Before I start this article, I want to say thank you to the brave men and women who serve as first responders and enter into unknown situations to serve as a first line of defense for the rest of us. From the bottom of my heart, thank you. Our industry is working hard to limit the number of unsafe situations you have to face, but our efforts so far are not yet sufficient to ensure your safety.

And so, before we dive in, let’s set the scene for all that we’ve covered so far in this Wholistic Battery Safety newsletter. From defining what a wholistic approach to battery safety looks like, to evaluating the estimated size of the risk, and to mitigation strategies that could help solve this problem. We’ve also analyzed the situation that NYC is currently facing, what we’re doing at Soteria to help work towards a solution, as well as discerning some of the ‘beautiful theories’ that people are referencing as the cause of these safety events. Which brings us to this article: what happens when the safety event turns from a fire, to an explosion?

Two Case Studies

Surprise, Arizona, April 19, 2019

In 2019, a lithium-ion battery storage fire occurred in the aptly named city, Surprise, Arizona. And it is at this time that four career firefighters had opened the door to the facility, yet were immediately, and ballistically propelled by a blast of flame—shooting over 75 feet horizontally and 20 feet vertically—all within 30 seconds of opening the door. The four firefighters had become seriously injured after tending to what they originally had only seen as smoke coming from the energy storage facility that had gone dormant. One firefighter was propelled into and under a chain-link fence, and came to rest 73 feet from the door under a bush. Another was also propelled through the fence and came to rest 30 feet from the door. The two others were blasted sideways. All firefighters were unconscious after the event. Three of them suffered very serious injuries, including broken bones, while the other had facial lacerations.

A full, detailed report was prepared by the UL Firefighters Safety Research Institute, entitled "Four Firefighters Injured in Lithium-ion Battery Energy Storage System Explosion – Arizona.” Our thanks also go to the authors of this report, who gave us every available detail so that we can learn from this event and prevent future events.

Birmingham, Alabama, March 31, 2023

An 18-wheeler truck carrying at least 25 fifty-five-gallon drums of used lithium-ion batteries exploded at a Flying-J Truck Stop on March 31, 2023. In the explosion, the four sides and the top of the trailer were launched away from the truck, and the truck frame was bent severely toward the ground. The driver was not near the truck, and nobody was hurt thankfully, allowing firefighters to put out the fire without incident.

There is far less information available about this more-recent event, but one report is here: CTIF: Barrels of discarded lithium batteries exploded during transport on their way to recycling. As you can see from the pictures, the blast flattened the walls of the trailer and severely bent the frame.

Some Common Features of the Two Events

There are several overlapping features of the two events that can help us think about design considerations within battery installations of various sizes, from cell phones to acres-wide energy storage facilities.

• Container: Both events had a container around the batteries.
• Large gas space: Both events had large, open gas space inside the container, which could retain explosive gases prior to ignition.
• Large quantity of batteries: Both events involved a large quantity of batteries.
• No venting: neither had anydesigned venting, which would trigger in the case of a non-thermal runaway venting of the batteries.
• Explosions: both events resulted in explosions that were at magnitudes of at least 10s of feet, both horizontally and vertically.

Cause of the Explosions: a Plausible Conjecture

Just labeling this what it is—a plausible conjecture—because as is the case with most lithium-ion battery fires, the fire consumes most of the evidence. But what likely happened was:

• One or more cells had a thermal event that did NOT result in thermal runaway, but rather resulted in venting of the battery into the nearby space.
• Likely, this occurred in more than one cell in each case, so there was a significant level of heating involved.
• The space immediately outside the cells filled with flammable gases—hydrogen, evaporated electrolyte, and others.
• In the absence of a spark, or in the absence of oxygen, the gases continue to accumulate until they reach a dangerous proportion.
• Then, either a spark occurs, or oxygen is introduced into the system, and it causes an explosion.

The explosion differs from a standard thermal runaway because the gas is initially contained, which allows pressure wave to build as the flame front accelerates away from the ignition source. Once this pressure wave blast ruptures the container it then forces a massive release of pressure, which in the case of lithium-ion batteries, is coupled with the introduction of more oxygen into the explosive system.

Implications on Battery Pack Design and Battery Transport Packaging

These two events have immediate implications on battery pack and battery transport packaging design.

Battery Pack Design

Simply put, design your battery pack to avoid the accumulation of vent gases from a cell that vented without going into thermal runaway. The two most important considerations are:

• Void space: reduce or eliminate void space in the battery enclosure. Cell phones have almost none. Crudely designed packs using 18650s may have a lot.
• Vent: Design an outward opening vent into the pack that will open up when the pressure of a cell venting event occurs, but obviously will also remain otherwise, to protect the batteries from the elements. This can be with a plastic membrane or a breakaway vent.

One may think this will only affect big batteries, such as electric vehicles or energy storage. But small battery packs of even less than 1 kWh can contain a lot of flammable gases if they are not designed well. And a tight enclosure can create sufficient pressure to turn the battery itself into a projectile, or create a blast of flame large and severe enough to create significant injuries. Cell phones are small batteries with very low gas space. E-bikes, motorcycles and light electric vehicles can contain a large amount of void space, and thus have the potential to become very explosive.

Battery Transport Packaging

In order to protect the batteries, the transport will likely have a lot of void space, which may even be filled with flammable materials like foam, paper or other packing materials. Thus, they must have proper ventilation, and may even be equipped with forced air throughput to prevent the accumulation of flammable gases. Additionally, other materials that could contribute fuel to the fire should be minimized if possible.

Conclusions

Battery fires are themselves very dangerous and can ignite other materials and structures. But when the fire is delayed, and the delay causes the accumulation of flammable gases in a closed container, the potential for explosion can be present, which can present extreme safety implications.

For those of you with your thinking caps on, the recommendations above do not go beyond the obvious, and there are likely some innovations here that can both reduce fire and reduce the open space without too much added cost or weight to the system. I encourage you to share your ideas in the comments below, or better yet, build them and make them available to the market!