Fire Mitigation Strategies

Author: Nrusimhan Seshadri, Balance Batteries

Having looked at the challenges and regulatory standards relating to fire hazards posed by Li-ion batteries in the previous article, in this article we will have a look at some of the fire mitigation strategies deployed across 10 BEVs in the current market.

The choice of thermal barrier materials (TBMs) for a battery pack depends on number of factors such as:

• type of cell chemistry
• form factor
• pack architecture (modular or cell to pack)
• most importantly the legislative requirements.

It is possible to pass the ECE 324 UN R100 regulation with very little in the way of thermal barrier material but to pass the Chinese GB38031-2020 a robust design factoring passive fire safety is vital.

Figure 1 shows the structure of a typical modular battery architecture, it consists of a pack top cover, modules, cross-members (mostly aluminium extrusions) for structural reasons, battery tray which houses the cold plate and the external bottom cover (omitted from figure) and finally reinforcements for protection against side impact. The top cover becomes crucial for occupant protection in an event of thermal runaway while the battery tray with bottom cover is crucial for protection against external fires. The cross members which act as a module housing need to withstand the jet fire with hot particle ejecta to delay thermal propagation between modules.

The modular/skateboard architecture is still the go-to choice for many large OEMs while companies like Tesla, which uses cylindrical cells, are trying to shift towards cell to pack design. In such battery architecture it common to find cell-cell thermal barriers which prevent heat flow between cells in an event of thermal runaway.

There are seven categories of TBMs used for passive fire protection of Li-ion battery packs as shown in Figure 2.

In the following table, we see the different types of TBMs used by different OEMs.

From this table, it can be seen that there is a trend among newer vehicles to focus more on passive thermal safety of battery packs, thanks mainly to new fire safety regulations in China which is one of the key markets for companies like Tesla.

We see from the benchmarked data that for cylindrical cells encapsulating foams and potting are widely adopted propagation mitigation measures (Lucid is an exception) while for prismatic and pouch cells a combination of module and cell level thermal barriers are used, for instance the GM’s Ultium platform uses aerogel-based cell barriers for propagation mitigation.

Figure 3 shows Tesla Model Y’s structural battery pack with the latest 4680 cylindrical cells and the pink material, which fills the battery pack, is the encapsulant which is highly insulative and can reduce heat flow between the cells in an event of thermal runaway.

Lucid Air’s battery pack is an exception which doesn’t use any potting materials though it is based on cylindrical cells and uses a thermoformed mica sheet in between every parallel layer of cells (Figure 4).

Mica has excellent insulating properties even at high temperatures and with its high dielectric strength it often finds applications inside the HV battery pack. Another use case of mica between modules or cells as propagation mitigation measure is in GM’s Hummer EVs vent cover as shown in Figure 5.

Here mica sheets are used as a cover above the cell vents possibly to counter against the jet fire and hot particle ejecta coming out of the cell during thermal runaway. For occupant protection against cell venting materials, OEMs like Tesla, Lucid and Rivian (see Figure 6) use a rigid mica shield in between the battery top cover and modules while the other strategy is to have top cover made up of SMC (sheet moulding compound) such as the one used by Ford as seen in Figure 8.

Typically, composites are lighter while the speed of manufacture is challenging compared to the likes of steel or aluminium. In the above table steel is considered as a fire protection measure as it has much higher temperature resistance than aluminium although that might not be the primary reason for material selection. None implies there isn’t any special TBMs other than the standard aluminium alloys.

There isn’t a definitive answer towards what is the ideal material for thermal runaway mitigation, every pack has its own unique set of requirements and challenges, and the solution must be tailor made.

At Balance Batteries we have invested in material selection and validation processes for meeting both the internal and external fire safety targets and have worked with multiple suppliers of TBMs across Europe so we can help you meet your design requirements while enhancing the overall safety of the battery pack.

About Balance Batteries – Founded in 2020, Balance Batteries consult on battery pack and module design and validation. Using a systems engineering approach we ensure your product requirements are met.

References

1. Munro Live, YouTube Channel