One of the great technical challenges of manufacturing electric vehicles is ensuring the performance and safety of the large battery packs which power them.
A leaking battery is more than just an inconvenience. The most common type of EV battery, lithium ion, can burst into flame or even explode if there is a leak.
All the components of an EV battery are vulnerable to leaks – the cells, the modules, the cooling components and the packs that make up the final assembly. In all cases, part size, accepted leak rate, and temperature are key variables.
For this post, let’s focus on the leak test at the end of the battery pack assembly process to ensure the integrity of the fully assembled pack. This leak test is often more challenging than any tests performed at the component or subassembly level, due to the myriad of factors at play.
The unique test challenges of EV battery pack leak testing:
- These assemblies are generally designed to be as light and cost-effective as possible. This poses challenges with ballooning and general part instability.
- These packs have large internal volume and surface area. Parts with large surface area are particularly susceptible to environmental influences.
As a first step in ensuring a reliable leak test for EV battery packs, the manufacturer must understand the impact which product design and material selection choices will have on establishing the proper test specification and test method.
Different leak test methods have their own testing strengths and weaknesses as they pertain to the unique physical characteristics of a pack. We are still governed by the ideal gas law: pressure (P) x volume (V) = amount of gas in moles (n) x universal gas constant (R) x absolute temperature (T) of the gas or PV=nRT.
Any test method that builds pressure (with air) inside the pack can cause it to physically expand like a balloon. This will influence the measured leak rate and potentially lead to a false pass or fail condition depending upon the amount of expansion and contraction that occurs.
Changes in ambient air temperature or barometric pressure, can create a unique situation that is particularly problematic when attempting to use a pressure or flow-based measurement system. Even small changes in ambient temperature or barometric pressure are amplified by the flexible nature of the components in today’s battery packs.
Test methods for EV battery pack leak testing: Pros and cons
Pressure decay
With pressure decay, leak rate must be based on a PD cal factor (the slope and offset to convert a pressure change to a leak rate). That’s because this leak test method relies on the calibration factor to give an accurate measurement. If you use a pressure decay test for flexible battery packs, volume references for calculation of leak rate will result in incorrect results.
Mass flow
Mass flow does not require calibration factors or PD cals. It will read the same leak rate regardless of how large the pack. For this reason, mass flow could likely be a better choice for large and flexible battery packs. This test method is less susceptible to variations in part stiffness.
However, always bear in mind that a measured leak will still vary between packs if the measurement is taken before each pack has stabilized and reached thermal equilibrium.
Tracer gas testing
Tracer gas testing is also an option, ideal for testing to IP67-IP69 standards. In this case, the pack is placed in a leak-tight chamber and bombarded/saturated with helium. If there are leak paths present, the helium will be forced through them and into the pack. A mass spectrometer is then used to “sniff” for any evidence that this helium is then leaking from the pack.
Learn more about how we can help you find the best approach for your EV battery pack testing needs.