“Can you design and build a battery pack for our product?”
“Absolutely. Where are you in the project? The planning phase?”
“Actually, no. The project is just about complete. All that’s left is to design a Li-ion battery pack that will prevent thermal runaway. We left a small space in the product to fit this solution. That’s enough, right?”
“ Well…”
I can’t tell you how many times I’ve had this exact conversation, or some version of it.
Too often when designing Li-Ion battery packs, brands make the mistake of trying to squeeze a battery safety solution in at the end of the product phase. When that happens, there’s often no room for solution—or at least, not ample room for a solution that will actually safely mitigate the risk.
I totally understand that brands need to try to make these battery packs as compact, light, and low-cost as possible to fit their products. But when it comes to thermal runaway, you can’t ignore the physics of the problem.
That’s not to say that we can’t do it. It just becomes more challenging, and the risks can become greater.
That’s why it’s so important to design a solution that will prevent thermal runaway. To do that properly, you first need to understand the circumstances that create these risks.
Thermal runaway isn’t always a preventable problem. Li-ion batteries are high-energy storage devices. If they go to failure, there’s a lot of heat that gets generated within the battery pack. It has to go somewhere.
That said, thermal runaway is almost always caused by specific circumstances that we see all too often—and they are all preventable.
As a result, these components within the modules are often too tightly packed—in many cases, literally touching each other, including:
This leaves no room for battery safety requirements.
Most of the time, it’s only a matter of time before Li-ion cells go into thermal runaway. If the cells are too tightly packed, the propagation will spread quickly from one component to another. No amount of material will stop it from propagating if there’s a conduction pathway like that.
First, it’s important to understand the difference between flame retardant and flame suppressant materials. We talked about this in our recent blog post, Flame Retardant vs Flame Suppressant: Does the Difference Really Matter for Battery Safety?
What’s key to know is that flame retardant is not the end-all-be-all solution that it’s often marketed as. It’s not just about preventing thermal runaway, because sometimes it can’t be prevented.
To ensure safety, heat needs to be removed from the pack, and flame retardant material doesn’t do that effectively.
This is especially the case when, as we often see, the components immediately surrounding the battery pack aren’t sufficiently flame-resistant. And, in many cases, they are actually dangerously flammable, such as an outer casing made of cheap plastic. These shortcuts—often made for spacing, timing, or budgetary constraints—can cause either head conduction or venting into other cells or modules within the pack.
Ideally, your battery pack should be designed with an approach to quickly remove or convert that heat, which includes appropriate venting to quickly remove flammable gases from building up in the battery pack.
Simply put, the most effective method for preventing thermal runaway is by transferring sufficient heat out of the battery pack system. That’s the physics you can’t ignore.
So, if you’re venting right into another component or other cells, you’re basically creating a torch without a way to stop it from propagating.
To configure proper venting pathways, you can orient the cells so they always vent a certain way that you can control, or put a place where the hot gases can be directed outside of the battery pack.
Many times, engineers use mica or ceramic sheets to block the venting, but usually this just directs the heat back into the cells.
There’s a delicate balance that needs to be reached: you need enough headspace for proper ventilation, but don’t want too much headspace because oxygen can get pulled in and create more flammable conditions. That’s why it’s important to work with a thermal management expert.
So, how do you prevent thermal runaway?
First, you need a flame suppressant solution that can extinguish flames, stop propagation, and neutralize the danger.
Second, you need ample cell spacing so they don’t vent into one another. Obviously, the more the better—but 2mm is typically the sufficient minimum. Ideally, it’s 3-4mm.
Third, you need to capture and convert the heat through proper venting, which is challenging because there typically isn’t much room for additional material inside the battery pack—especially when battery safety isn’t part of the product design from the beginning.
It’s also challenging to dissipate or transfer these levels of extreme heat out of the battery pack quickly without it catastrophically impacting other cells, components, or sub-modules.
That’s where XTS® comes in.
XTS® can be configured by cell format, spacing, and architecture to fit your unique application:
Here’s how it works: XTS® is a proprietary phase change gel with very high thermal energy absorption around the onset temperature of thermal runaway. When it vaporizes, it has significant cooling capacity while also generating non-flammable gas that acts as a flame suppressant.
Heat conversion properties that happen almost immediately when the gel reaches 110-120 °C, in which the gel goes through a liquid-to-gas phase transition.
In other words, under thermal runaway conditions, the extreme temperatures heat the gel, which turns into gas that instantly suppresses and extinguishes the flames—in less than a second.
3g of gel per cell is the minimum for a typical 21700 cylindrical format battery pack to ensure enough heat is converted during thermal runaway to prevent propagation.
This phase change happens so quickly that the extreme heat event within the battery pack is immediately addressed, ensuring that any heat transferred to adjacent cells or pack components is extinguished before it can cause damage to the product or the end user.
The Science Behind the Safety: How XTS® Works