising tracer concentration inside a sealed chamber near atmospheric pressure. If the measurement exceeds the limit, engineering teams scan welds and sealing joints with a sniffer probe to isolate the leak.

Cooling circuit leak testing

Even a small internal leak can compromise thermal management or electrical isolation. This risk drives separate screening of the cooling plate and its fluid connections before pack assembly, rather than relying on end-of-line testing.

The test method and rejection limit depend primarily on the operating fluid and pressure. Production lines use pressure decay for rapid screening, while tracer gas resolves smaller losses. Direct-refrigerant circuits generally require tighter limits than water-glycol loops because their cooling performance depends on maintaining the specified charge. If a component fails, tracer-gas sniffing follows the fluid path to isolate the leaking joint.

Electrical feedthrough leak testing

Electrical feedthroughs and connector pass-throughs pose a distinct challenge for automotive manufacturers. Each conductor passes through a compact enclosure interface that must maintain sealing integrity and electrical insulation.

Service ports and cable exits present similar leak paths. Pressure-decay screening assesses overall leak tightness across these interfaces. If screening detects excessive leakage, tracer-gas sniffing isolates the source at conductor seals or mating surfaces.

Production screening establishes only the assembly’s initial leak tightness. Once the pack enters service, vibration repeatedly loads the feedthrough assembly, and connector mating wears the same interfaces.

Temperature cycling creates a separate failure mechanism: differential thermal expansion stresses bonded interfaces. After exposing the assembly to these service conditions, engineers repeat leak testing to determine whether the seals retain their integrity.

Choosing between helium and forming gas

As shown in Figure 2, a single detector platform may support leak testing with either helium or hydrogen.

Figure 2. The INFICON LDS3000 detects helium and hydrogen tracer gases, with sensitivity down to 10⁻¹¹ mbar·l/s and an EcoBoost mode that reduces helium consumption by up to 90% for cost-sensitive production runs. (Image: INFICON)

The required detection limit provides the starting point for tracer selection:

  • Helium: Its low atmospheric background supports high-sensitivity mass-spectrometry and accumulation testing. Helium is also inert and nonflammable. Higher tracer concentrations improve detector response and may shorten cycle time, depending on the instrument and test configuration.
  • Forming gas: This 5% hydrogen and 95% nitrogen blend reduces test-gas cost and dependence on helium when the application allows lower sensitivity. Although classified as nonflammable, it’s a compressed gas that requires ventilated, gas-rated equipment. The diluted hydrogen fraction raises the minimum detectable total leak rate, and hydrogen already present in the test area can elevate background readings.

Battery-pack production lines typically use forming gas for sniffer localization. Lower-limit integral tests generally favor helium. Ultimately, the rejection limit should guide the choice.

Keeping water out while equalizing pressure

Leak-rate measurements alone do not establish water-ingress protection, which begins with the enclosure design. Welded or bonded seams provide a continuous barrier, while gaskets and sealed connectors protect serviceable interfaces.

Validation then verifies performance under direct water exposure. Depending on the application, this may include IP67 temporary-immersion testing or vehicle-specific IP6K9K spray testing. Notably, production leak testing does not subject every pack to these exposures. Instead, it compares measured leakage against a threshold correlated with the validation results.

Even after passing ingress validation, a sealed enclosure must accommodate pressure changes caused by temperature and altitude. Without equalization, the pressure differential can deform the cover or draw water through a marginal seal.

An expanded polytetrafluoroethylene (ePTFE) membrane vent allows air exchange while blocking liquid water and particulates under rated conditions. After vent installation, test fixtures must temporarily seal the airflow path or compensate for its rated flow. Otherwise, intentional vent flow can mask enclosure leakage.

Routine pressure equalization and emergency gas release require different flow capacities. As shown in Figure 3, a dual-stage vent can integrate both functions.

Figure 3. A dual-stage battery-pack vent uses a membrane to equalize internal pressure while blocking water and particulates during normal operation. During thermal runaway, the relief stage opens a higher-flow degassing path to limit enclosure pressure. (Image: SAE International)

Separating these functions maintains ingress protection during routine operation without restricting emergency gas release.

Summary

EV battery pack leak testing combines production screening with environmental validation. Pressure-decay and integral tracer-gas tests establish whether an assembly meets its leak limit. When leakage exceeds that limit, sniffing isolates the failure for repair.

Required sensitivity guides tracer selection, while post-exposure testing determines whether the seals retain their integrity. Dual-stage venting maintains ingress protection during normal operation and provides a separate path for emergency gas release.

References

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