A model of safety

Battery Safety Simulation

AVL’s simulation approach for safer batteries

As the electrification of the powertrain has become more prominent, the importance of the battery has and continues to grow. So too have the demands placed on it, and so the issue of safety has become a major factor during development and homologation of electric vehicles with Lithium-Ion battery systems.

Different types of battery cell abuse can lead to a condition known as ‘thermal runaway’. This is where the temperature increase within a battery cell triggers a chain of self-accelerating exothermic chemical reactions, leading to a sudden release of energy and the venting of highly toxic and flammable gas. When the thermal runaway propagates to the neighboring cells and modules, the entire energy stored in the battery pack, if released can lead to massive fire and explosions. That’s why it is fundamental to contain thermal runaway to prevent propagation from one cell to the next – and new safety requirements demand this level of protection for batteries in electric vehicles.

Currently almost all OEMs and battery manufacturers are facing the same problem: how to ensure they meet and surpass the new regulations regarding protection and safety measures in case of thermal runaway. From this year onwards Global Transportation Regulations will become mandatory in almost all countries around the globe. OEMs will need to prove that passengers will have enough time to safely escape from a vehicle which is experiencing thermal runaway in a single battery cell. In numbers, that means that there must be at least a safe time period of five minutes between warning the driver (and therefore the detection of a safety critical event) and visible flames and fire outside the battery. To realize this goal, we investigate different countermeasure concepts to determine which are the most sensitive and effective.

However, exploring any new technical concept can be costly and time consuming. And so, to cut time and costs in development, at AVL we have developed our own methodology for the simulation of thermal runaway events and venting behavior in a pack, allowing fast and easy analysis of these phenomena.

Simulation for Quick and Cost-Effective Battery Development

The AVL approach conducts tests to characterize a single cell in the test chamber. The cell’s behavior is then transferred to and integrated into module and pack-level simulation models for the assessment of:

  • Propagation time of thermal runaway, between first cell and neighboring cells
  • Venting flow distribution and thermal risk to other modules or the pack sealing
  • Melting of housing, cover or high voltage isolations, which would lead to electrical failures
  • Distribution of toxic or flammable gases within the pack
  • Assessment of the flammability of the venting gas, including the actual combustion of the flammable gas
  • Layout and optimization of burst discs for the controlled release of gases from the pack

To achieve these simulations, we use our thermodynamic modelling tool AVL FIRE™ M. This dynamic tool offers several unique features which allow the accurate modelling of those harmful and extreme conditions. In occurrences such as the melting of thin aluminum covers or plastic parts, we investigate the influence on the gas flow distribution before and after the melting of the pack cover can be assessed.

This kind of simulation offers a wide range of benefits, some of which would not be possible any other way. For example, to gain insight into internal phenomena during fire in a battery pack. This type of knowledge cannot be obtained via post-mortem analysis as the pack is normally completely destroyed. Additionally, this approach allows us to carry out sensitivity analysis of different countermeasure possibilities, since experience has shown that even small changes can have a significant impact on the overall behavior in a case of thermal runaway. Furthermore, the simulation allows to quickly and cost-effectively explore the effectiveness of fire-retardant materials.

Simulation allows development teams to investigate how thermal propagation behavior changes depending on which cells are triggered inside the pack. These specific kinds of explorations are otherwise only possible with very expensive destructive tests. Thus, simulation offers benefits in time and costs for development engineers, and increased safety for the passengers of electrified vehicles. Last but not least, simulation ensures design maturity and facilitates the understanding of test results.

Key Benefits

  • Hazard limitation and safety assessment
  • Lower production costs and a shorter time to market thanks to simulation technology
  • Protection and safety measures in case of thermal runaway
  • Fast and easy analysis of phenomena
  • Simulation allows you to evaluate the effectiveness of fire-retardant materials